Major Sectors and Their Hazards
The term construction industry is used worldwide to cover what is a collection of industries with very different practices, brought together temporarily on the site of a building or civil engineering job. The scale of operations ranges from a single worker carrying out a job lasting minutes only (e.g., replacing a roof tile with equipment consisting of a hammer and nails and possibly a ladder) to vast building and civil engineering projects lasting many years that involve hundreds of different contractors, each with their own expertise, plant and equipment. However, despite the enormous variation in scale and complexity of operations, the major sectors of the construction industry have a great deal in common. There is always a client (known sometimes as the owner) and a contractor; except for the very smallest jobs, there will be a designer, either an architect or engineer, and if the project involves a range of skills, it will inevitably require additional contractors working as subcontractors to the main contractor (see also the article “Organizational factors affecting health and safety” in this chapter). While small-scale domestic or agricultural buildings may be built on the basis of an informal agreement between the client and builder, the vast majority of building and civil engineering work will be carried out under the terms of a formal contract between the client and contractor. This contract will set out details of the structure or other work that the contractor is to provide, the date by which it is to be built and the price. Contracts may contain a great deal besides the job, the time and the price, but those are the essentials.
The two broad categories of construction projects are building and civil engineering. Building applies to projects involving houses, offices, shops, factories, schools, hospitals, power and railway stations, churches and so on—all those kinds of structures that in everyday speech we describe as “buildings”. Civil engineering applies to all the other built structures in our environment, including roads, tunnels, bridges, railways, dams, canals and docks. There are structures that appear to fall into both categories; an airport involves extensive buildings as well as civil engineering in the creation of the airfield proper; a dock may involve warehouse buildings as well excavation of the dock and raising of the dock walls.
Whatever the type of structure, building and civil engineering both involve certain processes such as building or erection of the structure, its commissioning, maintenance, repair, alteration and ultimately its demolition. This cycle of processes occurs regardless of the type of structure.
Small Contractors and the Self-employed
While there are variations from country to country, construction is typically an industry of small employers. As many as 70 to 80% of contractors employ less than 20 workers. This is because many contractors start out as a single tradesperson working alone on small-scale jobs, probably domestic ones. As their business expands, such tradespeople start to employ a few workers themselves. The workload in construction is rarely consistent or predictable, as some jobs finish and others start up at different times. There is a need in the industry to be able to move groups of workers with particular skills from job to job as the work requires. Small contractors fulfil this role.
Alongside the small contractors there is a population of self-employed workers. Like agriculture, construction has a very high proportion of self-employed workers. These again are usually tradespeople, such as carpenters, painters, electricians, plumbers and bricklayers. They are able to find a place in either small-scale domestic work or as part of the workforce on bigger jobs. In the boom construction period of the late 1980s, there was an increase in workers claiming to be self-employed. This was partly because of tax incentives for the individuals concerned and use by contractors of so-called self-employed who were cheaper than employees. Contractors were not faced with the same level of social security costs, were not required to train self-employed persons and could get rid of them more easily at the end of jobs.
The presence in construction of so many small contractors and self-employed individuals tends to militate against effective management of health and safety for the job as a whole and, with such a transitory workforce, certainly makes it more difficult to provide proper safety training. Analysis of fatal accidents in the United Kingdom over a 3-year period showed that about half the fatal accidents happened to workers who had been onsite for a week or less. The first few days on any site are especially hazardous to construction workers because, however experienced they may be as tradespeople, each site is a unique experience.
Public and Private Sectors
Contractors may be part of the public sector (e.g., the works department of a city or district council) or they are part of the private sector. A considerable amount of maintenance used to be done by such public works departments, especially on housing, schools and roads. Recently there has been a move to encourage greater competition in such work, partly as a result of pressures for better value for money. This has led firstly to a reduction in the size of public works departments, even their total disappearance in some places, and to the introduction of mandatory competitive tendering. Jobs previously done by public works departments are now done by private-sector contractors under severe “lowest tender wins” conditions. In their need to cut costs, contractors may be tempted to reduce what are seen as overheads such as safety and training.
The distinction between public and private sectors may also apply to clients. Central and local government (along with transportation and public utilities if under the control of central or local government) may all be clients for construction. As such they would generally be thought to be in the public sector. Transportation and utilities run by corporations would usually be considered to be in the private sector. Whether a client is in the public sector sometimes influences attitudes towards inclusion of some items of safety or training in the cost of construction work. Recently public- and private-sector clients have been under similar constraints as regards competitive tendering.
Work across National Boundaries
An aspect of public-sector contracts of increasing importance is the need for tenders to be invited from beyond national boundaries. In the European Union, for example, large-scale contracts beyond a value set out in Directives, must be advertised within the Union so that contractors from all member countries may tender. The effect of this is to encourage contractors to work across national boundaries. They are then required to work in accordance with the local national health and safety laws. One of the aims of the European Union is to harmonize standards between member states in health and safety laws and their application. Major contractors working in parts of the world subject to similar regimes must therefore be familiar with health and safety standards in those countries where they carry out work.
In buildings, the designer is usually an architect, although on small-scale domestic housing, contractors sometime provide such design expertise as is necessary. If the building is large or complex, there may be architects dealing with design of the overall scheme as well as structural engineers concerned with design of, for example, the frame, and specialist engineers involved with design of the services. The architect for the building will ensure that sufficient space is provided in the right places in the structure to permit installation of plant and services. Specialist designers will be concerned to ensure that the plant and services are designed to operate to the required standard when installed in the structure in the places provided by the architect.
In civil engineering, the lead in design is more likely to be taken by a civil or structural engineer, although in high-profile jobs where visual impact may be an important factor, an architect may have an important role in the design team. In tunnelling, railways and highways, the lead in design is likely to be taken by structural or civil engineers.
The role of the developer is to seek to improve the utilization of land or buildings and profit from that improvement. Some developers simply sell the improved land or buildings and have no further interest; others may retain ownership of land or even buildings and reap a continuing interest in the form of rents that are greater than before the improvements.
The skill of the developer is to identify sites either as empty land or under-utilized and out-of-date buildings where application of construction skills will improve their value. The developer may use his or her own finances, but perhaps more often exercises further skills in identifying and bringing together other sources of finance. Developers are not a modern phenomenon; the expansion of cities over the last 200 years owes a great deal to developers. Developers may themselves be clients for the construction work, or they may simply act as agents for other parties who provide finance.
Types of Contract
In the traditional contract, the client arranges for a designer to prepare a full design and specifications. Contractors are then invited by the client to tender or bid for doing the job in accordance with the design. The role of the contractor is largely confined to construction proper. The contractor’s involvement in questions of design or specification is then mainly a matter of seeking such changes as will make it easier or more efficient to build—to improve “buildability”.
The other common arrangement in construction is the design and build contract. The client requires a building (perhaps an office block or shopping development) but has no firm ideas on detailed aspects of its design other than the size of site, number of persons to be accommodated or scale of activities to be carried out in it. The client then invites tenders from either designers or contractors to submit both design and construction proposals. Contractors working in design and build either have their own design organization or have close links with an external designer who will work for them on the job. Design and build may involve two stages in design: an initial stage where a designer prepares an outline scheme which is then put out to tender; and a second stage where the successful design and build contractor will then carry out further design on detailed aspects of the job.
Maintenance and emergency contracts cover a wide variety of arrangements between clients and contractors and represent a significant proportion of the work of the construction industry. They generally run for a fixed period, require the contractor to do certain types of work or to work on a “call-off” basis (i.e., work that the client calls the contractor in to do). Emergency contracts are widely used by public authorities who are responsible for providing a public service that ought not to be interrupted; government agencies, public utilities and transportation systems make wide use of them. Operators of factories, particularly those with continuous processes such as petrochemicals, also make wide use of emergency contracts to deal with problems in their facilities. Having entered such a contract, the contractor undertakes to make available suitable workers and plant to carry out the work, often at very short notice (e.g., in the case of emergency contracts). The advantage to the client is that he or she does not need to retain workers on payroll or have plant and equipment that may only occasionally be used to deal with maintenance and emergencies.
Pricing of maintenance and emergency contracts may be on the basis of a fixed sum per annum, or on the basis of time spent carrying out work, or some combination.
Perhaps the most common publicly known example of such contractors is maintenance of roads and emergency repairs to gas main or power supplies that have either failed or been accidentally damaged.
Whatever the form of contract, the same possibilities arise for clients and designers to influence the health and safety of contractors by decisions made in the early stage of the job. Design and build perhaps permits closer liaison between the designer and contractor on health and safety.
Price is always an element in a contract. It may simply be a single sum for the cost of doing the job, such as building a house. Even with a single lump sum, the client may have to pay part of the price in advance of the job starting, to enable the contractor to buy materials. The price may, however, be on a cost-plus basis, where the contractor is to recover his or her costs plus an agreed amount or percentage for profit. This arrangement tends to work to the disadvantage of the client, since there is no incentive for the contractor to keep costs down. The price may also have bonuses and penalties attached to it, so that the contractor will receive more money if, for example, the job is completed earlier than the agreed time. Whatever form the price takes for the job, it is usual for payments to be made in stages as the work progresses, either on completion of certain parts of the job by agreed dates or on the basis of some agreed method of measuring the work. At the end of construction proper, it is common for an agreed proportion of the price to be kept back by the clients until “snags” have been put right or the structure has been commissioned.
During the course of the job, the contractor may encounter problems that were not foreseen when the contract was made with the client. These might require changes to the design, the construction method or the materials. Usually such changes will create extra costs for the contractor, who then seeks to recover from the client on the basis that these items become agreed “variations” from the original contract. Sometimes recovery of the cost of variations can make the difference for the contractor between doing the job at a profit or loss.
The pricing of contracts can affect health and safety if inadequate provision is made in the contractor’s tender to cover the costs of providing safe access, lifting equipment and so on. This becomes even more difficult where, in an attempt to ensure that they obtain value for money from contractors, clients pursue a vigorous policy of competitive tendering. Governments and local authorities apply policies of competitive tendering to their own contracts, and indeed there may be laws requiring that contracts can be awarded only on the basis of competitive tendering. In such a climate, there is always a risk that the health and safety of construction workers will suffer. In cutting costs, clients may resist a reduction in the standard of construction materials and methods, but at the same time be totally unaware that in accepting the lowest tender, they have accepted working methods that are more likely to endanger construction workers. Even in a situation of competitive tendering, contractors submitting tenders should have to make clear to the client that their bid adequately covers the cost of health and safety involved in their proposals.
Developers can influence health and safety in construction in ways similar to clients, firstly by using contractors who are competent in health and safety and architects who take health and safety into account in their designs, and secondly in not automatically accepting the lowest tenders. Developers generally want to be associated only with successful developments, and one measure of success ought to be projects where there are no major health and safety problems during the construction process.
Building Standards and Planning
In the case of buildings, whether housing, commercial or industrial, projects are subject to planning laws that dictate where certain types of development may take place (e.g., that a factory may not be built among houses). Planning laws may be very specific about the appearance, materials and size of buildings. Typically areas identified as industrial zones are the only places where factory buildings may be erected.
Often there are also building regulations or similar standards that specify in precise detail many aspects of the design and specification of buildings—for example, the thickness of walls and timbers, depth of foundations, insulation characteristics, size of windows and rooms, layout of electrical wiring and earthing, layout of plumbing and pipework and many other issues. These standards have to be followed by clients, designers, specifiers and contractors. They limit their choices but at the same time ensure that buildings are built to an acceptable standard. Planning laws and building regulations thus affect the design of buildings and their cost.
Projects to build housing may consist of a single house or vast estates of individual houses or flats. The client may be each individual householder, who will then normally be responsible for maintenance of his or her own house. The contractor will usually remain responsible for correcting defects in construction for a period of months after building is finished. However, if the project is for many houses, the client may be a public body, either in local or national government, with responsibility for providing housing. There are also large private bodies like housing associations for whom numbers of houses may be built. Public or private bodies with responsibilities for providing housing generally rent the finished houses to occupants, retaining a greater or lesser degree of responsibility for maintenance also. Building projects involving blocks of flats usually have a client for the block as a whole, who then lets out individual flats under a leasing arrangement. In this situation the owner of the block has responsibility for carrying out maintenance but passes on the cost to the tenants. In some countries ownership of individual flats in a block can rest with the occupants of each flat. There has to be some arrangement, sometimes through an estate management contractor, whereby maintenance can be carried out and the necessary costs raised among the occupants.
Often houses are built on a speculative basis, by a developer. Specific clients or occupants of those houses may not have been identified at the outset but come on the scene after construction has begun and purchase or rent the property like any other article. Houses are usually fitted out with electrical, plumbing and drainage services and heating systems; a gas supply may also be laid on. Sometimes in an attempt to cut costs, houses are only partially finished, leaving it to the purchaser to install some of the fittings and to paint or decorate the building.
Commercial buildings include offices, factories, schools, hospitals, shops—an almost endless list of different types of buildings. In most cases these buildings are constructed for a particular client. However, offices and shops are often built on a speculative basis like housing, with the hope of attracting buyers or tenants. Some clients require an office or shop to be totally fitted out to their requirements, but very often the contract is for the structure and main services, with the client making arrangements to fit out the premises using specialist contractors in office and shop fitting.
Hospitals and schools are built for clients who have a clear idea of precisely what they want, and the clients often provide design input into the project. Plant and equipment in hospitals may cost more than the structure and involve a great deal of design that has to satisfy stringent medical standards. National or local government may also play a part in the design of schools by laying down very detailed requirements on space standards and equipment as part of its wider role in education. National governments usually have very detailed standards as to what is acceptable in hospital buildings and plant. Fitting out of hospitals and similarly complex buildings is a form of construction work usually carried out by specialist subcontractors. Such contractors not only require knowledge of health and safety in construction in general, but also need expertise in ensuring that their operations do not adversely affect the hospital’s own activities.
Industrial building or construction involves use of the mass- production techniques of manufacturing industry to produce parts of buildings. The ultimate example is the house brick, but normally the expression is applied to building using concrete parts or units that are assembled onsite. Industrial construction expanded rapidly after the Second World War to meet the demand for cheap housing, and it is more commonly found in mass housing developments. Under factory conditions it is possible to mass produce cast units that are consistently accurate in a way that would be virtually impossible under normal site conditions.
Sometimes units for industrial construction are manufactured away from the construction site in factories that may supply a wide area; sometimes, where the individual development is itself very large, a factory is set up onsite to serve that sole site.
Units designed for industrial construction must be structurally strong enough to stand up to being moved, lifted and lowered; they must incorporate anchorage points, or slots to permit safe attachment of lifting tackle, and must also include appropriate lugs or recesses to permit the units to fit together both easily and strongly. Industrial construction demands plant for transporting and lifting units into position and space and arrangements to store units safely when delivered to site, so that units are not damaged and workers are not injured. This technique of building tends to produce visually unattractive buildings, but on a large scale it is cheap; a whole room can be assembled from six cast units with window and door openings in place.
Similar techniques are used to produce concrete units for civil engineering structures like elevated motorways and tunnel linings.
Some clients for industrial or commercial buildings containing extensive complex plant wish simply to walk into a facility that will be up and running from their first day in the premises. Laboratories are sometimes constructed and fitted out on this basis. Such an arrangement is a “turn-key” project, and here the contractor will ensure that all aspects of plant and services are fully operational before handing the project over. The job may be done under a design and build contract so that, in effect, the turn-key contractor deals with everything from design to commissioning.
Civil Engineering and Heavy Construction
The civil engineering of which the public is most aware is work on highways. Some highway work is the creation of new roads on virgin land, but much of it is the extension and repair of existing highways. Contracts for highway work are usually for state or local government agencies, but sometimes roads remain under the control of contractors for some years after completion, during which time they are permitted to charge tolls. If civil engineering structures are being financed by government, then both the design and actual construction will be subject to a high degree of supervision by officials on behalf of government. Contracts for construction of highways are usually let to contractors on the basis of a contractor being responsible for a section of so many kilometres of the highway. There will be a main contractor for each section; but highway construction involves a number of skills, and aspects of the job such as steel work, concrete, shuttering and surfacing may be subcontracted by the main contractor to specialist firms. Highway construction is also sometimes carried out under management contract arrangements, where a civil engineering consultancy will provide management for the job, with all the work being done by subcontractors. Such a management contractor may also have been involved in design of the highway.
Construction of highways requires the creation of a surface whose gradients are suitable for the sort of traffic that will use it. In a generally level terrain, creation of the foundation of the highway may involve earthmoving—that is, shifting soil from cuttings to create embankments, building bridges across rivers and driving tunnels through mountainsides where it is not possible to go round the obstruction. Where labour costs are higher, such operations are carried out using mechanically powered plant such as excavators, scrapers, loaders and lorries. Where labour costs are lower, these processes may be carried out manually by large numbers of workers using hand tools. Whatever the actual methods adopted, highway construction requires high standards of route surveying and planning of the job.
Highway maintenance frequently requires roads to remain in use whilst repairs or improvements are carried out in part of the road. There is thus a hazardous interface between traffic movement and construction operations which makes good planning and management of the job even more important. There are often national standards for signage and coning off of roadworks and requirements as to the amount of separation there should be between construction and traffic, which may be difficult to achieve in a confined area. Control of traffic approaching roadworks is usually the responsibility of the local police, but requires careful liaison between them and the contractors. Highway maintenance creates traffic hold-ups, and accordingly contractors are put under pressure to finish jobs quickly; sometimes there are bonuses for finishing early and penalties for finishing late. Financial pressures must not undermine safety on what is very dangerous work.
Surfacing of highways may involve concrete, stone or tarmacadam. This requires a substantial logistical train to ensure that the required quantities of surfacing materials are in place in the right condition to ensure that surfacing proceeds without interruption. Tarmacadam requires special purpose spreading plant that keeps the surfacing material plastic while spreading it. Where the job is re-surfacing, plant will be required including picks and breakers so that the existing surface is broken up and removed. A final finish is usually applied to the surfaces of highways involving use of heavy powered rollers.
Creation of cuttings and tunnels may require use of explosives and then arrangements to shift the muck displaced by the blasting. The sides of cuttings may require permanent supports to prevent landslides or falls of ground onto the finished road.
Elevated highways often require structures similar to bridges, especially if the elevated section passes through an urban area when space is limited. Elevated highways are often constructed from cast reinforced concrete sections that are either cast in situ or cast in a fabrication area and then shifted to the required position onsite. The work will require large-capacity lifting machinery to lift cast sections, shuttering and reinforcing.
Temporary support arrangements or “falsework” to support sections of either elevated highways or bridges while they are being cast in position need to be designed to take into account the uneven loads imposed by concrete as it is poured. Design of falsework is as important as design of the structure proper.
Bridges in remote areas may be simple constructions from timber. More commonly today bridges are from reinforced concrete or steel. They may also be clad in brickwork or stone. If the bridge is to span a considerable gap, whether above water or not, its design will require specialist designers. Using today’s materials, the strength of the bridge span or arch is not achieved by mass material, which would be simply too heavy, but by skilful design. The main contractor for a bridge building job is usually a major general civil-engineering contractor with management expertise and plant. However, specialist subcontractors may deal with major aspects of the job like erection of steel work to form the span or casting or placing cast sections of the span in place. If the bridge is over water, one or both abutments that support the ends of the bridge may themselves have to be constructed in water, involving piling, coffer dams, mass concrete or stone work. A new bridge may be part of a new highway system, and approach roads may have to be built, themselves possibly elevated.
Good design is especially important in bridge building, so that the structure is strong enough to withstand the loads imposed on it in use and to ensure that it will not require maintenance or repair too frequently. The appearance of a bridge is often a very important factor, and again good design can balance the conflicting demands of sound engineering and aesthetics. Provision of safe means of access for maintenance of bridges needs to be taken into account during design.
Tunnels are a specialized form of civil engineering. They vary in size from the Channel Tunnel, with over 100 km of bores from 6 to 8 m in diameter, to mini-tunnels whose bores are too small for workers to enter and which are created by machines launched from access shafts and controlled from the surface. In urban areas, tunnels may be the only way to provide or improve transport routes or to provide water and drainage facilities. The proposed route of the tunnel requires as detailed a survey as possible to confirm the kind of ground that the tunnel workings will be in and whether there will be groundwater. The nature of the ground, the presence of groundwater and the end use of the tunnel all influence the choice of tunnelling method.
If the ground is consistent, like the chalk-clay beneath the English Channel, then machine digging may be possible. If high groundwater pressures are not encountered during pre- construction survey, then it is usually unnecessary for the workings to be pressurized to keep out the water. If working in compressed air cannot be avoided, this adds considerably to costs because airlocks have to be provided, workers need to be allowed time to decompress, and access to workings for plant and materials may be made more difficult. A large tunnel for a road or railway in consistent non-hard-rock ground might be dug using a full-face tunnel-boring machine (TBM). This is really a train of different machines linked together and moving forward on rails under its own power. The front face is a circular cutting head that rotates and feeds spoil back through the TBM. Behind the cutting head are various sections of the TBM that place the segments of tunnel lining rings in position around the surface of the tunnel, grout behind the lining rings and, in a very confined space, provide all the machinery to handle and place ring segments (each weighing some tonnes), remove spoil, bring grout and extra segments forward and house electric motors and hydraulic pumps to power the cutting head and segment-placing mechanisms.
A tunnel in non-hard-rock ground which is not consistent enough to use a TBM, may be dug using equipment such as roadheaders that bite into the face of the heading. Spoil falling from the roadheader onto the tunnel floor are to be collected by diggers and removed by lorry. This technique permits digging of tunnels that are not circular in section. The ground in which such a tunnel is dug will not usually have sufficient strength for it to remain unlined; without some form of lining there might be falls from roof and walls. The tunnel may be lined by liquid concrete sprayed onto a steel mesh held in position by rock bolts (the “New Austrian tunnelling method”) or by cast concrete.
If the tunnel is in hard rock, the heading will be dug by means of blasting, using explosives placed into shot holes drilled into the rock face. The trick here is to use the minimum of blast to achieve a fall of rock in the position and sizes required, thereby making it easier to remove the spoil. On bigger jobs, multiple drills mounted on tracked bases will be used along with diggers and loaders to remove spoil. Hard rock tunnels are often simply trimmed to provide an even surface, but are not then further lined. If the rock surface remains friable with a risk of pieces falling, then a lining will be applied, usually some form of sprayed or cast concrete.
Whatever the method of construction adopted for the tunnel, the effective supply of tunnelling materials and removal of spoil are vital to the successful progress of the job. Large tunnelling jobs may require extensive narrow-gauge construction rail systems to provide logistical support.
Dams invariably contain large quantities of earth or rock to provide mass to resist the pressure from water behind them; some dams are also covered in masonry or reinforced concrete. Depending on the length of the dam, its construction often requires earthmoving on the very largest scale. Dams tend to be built in remote locations dictated by the need to ensure that water is available at a position where it is technically possible to restrict the flow of the river. Thus temporary roads may have to be built before dam building may start in order to get plant, materials and personnel to the site. Workers on dam projects may be so far from home that full-scale living accommodations have to be provided along with the usual construction site facilities. It is necessary to divert the river away from the site of the workings, and a coffer dam and temporary riverbed may have be created.
A dam constructed simply from earth or rock that has been shifted will require large scale excavation, digging and scraping plant as well as lorries. If the dam wall is covered by masonry or cast concrete, it will be necessary to employ high or long-reach cranes capable of depositing masonry, shuttering, reinforcing and concrete in the right places. A continuous supply of good-quality concrete will be necessary, and a concrete-mixing plant will be necessary alongside the dam workings, with the concrete either handled in batches by crane or pumped to the job.
Canals and docks
Construction and repair of canals and docks contain some aspects of other jobs that have been described, such as roadworks, tunnels and bridges. It is particularly important in canal building for surveying to be to the highest standard before work begins, especially regarding levels and to ensure that material that has had to be dug out can economically be used elsewhere in the job. Indeed the early railway engineers owed a great deal to the experience of canal builders a century before. The canal will require a source for its water and will either tap into a natural source such as a river or lake or create an artificial one in the form of a reservoir. Digging of docks may start on dry land, but sooner or later has to link up to either a river, a canal, the sea or another dock.
Canal and dock building requires excavators and loaders to open up the ground. Spoil may be removed by lorry or water transport may be used. Docks are sometimes developed on ground that has a long history of industrial use. Industrial wastes may have escaped into such ground over many years, and spoil removed in digging or extending the docks will be heavily contaminated. Work in repairing a canal or dock is likely to have to be carried out while adjacent parts of the system are kept in use. The workings may have to rely on coffer dams for protection. Failure of a coffer dam during extension of Newport Docks in Wales in the early years of this century resulted in nearly 100 deaths.
Clients for canals and docks are likely to be public authorities. However, sometimes docks are constructed for corporations alongside their major production plants or for corporate clients to handle a particular type of incoming or outgoing goods (e.g., motor cars). Repair and renovation of canals is nowadays often for the leisure industry. Like dams, both canal and dock construction may be in very remote situations, requiring provision of facilities for workers beyond those of a normal construction site.
Construction of railroads or railways historically came after canals and before major highways. Clients in railway construction contracts may be rail operators themselves or governmental agencies, if the railways are financed by government. As with highways, design of a railroad that is economical and safe to build and operate depends on good surveying beforehand. In general, locomotives do not operate effectively on steep gradients, and therefore those designing layout of the track are concerned with avoiding changes in levels, going round or through obstacles rather than over them.
Designers of railroads are subject to two constraints unique to the industry: first, curves in the track layout must generally conform to very large radii (otherwise trains cannot negotiate them); second, all the structures connected with the railway—its bridge arches, tunnels and stations—must be capable of accommodating the envelope of the largest locomotives and rolling stock that will use the track. The envelope is the silhouette of the rolling stock plus clearance to allow safe passage through bridges, tunnels and so on.
Contractors involved in building and repair of railroads require the usual construction plant and effective logistical arrangements to ensure that rail track and ballast as well as construction materials are always available in what may be remote locations. Contractors may use the track they have just laid to run trains supplying the works. Contractors involved in maintenance of existing operational railways have to ensure that their work does not interfere with the operations of the railway and endanger workers or the public.
The rapid expansion of air transportation since the middle of the 20th century has resulted in one of the biggest and most complex forms of construction: the building and extension of airports.
Clients for airport construction are usually governments at the national or local level or agencies representing the government. Some airports are built for major cities. Airports are rarely for private clients such as business corporations.
Planning the work is sometimes made more difficult because of environmental constraints that have been placed on the project in relation to noise and pollution. Airports require a lot of space, and if they are located in more heavily populated areas, creation of the runways and space for terminal buildings and car parks may require reinstatement of derelict or otherwise difficult land. Building an airport involves levelling a large area, which may require earth moving and even land reclamation, and then construction of a wide variety of often very large buildings, including hangars, maintenance workshops, control towers and fuel storage facilities, as well as terminal buildings and parking.
If the airport is being built on soft ground, buildings may require piled foundations. Actual runways require good foundations; hardcore supporting the surface layers of concrete or tarmac needs to be heavily compacted. Plant used on airport construction is similar in scale and type to that used in major highway projects, except that it is concentrated within a limited area rather than over the many miles of a highway.
Airport maintenance is a particularly difficult type of work where resurfacing the runways has to be integrated with continuing operation of the airport. Usually the contractor is allowed an agreed number of hours during the night when he or she can work on a runway that is temporarily taken out of use. All the contractor’s plant, materials and workforce have to be marshalled off the runways, prepared to move immediately to the work site at the agreed start time. The contractor must finish his or her work and get off the runways again at the agreed time when flights may resume. Whilst working on the runway, the contractor must not impede or otherwise endanger aircraft movement on other runways.
All new buildings and civil engineering structures go through the same cycle of conception or design, groundworks, building or erection (including the roof of a building), finishing and provision of utilities and final commissioning before being brought into use. In the course of years, those once new buildings or structures require maintenance including re-painting and cleaning; they are likely to be renovated by being updated or changed or repaired to correct damage by weather or accident; and finally they will need to be demolished to make way for a more modern facility or because their use is no longer required. This is true of houses; it is also true of large, complex structures like power stations and bridges. Each stage in the life of a building or civil engineering structure presents hazards, some of which are common to all work in construction (like the risk from falls) or unique to the particular type of project (such as the risk from collapse of excavations during preparation of foundations in either building or civil engineering).
For each type of project (and, indeed, each stage within a project) it is possible to forecast what will be the principal hazards to the safety of construction workers. The risk from falls is common to all construction projects, even those at ground level. This is supported by the evidence of accident data which show that up to half of fatal accidents to construction workers involve falls.
Physical hazards to those engaged in design of new facilities normally arise from visits by professional staff to carry out surveys. Visits by unaccompanied staff to unknown or abandoned sites may expose them to risks from dangerous access, unguarded openings and excavations and, in a building, to electrical wiring and equipment in a dangerous condition. If the survey requires entry into rooms or excavations that have been closed for some time, there is the risk of being overcome by carbon dioxide or reduced oxygen levels. All hazards are increased if visits are made to an unlit site after dark or if the lone visitor has no means of communicating with others and summoning aid. As a general rule, professional staff should not be required to visit sites where they will be on their own. They should not visit after dark unless the site is well lit. They should not enter enclosed spaces unless these have been tested and shown to be safe. Lastly, they should be in communication with their base or have an effective means of getting help.
Conception or design proper should play an important part in influencing safety when contractors are actually working onsite. Designers, be they architects or civil engineers, should be expected to be more than mere producers of drawings. In creating their design, they should, by reason of their training and experience, have some idea how contractors are likely to have to work in putting the design into effect. Their competence should be such that they are able to identify to contractors the hazards that will arise from those methods of working. Designers should try to “design out” hazards arising from their design, making the structure more “buildable” as regards health and safety and, where possible, substituting safer materials in the specifications. They should improve access for maintenance at the design stage and reduce the need for maintenance workers to be put at risk by incorporating features or materials that will require less frequent attention during the life of the building.
In general, designers are able to design out hazards only to a limited extent; there will usually be significant residual risks that the contractors will have to take into account when devising their own safe systems of work. Designers should provide contractors with information on these hazards so that the latter are able to take both the hazards and necessary safety procedures into account, firstly when tendering for the job, and secondly when developing their systems of work to do the job safely.
The importance of specifying materials with better health and safety properties tends to be underestimated when considering safety by design. Designers and specifiers should consider whether materials are available with better toxic or structural properties or that can be used or maintained more safely. This requires designers to think about the materials that will be used and to decide whether following previous practice will adequately protect construction workers. Often cost is the determining factor in choice of materials. However, clients and designers should realize that while materials with better toxic or structural properties may have a higher initial cost, they often yield much bigger savings over the life of the building because construction and maintenance workers require less expensive access or protective equipment.
Usually the first job to be done on the site after site surveys and laying out of the site once the contract has been awarded (assuming there is no need for demolition or site clearance) is groundworks for the foundations. In the case of domestic housing, the footings are unlikely to require excavations greater than half a metre and may be dug by hand. For blocks of flats, commercial and industrial buildings and some civil engineering, the foundations may need to be several metres below ground level. This will require the digging of trenches in which work will have to carried out to lay or erect the foundations. Trenches deeper than 1 m are likely to be dug using machines such as excavators. Excavations are also dug to permit laying of cables and pipes. Contractors often use special-purpose excavators capable of digging deep but narrow excavations. If workers have to enter these excavations, the hazards are essentially the same as those encountered in excavations for foundations. However, there is usually more scope in cable and pipe excavations or trenches to adopt methods of working that do not require workers to enter the excavation.
Work in excavations deeper then 1 m needs especially careful planning and supervision. The hazard is the risk of being struck by earth and debris as the ground collapses along the side of the excavation. Ground is notoriously unpredictable; what looks firm can be caused to slip by rain, frost or vibration from other construction activities nearby. What looks like firm, stiff clay dries out and cracks when exposed to the air or will soften and slip after rain. A cubic metre of earth weighs more than 1 tonne; a worker struck by only a small fall of ground risks broken limbs, crushed internal organs and suffocation. Because of the vital importance to safety of selecting a suitable method of support for the sides of the excavation, before work starts, the ground should be surveyed by a person experienced in safe excavation work to establish the type and condition of the ground, especially the presence of water.
Support for trench sides
Double-sided support. It is not safe to rely on cutting or “battering” back the sides of the excavation to a safe angle. If the ground is wet sand or silt, the safe angle of batter would be as low as 5 to 10 above horizontal, and there is generally not enough room onsite for such a wide excavation. The most common method of providing safety for work in excavations is to support both sides of the trench through shoring. With double-sided support, the loads from the ground on one side are resisted by similar loads acting through struts between the opposing sides. Timber of good quality must be used to provide vertical elements of the support system, known as poling boards. Poling boards are driven into the ground as soon as excavation begins; the boards are edge to edge, and thus provide a timber wall. This is done on each side of the excavation. As the excavation is dug deeper, the poling boards are driven into the ground ahead of the excavation. When the excavation is a metre deep, a row of horizontal boards (known as walings or wales) is placed against the poling boards and then held in position by timber or metal struts wedged between the opposing walings at regular intervals. As digging proceeds, the poling boards are driven further into the ground with their walings and struts, and it will be necessary to create a second row of walings and struts if the excavation is deeper than 1.2 m. Indeed, an excavation of 6 m could require up to four rows of walings.
The standard timber methods of support are unsuitable if the excavation is deeper than 6 m or the ground is water bearing. In these situations, other types of support for the sides of excavations are required, such as vertical steel trench sheets, closely spaced with horizontal timber walings and metal adjustable struts, or full-scale steel sheet piling. Both methods have the advantage that the trench sheets or sheet piles can be driven by machine before excavation proper starts. Also, trench sheets and sheet piles can be withdrawn at the end of the job and re-used. Support systems for excavations deeper than 6 m or in water-bearing ground should be custom designed; standard solutions will not be adequate.
Single-sided support. An excavation that is rectangular in shape and too large for the support methods described above to be practicable may have one or more of its sides supported by a row of poling boards or trench sheets. These are themselves supported first by one or more rows of horizontal walings which are themselves then held in place by angled rakes back to a strong anchorage or support point.
Other systems. It is possible to use manufactured steel boxes of adjustable width that may be lowered into excavations and within which work can be carried out safely. It is also possible to use proprietary waling frame systems, whereby a horizontal frame is lowered into the excavation between the poling boards or trench sheets; the waling frame is forced apart and applies pressure to keep the poling boards upright by the action of hydraulic struts across the frame which can be pumped from a position of safety outside the excavation.
Training and supervision. Whatever method of support is adopted, the work should be carried out by trained workers under supervision of an experienced person. The excavation and its supports should be inspected each day and after each occasion that they have been damaged or displaced (e.g., after a heavy rain). The only assumption one is entitled to make regarding safety and work in excavations is that all ground is liable to fail and therefore no work should ever be carried out with workers in an unsupported excavation deeper than 1 m. See also the article “Trenching” in this chapter.
Erection of the main part of the building or civil engineering structure (the superstructure) takes place after completion of the foundation. This part of the project usually requires work at heights above ground. The biggest single cause of fatal and major injury accidents is falls from heights or on the same level.
Even if the job is simply building a house, the number of workers involved, the amount of building materials to be handled and, in later stages, the heights at which work will have to be carried out all require more than simple ladders for access and safe places of work.
There are limitations on the sort of work that can be done safely from ladders. Work more than 10 m above ground is usually beyond the safe reach of ladders; lengthy ladders themselves become dangerous to handle. There are limitations on the reach of workers on ladders as well as on the amount of equipment and materials they can safely carry; the physical strain of standing on ladder rungs limits the time they can spend on such work. Ladders are useful for carrying out short-duration, light-weight work within safe reach of the ladder; typically, inspection and repair and painting of small areas of the building’s surface. Ladders also provide access in scaffolds, in excavations and in structures where more permanent access has not yet been provided.
It will be necessary to use temporary working platforms, the most common of which is scaffolding. If the job is a multi-storey block of flats, office building or structure like a bridge, then scaffolding of varying degrees of complexity will be required, depending on the scale of the job.
Scaffolds consist of easily assembled frameworks of steel or timber on which working platforms may be placed. Scaffolds may be fixed or mobile. Fixed scaffolds—that is, those erected alongside a building or structure—are either independent or putlog. The independent scaffold has uprights or standards along both sides of its platforms and is capable of remaining upright without support from the building. The putlog scaffold has standards along the outer edges of its working platforms, but the inner side is supported by the building itself, with parts of the scaffold frame, the putlogs, having flattened ends that are placed between courses of brickwork to gain support. Even the independent scaffold needs to be rigidly “tied” or secured to the structure at regular intervals if there are working platforms above 6 m or if the scaffold is sheeted for weather protection, thus increasing wind-loadings.
Working platforms on scaffolds consist of good-quality timber boards laid so that they are level and both ends are properly supported; intervening supports will be necessary if the timber is liable to sag due to loading by people or materials. Platforms should never be less than 600 mm in width if used for access and working or 800 mm if used also for materials. Where there is a risk of falling more than 2 m, the outer edge and ends of a working platform should be protected by a rigid guard rail, secured to the standards at a height of between 0.91 and 1.15 m above the platform. To prevent materials falling off the platform, a toe board rising at least 150 mm above the platform should be provided along its outer edge, again secured to the standards. If guard rails and toe-boards have to be removed to permit passage of materials, they should be replaced as soon as possible.
Scaffold standards should be upright and properly supported at their bases on base plates, and if necessary on timber. Access within fixed scaffolds from one working platform level to another is usually by means of ladders. These should be properly maintained, secured at top and bottom and extend at least 1.05 m above the platform.
The principal hazards in the use of scaffolds—falls of person or materials—usually arise from shortcomings either in the way the scaffold is first erected (e.g., a piece such as a guard rail is missing) or in the way it is misused (e.g., by being overloaded) or adapted during the course of the job for some purpose that is unsuitable (e.g., sheeting for weather protection is added without adequate ties to the building). Timber boards for scaffold platforms become displaced or break; ladders are not secured at top and bottom. The list of things that can go wrong if scaffolds are not erected by experienced persons under proper supervision is almost limitless. Scaffolders are themselves particularly at risk from falls during erection and dismantling of scaffolds, because they are often obliged to work at heights, in exposed positions without proper working platforms (see figure 1).
Figure 1. Assembling scaffolding at a Geneva, Switzerland, construction site without adequate protection.
Tower scaffolds. Tower scaffolds are either fixed or mobile, with a working platform on top and an access ladder inside the tower frame. The mobile tower scaffold is on wheels. Such towers easily become unstable and should be subject to height limitations; for the fixed tower scaffold the height should not be more than 3.5 times the shortest base dimension; for mobile, the ratio is reduced to 3 times. The stability of tower scaffolds should be increased by use of outriggers. Workers should not be permitted on the top of mobile tower scaffolds while the scaffold is being moved or without the wheels being locked.
The principal hazard with tower scaffolds is overturning, throwing people off the platform; this may be due to the tower being too tall for its base, failure to use outriggers or lock wheels or unsuitable use of the scaffold, perhaps by overloading it.
Slung and suspended scaffolds. The other main category of scaffold is those that are slung or suspended. The slung scaffold is essentially a working platform hung by wire ropes or scaffold tubes from an overhead structure like a bridge. The suspended scaffold is again a working platform or cradle, suspended by wire ropes, but in this case it is capable of being raised and lowered. It is often provided for maintenance and painting contractors, sometimes as part of the equipment of the finished building.
In either case, the building or structure must be capable of supporting the slung or suspended platform, the suspension arrangements must be strong enough and the platform itself should be sufficiently robust to carry the intended load of people and materials with guard sides or rails to prevent them from falling out. For the suspended platform, there should be at least three turns of rope on the winch drums at the lowest position of the platform. Where there are no arrangements to prevent the suspended platform from falling in the event of failure of a rope, workers using the platform should wear a safety harness and rope attached to a secure anchorage point on the building. Persons using such platforms should be trained and experienced in their use.
The principal hazard with slung or suspended scaffolds is failure of the supporting arrangements, either of the structure itself or the ropes or tubes from which the platform is hung. This can arise from incorrect erection or installation of the slung or suspended scaffold or from overloading or other misuse. Failure of suspended scaffolds has resulted in multiple fatalities and can endanger the public.
All scaffolds and ladders should be inspected by a competent person at least weekly and before being used again after weather conditions that may have damaged them. Ladders which have cracked styles or broken rungs should not be used. Scaffolders who erect and dismantle scaffolds should be given specific training and experience to ensure their own safety and the safety of others who may use the scaffolds. Scaffolds are often provided by one, perhaps the main, contractor for use by all contractors. In this situation, tradespeople may modify or displace parts of scaffolds to make their own job easier, without restoring the scaffold afterwards or realizing the hazard they have created. It is important that the arrangements for coordination of health and safety across the site deal effectively with the action of one trade on the safety of another.
Powered access equipment
On some jobs, during both construction and maintenance, it may be more practicable to use powered access equipment than scaffolding in its various forms. Providing access to the underside of a factory roof undergoing recladding or access to the outside of a few windows in a building may be safer and cheaper than scaffolding out the whole structure. Powered access equipment comes in a variety of forms from manufacturers, for example, platforms that may be raised and lowered vertically by hydraulic action or the opening and closing of scissor jacks and hydraulically-powered articulated arms with a working platform or cage on the end of the arm, commonly called cherry pickers. Such equipment is generally mobile and can be moved to the place it is required and brought into use in a matter of moments. Safe use of powered access equipment requires that the job be within the specification for the machine as described by the manufacturer (i.e., the equipment must not overreach or be overloaded).
Powered access equipment requires a firm, level floor on which to operate; it may be necessary to put out outriggers to ensure that the machine does not tip over. Workers on the working platform should have access to operating controls. Workers should be trained in safe use of such equipment. Properly operated and maintained, powered access equipment can provide safe access where it may be virtually impossible to provide scaffolding, for example, during the early stages of erection of a steel frame or to provide access for steel erectors to the connecting points between columns and beams.
The superstructure of both buildings and civil engineering structures often involves erection of substantial steel frames, sometimes of great height. While responsibility for ensuring safe access for steel erectors who assemble these frames rests principally with the management of steel erection contractors, their difficult job can be made easier by the designers of the steel work. Designers should ensure that patterns of bolt holes are simple and facilitate easy insertion of bolts; the pattern of joints and bolt holes should be as uniform as possible throughout the frame; rests or perches should be provided on columns at joints with beams, so that the ends of beams may rest still while steel erectors are inserting bolts. As far as possible, the design should ensure that access stairs form part of the early frame so that steel erectors have to rely less on ladders and beams for access.
Also, the design should provide for holes to be drilled in suitable places in the columns during fabrication and before the steel is delivered to site, which will permit securing of taut wire ropes, to which steel erectors wearing safety harnesses may secure their running lines. The aim should be to get floor plates in place in steel frames as soon as possible, to reduce the amount of time that steel erectors have to rely on safety lines and harnesses or ladders. If the steel frame has to remain open and without floors while erection continues to higher levels, then safety nets should be slung below the various working levels. As far as possible, the design of the steel frame and the working practices of the steel erectors should minimize the extent to which workers have to “walk steel”.
While raising the walls is an important and hazardous stage in erecting a building, putting the roof in place is equally important and presents special hazards. Roofs are either flat or pitched. With flat roofs the principal hazard is of persons or materials falling either over the edge or down openings in the roof. Flat roofs are usually constructed either from wood or cast concrete, or slabs. Flat roofs must be sealed against entry of water, and various materials are used, including bitumen and felt. All materials required for the roof have to be raised to the required level, which may require goods hoists or cranes if the building is tall or the quantities of covering and sealant are substantial. Bitumen may have to be heated to assist spreading and sealing; this may involve taking on to the roof a gas cylinder and melting pot. Roof-workers and persons beneath can be burned by the heated bitumen and fires can be started involving the roof structure.
The hazard from falls can be prevented on flat roofs by erecting temporary edge protection in the form of guard rails of dimensions similar to the guard rails in scaffolds. If the building is still surrounded by external scaffolding, this can be extended up to roof level, to provide edge protection for roof-workers. Falls down openings in flat roofs can be prevented by covering them or, if they have to remain open, by erecting guard rails round them.
Pitched roofs are most commonly found on houses and smaller buildings. The pitch of the roof is achieved by erecting a wooden frame to which the outer covering of the roof, usually clay or concrete tiles, is attached. The pitch of the roof may exceed 45 above horizontal, but even a shallower pitch presents hazards when wet. To prevent roof-workers from falling while fixing battens, felt and tiles, roof ladders should be used. If the roof ladder cannot be secured or supported at its bottom end, it should have a properly designed ridge-iron that will hook over the ridge tiles. Where there is doubt about the strength of ridge tiles, the ladder should be secured by means of a rope from its top rung, over the ridge tiles and down to a strong anchorage point.
Fragile roofing materials are used on both pitched and curved or barrel roofs. Some roof lights are made of fragile materials. Typical materials include sheets of asbestos cement, plastic, treated chipboard and wood-wool. Because roof-workers frequently step through sheets they have just laid, safe access to where the sheets are to be laid, and a safe position from which to do it, are required. This is usually in the form of a series of roof ladders. Fragile roofing materials present an even greater hazard to maintenance workers, who may be unaware of their fragile nature. Designers and architects can improve the safety of roof-workers by not specifying fragile materials in the first place.
Laying of roofs, even flat roofs, can be dangerous in high winds or heavy rain. Materials such as sheets, normally safe to handle, become dangerous in such weather. Unsafe roof-work not only endangers roof-workers, but also presents hazards to the public beneath. Erection of new roofs is hazardous, but, if anything, maintenance of roofs is even more dangerous.
Renovation includes both maintenance of the structure and changes to it during its life. Maintenance (including cleaning and repainting of woodwork or other exterior surfaces, repointing of cement and repairs to walls and the roof) presents hazards from falling similar to those of erection of the structure because of the need to gain access to high parts of the structure. Indeed, the hazards may be greater because during smaller, short-duration maintenance jobs, there is a temptation to cut costs on provision of safe access equipment, for example, by trying to do from a ladder what can be safely done only from a scaffold. This is especially true of roof work, where replacement of a tile may take only minutes but there is still the possibility of a worker falling to his or her death.
Maintenance and cleaning
Designers, especially architects, can improve safety for maintenance and cleaning workers by taking into account in their designs and specifications the need for safe access to roofs, to plant rooms, to windows and to other exposed positions on the outside of the structure. Avoiding the need for access at all is the best solution, followed next by permanent safe access as part of the structure, perhaps stairs or a walkway with guard rails or a powered access platform permanently slung from the roof. The least satisfactory situation for maintenance personnel is where a scaffold similar to that used to erect the building is the only way to provide safe access. This will be less of a problem for major, longer duration renovation work, but on short-duration jobs, the cost of full scaffolding is such that there is a temptation to cut corners and use mobile powered access equipment or tower scaffolds where they are unsuitable or inadequate.
If renovation involves major re-cladding of the building or wholesale cleaning using high-pressure water jetting or chemicals, total scaffolding may be the only answer that will not only protect the workers but also allow the hanging of sheeting to protect the public nearby. Protection of workers involved in cleaning using high-pressure water jets includes impervious clothing, boots and gloves, and a face screen or goggles to protect the eyes. Cleaning involving chemicals such as acids will require similar but acid-resistant protective clothing. If abrasives are used to clean the structure a silica-free substance should be used. Since use of abrasives will give rise to dust that may be injurious, approved respiratory equipment should be worn by the workers. Repainting of windows in a tall office building or block of flats cannot be done safely from ladders, although this is usually possible on domestic housing. It will be necessary to provide either scaffolding or to hang suspended scaffolds such as cradles from the roof, ensuring that suspension points are adequate.
Maintenance and cleaning of civil engineering structures, like bridges, tall chimneys or masts may involve working at such heights or in such positions (e.g., above water) that prohibit the erection of a normal scaffold. As far as possible, work should be done from a fixed scaffold slung or cantilevered from the structure. Where this is not possible, work should be done from a properly suspended cradle. Modern bridges often have their own cradles as parts of the permanent structure; these should be checked fully before being used for a maintenance job. Civil engineering structures are often exposed to the weather, and work should not be permitted in high winds or heavy rain.
Window cleaning presents its own hazards, especially where it is done from the ground on ladders, or with improvised arrangements for access on taller buildings. Window cleaning is not usually regarded as part of the construction process, and yet is a widespread operation that can endanger both the window cleaners and the public. Safety in window cleaning is, however, influenced by one part of the construction process-design. If architects fail to take into account the need for safe access, or alternatively to specify windows of a design that can be cleaned from inside, then the job of the window cleaning contractor will be much more hazardous. Whilst designing out the need for external window cleaning or installing proper access equipment as part of the original design may initially cost more, there should be considerable savings over the life of the building in maintenance costs and a reduction in hazards.
Refurbishment is an important and hazardous aspect of renovation. It takes place when for example, the essential structure of the building or bridge is left in place but other parts are repaired or replaced. Typically in domestic housing, refurbishment involves stripping out windows, possibly floors and stairs, along with wiring and plumbing, and replacing them with new and usually upgraded items. In a commercial office building, refurbishment involves windows and possibly floors, but also is likely to involve stripping out and replacing cladding to a framed building, installing new heating and ventilation equipment and lifts or total rewiring.
In civil engineering structures such as bridges, refurbishment may involve stripping the structure back to its basic frame, strengthening it, renewing parts and replacing the roadway and any cladding.
Refurbishment presents the usual hazards to construction workers: falling and falling materials. The hazard is made more difficult to control where the premises remain occupied during refurbishment, as is often the case in domestic premises such as blocks of flats, when alternative accommodations to house occupants are simply not available. In that situation the occupants, especially children, face the same hazards as construction workers. There may be hazards from power cables to portable tools such as saws and drills required during refurbishment. It is important that the work be carefully planned to minimize hazards to both workers and the public; the latter need to know what will be going on and when. Access to rooms, stairs or balconies where work is to be carried out should be prevented. Entrances to blocks of flats may have to be protected by fans to protect persons from falling materials. At the close of the working shift, ladders and scaffolds should be removed or closed off in a manner that does not allow children to get onto them and endanger themselves. Similarly, paints, gas cylinders and power tools should be removed or stored safely.
In occupied commercial buildings where services are being refurbished, it should not be possible for liftway doors to be opened. If refurbishment interferes with fire and emergency equipment, special arrangements need to be made to warn both occupants and workers if fire breaks out. Refurbishment of both domestic and commercial premises may require removal of asbestos-containing materials. This presents major health risks to the workers and the occupants when they return. Such asbestos removal should be carried out only by specially trained and equipped contractors. The area where asbestos is being removed will need to be sealed off from other parts of the building. Before the occupants return to areas from which asbestos has been stripped, the atmosphere in those rooms should be monitored and the results evaluated to ensure that asbestos fibre levels in air are below permissible levels.
Usually the safest way to carry out refurbishment is to totally exclude occupants and members of the public; however, this is sometimes simply not practicable.
Provision of utilities in buildings, such as electricity, gas, water and telecommunications, is usually carried out by specialist subcontractors. Principal hazards are falls due to poor access, dust and fumes from drilling and cutting and electric shock or fire from electrical and gas services. The hazards are the same in houses, only on a smaller scale. The job is easier for contractors if proper allowance has been made by the architect in designing the structure to accommodate the utilities. They require space for ducts and channels in walls and floors plus sufficient additional space for installers to operate effectively and safely. Similar considerations apply to maintenance of utilities after the building has been taken into use. Proper attention to the detailing of ducts, channels and openings in the initial design of the structure should mean that these are either cast or built into the structure. It will then not be necessary for construction workers to chase out channels and ducts or to open up holes using power tools, which create large quantities of harmful dust. If adequate space is provided for heating and air conditioning ducts and equipment, the job of the installers is both easier and safer because it is then possible to work from safe positions rather than, for example, standing on boards wedged across the inside of vertical ducts. If lighting and wiring have to be installed overhead in rooms with high ceilings, contractors may need scaffolding or tower scaffolds in addition to ladders.
Installation of utility services should be conform to recognized local standards. These should, for example, cover all safety aspects of electrical and gas installations so that contractors are in no doubt as to standards required for wiring, insulation, earthing (grounding), fusing, isolation and, for gas, protection for pipework, isolation, adequate ventilation and fitting of safety devices for flame failure and loss of pressure. Failure by contractors to deal adequately with these matters of detail in the installation or maintenance of utilities will create hazards for both their own workers and the occupants of the building.
If the structure is of brick or concrete, the interior finish may require initial plastering to provide a surface which can be painted. Plastering is a traditional craft trade. The principal hazards are severe strain to the back and arms from handling bagged material and plaster boards and then the actual plastering process, especially when the plasterer is working overhead. After plastering, surfaces may be painted. The hazard here is from vapours given off by thinners or solvents and sometimes from the paint itself. If possible, water-based paints should be used. If solvent-based paints have to be used, the rooms should be well ventilated, if necessary by the use of fans. If materials used are toxic and adequate ventilation cannot be achieved, then respiratory and other personal protection should be worn.
Sometimes interior finishing may require the fixing of cladding or linings to the walls. If this involves use of cartridge guns to secure the panels to timber studding the hazard will principally arise from the way the gun is operated. Cartridge-driven nails can easily be fired through walls and partitions or can ricochet on striking something hard. Contractors need to plan this work carefully, if necessary excluding other persons from the vicinity.
Finishing may require tiles or slabs of various materials to be fixed to walls and floors. Cutting large quantities of ceramic tiles or stone slabs using powered cutters gives rise to great quantities of dust and should either be done wet or in an enclosed area. The principal hazard with tiles, including carpet tiles, arises from the need to stick them in position. Adhesives used are solvent based and give off vapours that are harmful, and in an enclosed space they can be flammable. Unfortunately, those laying tiles are kneeling down low over the point where vapours are given off. Water-based adhesives should be used. Where solvent-based adhesives have to be used, rooms should be well ventilated (fan assisted), the quantity of adhesives brought into the workroom should be kept to a minimum and drums should be decanted into smaller tins used by tilers outside the workroom.
If finishing requires installations of sound- or heat-insulation materials, as is often the case in blocks of flats and commercial buildings, these may be in the form of sheets or slabs that are cut, blocks that are laid and fixed together or to a surface by a cement or in a wet form that is sprayed. Hazards include exposure to dust that may both irritate and be harmful. Asbestos-containing materials should not be used. If artificial mineral fibres are used, respiratory protection and protective clothing should be worn to prevent skin irritation.
Fire hazards in interior finishing
Many of the finishing operations in a building involve use of materials that greatly increase the fire hazard. The basic structure may be relatively non-flammable steel, concrete and brick. However, the finishing trades introduce wood, possibly paper, paints and solvents.
At the same time that interior finishing is being performed work may be going on nearby using electric powered tools, or the electrical services may be being installed. Nearly always there is a source of ignition for flammable vapour and materials used in finishing. Many very costly fires have been ignited during finishing, putting workers at risk and usually damaging not only the finishing of the building but also its main structure. A building undergoing finishing is an enclosure in which possibly hundreds of workers are using flammable materials. The main contractor should ensure that proper arrangements are made to provide and protect means of escape, keep access routes clear from obstructions, reduce the quantity of flammable materials stored and in use inside the building, warn contractors of fire and, when necessary, evacuate the building.
Some of the materials used in internal finishing may also be used on the exterior, but exterior finishing is generally concerned with cladding, sealing and painting. The cement courses in brick and block work are generally “pointed” or finished as the bricks or blocks are laid and require no further attention. The exterior of walls may be cement that is to be painted or have an application of a layer of small stones, as in stucco or roughcast. Exterior finishing, like general construction work, is done outdoors and is subject to the effects of the weather. By far the greatest hazard is the risk of falling, often heightened by difficulties in handling components and materials. Use of paints, sealants and adhesives containing solvents is less of a problem than in internal finishing because natural ventilation prevents a build-up of harmful or flammable concentrations of vapour.
Again, designers can influence the safety of exterior finishing by specifying cladding panels that can be safely handled (i.e., not too heavy or large) and making arrangements so that cladding can be done from safe positions. The frames or floors of the building should be designed to incorporate features like lugs or recesses that permit easy landing of cladding panels, especially when placed in position by crane or hoist. Specification of materials such as plastics for window frames and fascias eliminates the need for painting and repainting and reduces subsequent maintenance. This benefits the safety of both construction workers and the occupants of house or flat.
Landscaping on a large scale may involve earth-moving similar to that involved in highway and canal works. It may require deep excavations to install drains; extensive areas may have to be slabbed or concreted; rocks may have to be moved. Finally, the client may wish to create the impression of a mature, well-established development, so that fully grown trees will be planted. All of this requires excavation, digging and loading. It often also requires considerable lifting capacity.
Landscape contractors are usually specialists who do not spend much of their time working as part of construction contracts. The main contractor should ensure that landscape contractors are brought to the site at an appropriate time (not necessarily towards the end of the contract). Major excavation and pipe laying may best be carried out early in the life of the project, when similar work is being done for the foundations of the building. Landscaping must not undermine or endanger the building or overload the structure by heaping earth on or against it and its outbuildings in a dangerous manner. If topsoil is to be removed and later placed back in position, sufficient space to heap it in a safe manner will have to be provided.
Landscaping may also be required at industrial premises and public utilities for safety and environmental reasons. Around a petrochemical plant it may be necessary to level off the ground or provide a particular direction of slope, possibly covering the ground with stone chips or concrete to prevent the growth of vegetation. On the other hand, if landscaping around industrial premises is intended to improve appearance or environmental reasons (e.g., to reduce noise or hide an unsightly plant), it may require embankments and erection of screens or planting of trees. Highways and railroad tracks today have to include features that will reduce noise if they are near urban areas or hide the operations if they are in environmentally sensitive areas. Landscaping is not just an afterthought, because as well as improving the appearance of the building or plant, it may, depending on the nature of the development, preserve the environment and improve safety generally. Therefore, it needs to be designed and planned as an integral part of the project.
Demolition is perhaps the most dangerous construction operation. It has all the hazards of working at heights and being struck by falling materials, but it is carried out in a structure that has been weakened either as part of the demolition, or as the result of storms, damage produced by flood, fire, explosions or simple wear and tear. The hazards during demolition are falls, being struck or buried in falling material or by the unintentional collapse of the structure, noise and dust. One of the practical problems with ensuring health and safety during demolition is that it can proceed very rapidly; with modern equipment a great deal can be demolished in a couple of days.
There are three principal ways of demolishing a structure: take it down piecemeal; knock it or push it down; or blast it down using explosives. Choice of method is dictated by the condition of the structure, its surroundings, the reasons for the demolition and cost. Use of explosives will usually not be possible when other buildings are close by. Demolition needs to be planned as carefully as any other construction process. The structure to be demolished should be thoroughly surveyed and any drawings obtained, so that as much information as possible on the nature of the structure, its method of construction and materials is available to the demolition contractor. Asbestos is commonly found in buildings and other structures that are to be demolished and requires contractors who are specialists in handling it.
Planning of the demolition process should ensure that the structure is not overloaded or unevenly loaded with debris and that there are suitable openings for chuting of debris for safe removal. If the structure is to be weakened by cutting parts of the frame (especially reinforced concrete or other highly stressed types of structure) or by removing parts of a building such as floors or internal walls, this must not so weaken the structure that it may collapse unexpectedly. Debris and scrap materials should be planned to fall in such a way that they can be removed or saved safely and appropriately; sometimes the cost of a demolition job depends on salvaging valuable scrap or components.
If the structure is to be demolished piecemeal (i.e., taken down bit by bit), without using remotely operated powered picks and cutters, workers will inevitably have to do the job using hand tools or hand-held powered tools. This means they may have to work at heights on exposed faces or above openings created to allow debris to fall. Accordingly, temporary scaffold working platforms will be necessary. The stability of such scaffolds should not be endangered by removal of parts of the structure or fall of debris. If stairs are no longer available for use by workers because the stairwell opening is being used to chute debris external ladders or scaffolds will be necessary.
Removal of points, spires or other tall features on the top of buildings is sometimes done most safely by workers operating from properly-designed buckets slung from the safety hook of a crane.
In piecemeal demolition, the safest method is to take the building down in a sequence opposite to the way it was put up. Debris should be removed regularly so that working places and access do not become obstructed.
If the structure is to be pushed or pulled over or knocked down, it is usually pre-weakened, with the attendant hazards. Pulling down is sometimes done by removing floors and internal walls, attaching wire ropes to strong points on the upper parts of the building and using an excavator or other heavy machine to pull on the wire rope. There is a real hazard from flying wire ropes if they break due to overload or failure of the anchorage point on the building. This technique is not suitable for very tall buildings. Pushing over, again after pre-weakening, involves use of heavy plant such as crawler-mounted grabs or pushers. The cabs of such equipment should be shielded to prevent drivers from being injured by falling debris. The site should not be allowed to become so obstructed by fallen debris as to create instability for machine used to pull or push the building down.
The most common form of demolition (and if done properly, in many ways the safest) is “balling” down, using a steel or concrete ball suspended from a hook on a crane with a jib strong enough to withstand the special strains imposed by balling. The jib is moved sideways and the ball swung against the wall to be demolished. The principal hazard is trapping the ball in the structure or debris, then trying to extricate it by raising the crane hook. This grossly overloads the crane, and either the crane cable or the jib may fail. It may be necessary for a worker to climb up to where the ball is wedged and free it. However, this should not be done if there is a risk of that part of the building collapsing on the worker. Another hazard associated with less skilled crane operators is balling too hard, so that unintended parts of the building are accidentally brought down.
Demolition using explosives can be done safely, but it must be carefully planned and carried out only by experienced workers under competent supervision. Unlike military explosives, the purpose of blasting to demolish a building is not to totally reduce the building to a heap of rubble. The safe way to do it is, after pre-weakening, to use no more explosive than will safely bring down the structure so that debris can be safely removed and scrap salvaged. Contractors carrying out blasting should survey the structure, obtain drawings and as much information as possible on its method of construction and materials. Only with this information is it possible to determine whether blasting is appropriate in the first place, where charges should be placed, how much explosive should be used, what steps may be necessary to prevent ejection of debris and what sort of separation zones will be required around the site to protect workers and the public. If there are a number of explosive charges, electrical shotfiring with detonators will usually be more practical, but electrical systems can develop faults, and on simpler jobs the use of detonator cord may be more practical and safer. Aspects of blasting that require careful preliminary planning are what is to be done if there is either a misfire or if the structure does not fall as planned and is left hanging in a dangerous state of instability. If the job is close to housing, highways or industrial developments, the people in the area should be warned; local police are usually involved in clearing the area and halting pedestrian and vehicular traffic.
Tall structures like television towers or cooling towers may be felled using explosives, providing they have been pre-weakened so that they fall safely.
Demolition workers are exposed to high noise levels because of noisy machinery and tools, falling debris or blasts from explosives. Hearing protection will usually be required. Dust is produced in large quantities as buildings are demolished. A preliminary survey should ascertain whether and where lead or asbestos are present; if possible, these should be removed before the start of the demolition. Even in the absence of such notable hazards, dust from demolition is often irritating if not actually injurious, and an approved dust mask should be worn if the work area cannot be kept wet to control the dust.
Demolition is both dirty and arduous, and a high level of welfare facilities should be provided, including toilets, washplaces, cloakrooms for both normal clothing and work clothes and a place to shelter and take meals.
Dismantling differs from demolition in that part of the structure or, more commonly, a large piece of machinery or equipment is disassembled and removed from site. For example, removal of part or the whole of a boiler from a power house in order to replace it, or replacement of a steel girder bridge span is dismantling rather than demolition. Workers involved in dismantling tend to do a great deal of oxyacetylene or gas cutting of steel work, either to remove parts of the structure or to weaken it. They may use explosives to knock over an item of equipment. They use heavy lifting machinery to remove large girders or pieces of machinery.
Generally, workers engaged in such activities face all the same hazards of falling, things falling on them, noise, dust and harmful substances that are met in demolition proper. Contractors who carry out dismantling require a sound knowledge of structures to ensure that they are taken apart in a sequence that does not cause a sudden and unexpected collapse of the main structure.
Work over and alongside water as in bridge building and maintenance, in docks and sea and river defence work presents special hazards. The hazard may be increased if the water is flowing or tidal, as opposed to still; rapid water movement makes it more difficult to rescue those who fall in. Falling in water presents the hazard of drowning (in even quite shallow water if the person is injured in the fall as well as hypothermia if the water is cold and infection if it is polluted).
The first precaution is to prevent workers from falling by ensuring that there are proper walkways and workplaces with guard rails. These should not be allowed to become wet and slippery. If walkways are not possible, as perhaps in the earlier stages of steel erection, the workers should wear harnesses and ropes attached to secure anchorage points. These should be supplemented with safety nets slung beneath the work position. Ladders and grablines should be provided to assist fallen workers to climb out of the water, as, for example, at the edges of docks and sea defences. While workers are not on a properly boarded out platform with guard rails or are travelling to and from their worksite, they should wear buoyancy aids. Lifebuoys and rescue lines should be placed at regular intervals along the edge of the water.
Work in docks, river maintenance and sea defences often involves use of barges to carry piling rigs and excavators to remove dredged out spoil. Such barges are equivalent to working platforms and should have suitable guard rails, lifebuoys and rescue and grab lines. Safe access from the shore, dock or river side should be provided in the form of walkways or gangways with guard rails. This should be so arranged as to adjust safely with the changing levels of tidal water.
Rescue boats should be available, fitted with grablines and with lifebuoys and rescue lines on board. If the water is cold or flowing, the boats should be continuously staffed, and should be powered and ready to carry out a rescue mission immediately. If water is polluted with industrial effluent or sewage, arrangements should be made to transport those who fall into such water to a medical centre or hospital for immediate treatment. Water in urban areas may be contaminated with the urine of rats, which may infect open skin abrasions, causing Weil’s disease.
Work over water is often carried out in locations that are subject to strong winds, driving rain or icing conditions. These increase the risk of falls and heat loss. Severe weather may make it necessary to stop work, even in the middle of a shift; to avoid excessive heat loss it may be necessary to supplement normal wet or cold weather protective clothing with thermal underclothing.
Diving is a specialized form of working underwater. The hazards faced by divers are drowning, decompression sickness (or the “bends”), hypothermia from the cold and becoming trapped below water. Diving may be required during construction or maintenance of docks, sea and river defences and at piers and abutments of bridges. It is often required in waters where visibility is poor or in locations where there is a risk of entanglement for the diver and his or her equipment. Diving may be carried out from dry land or from a boat. If the work requires only a single diver, then as a minimum a team of three will be required for safety. The team consists of the diver in the water, a fully equipped standby diver ready to enter the water immediately in the event of an emergency and a diving supervisor in charge. The diving supervisor should be at the safe position on land or in the boat from which the diving is to take place.
Diving at depths less than 50 m is usually carried out by divers wearing wet suits (i.e., suits that do not exclude water) and wearing self-contained underwater breathing apparatus with an open face mask (i.e., SCUBA diving gear). At depths greater than 50 m or in very cold water, it will be necessary for divers to wear suits that are heated by a supply of pumped warm water and closed diving masks, and equipment for breathing not compressed air but air plus a mixture of gases (i.e., mixed-gas diving). Divers must wear a suitable safety line and be able to communicate with the surface and in particular with their diving supervisor. The local emergency services should be advised by the diving contractor that diving is to take place.
Both divers and equipment require examination and testing. Divers should be trained to a recognized national or international standard, firstly and always for air diving and secondly for mixed-gas diving if this is to take place. They should be required to provide written evidence of successful completion of a diver training course. Divers should have an annual medical examination with a doctor experienced in hyperbaric medicine. Each diver should have a personal logbook in which a record of physicals and of his or her dives is kept. If a diver has been suspended from diving as a result of the physical, this also should be recorded in the logbook. A diver under suspension should not be allowed to dive or act as a standby diver. Divers should be asked by their diving supervisor if they are well, especially whether they have any respiratory illness, before being allowed to dive. Diving equipment, suits, belts, ropes, masks and cylinders and valves should be checked every day before use.
Satisfactory operation of cylinder and demand valves should be demonstrated by divers for their diving supervisor.
In the event of an accident or other reasons for the sudden ascent of a diver to the surface, he or she may experience the bends or be at risk of them and require to be recompressed. For this reason it is desirable that the whereabouts of a medical or other decompression chamber suitable for divers is located before diving starts. Those in charge of the chamber should be alerted to the fact that diving is taking place. Arrangements should be available for the rapid transport of divers requiring decompression.
Because of their training and equipment, plus all the backup required for safety, use of divers is very expensive, and yet the amount of time they are actually working on the riverbed may be limited. For these reasons there are temptations for diving contractors to use untrained or amateur divers or a diving team that is deficient in numbers and equipment. Only reputable diving contractors should be used for diving in construction, and particular care needs to be taken over the selection of divers who claim to have been trained in other countries where standards may be lower.
Caissons are rather like a large inverted saucepans whose rims sit on the bed of the harbour or river. Sometimes open caissons are used, which, as their name implies, have an open top. They are used on land to sink a shaft into soft ground. The bottom edge of the caisson is sharpened, workers excavate inside the caisson, and it sinks into the ground as soil is removed, thus creating the shaft. Similar open caissons are used in shallow water, but their depth may be extended by adding sections on top as the caisson sinks into the river or harbour bed. Open caissons rely on pumping to control the entry of water and soil into the base of the caisson. For deeper work still, a closed caisson will have to be used. Compressed air is pumped into it to displace the water, and workers are able to enter through an airlock, usually on top, and go down to work in air on that bed. Workers are able to work under water but are freed from the constraints of wearing diving equipment, and visibility is much better. The hazards in “pneumatic” caisson work are the bends and, as in all types of caisson including the simplest open caisson, drowning if water gets into the caisson through any structural failure or loss of air pressure. Because of the risk of entry of water, means of escape such as ladders up to the entry point should be available at all times in both open and pneumatic caissons.
Caissons should be inspected daily before they are used by someone competent and experienced in caisson work. Caissons may be raised and lowered as single units by heavy lifting equipment, or they may be constructed from components in the water. Construction of caissons should be under the supervision of a similarly competent person.
Tunnelling, when carried out in porous ground beneath water, may need to be done under compressed air. Driving tunnels for public transportation systems in city centres beneath rivers is a widespread practice, owing to lack of space above ground and environmental considerations. Compressed air working will be as limited as possible because of its danger and inefficiency.
Tunnels beneath water in porous ground will be lined with concrete or cast iron rings and grouted. But at the actual heading where the tunnel is being dug and in the short length where tunnel rings are being placed in position, there will not be a sufficiently water-tight surface for the work to proceed without some means of keeping out the water. Working under compressed air may still be used for the tunnel head and ring or segment placing part of the tunnel driving and lining process. Workers involved in driving the heading (i.e., on a TBM operating the rotating cutting head) or using hand tools, and those operating ring and segment placing equipment, will have to pass through an airlock. The rest of the now lined tunnel will not require to be compressed, and thus there will be easier transit of personnel and materials.
Tunnellers who have to work in compressed air face the same hazard of the bends as divers and caisson workers. The airlock giving access to the compressed-air workings should be supplemented by a second airlock through which workers pass at the end of the shift to be decompressed. If there is only a single airlock, this may create bottlenecks and also be dangerous. Hazards arise if workers are not decompressed sufficiently slowly at the end of their shift or if lack of airlock capacity holds up entry of vital equipment to the workings under pressure. Airlocks and decompression chambers should be under the supervision of a competent person experienced in compressed-air tunnelling and proper decompression.
Trenches are confined spaces usually dug to bury utilities or to place footings. Trenches are normally deeper than they are wide, as measured at the bottom, and are usually less than 6 m deep; they are also known as shallow excavations. A confined space is defined as a space that is large enough for a worker to enter and perform work, has limited means of entry and exit, and is not designed for continuous occupancy. Several ladders should be provided to enable workers to escape the trench.
Typically trenches are open only for minutes or hours. The walls of any trench will eventually collapse; it is merely a matter of time. Short-term apparent stability is a temptation for a contractor to send workers into a dangerous trench in hopes of rapid progress and financial gain. Death or serious injuries and mutilations can result.
In addition to being exposed to the possibility of collapsing trench walls, workers in trenches, can be harmed or killed by engulfment in water or sewage, exposure to hazardous gases or reduced oxygen, falls, falling equipment or materials, contact with severed electrical cables and improper rescue.
Cave-ins account for at least 2.5% of annual work-related deaths in the United States, for example. The average age of workers killed in trenches in the US is 33. Often a young person is trapped by a cave-in and other workers attempt a rescue. With failed rescue attempts, most of the dead are would-be rescuers. Emergency teams trained in trench rescue should be contacted immediately in the event of a cave-in.
Routine inspections of the trench walls and worker protection systems are essential. Inspections should occur daily before the start of work and after any occurrence—such as rainstorms, vibration or broken pipes—that may increase hazards. Following are descriptions of the hazards and how to prevent them.
Trench Wall Collapse
The main cause of deaths related to trenching is collapsed trench walls, which can crush or suffocate workers.
Trench walls may be weakened by activities outside but near a trench. Heavy loads must not be placed on the edge of the wall. Trenches should not be dug close to structures, such as buildings or railroads, because the trenching may undermine the structures and weaken the foundations, thus causing the structures and trench walls to collapse. Competent engineering assistance should be sought in the planning stages. Vehicles must not be permitted to approach too close to the sides of a trench; stop logs or soil berms should be in place to prevent vehicles from doing so.
Types of soil and environment
Proper selection of a worker protection system depends on soil and environmental conditions. Soil strength, the presence of water and vibration from equipment or nearby sources affect the stability of trench walls. Previously excavated soils never regain their strength. Accumulation of water in a trench, regardless of depth, signals the most dangerous situation.
The soil must be classified and the construction scene evaluated before an appropriate worker protection system is selected. A project safety and health plan should address unique conditions and hazards related to the project.
Soils can be divided into two main groups: cohesive and granular. Cohesive soils contain a minimum of 35% clay and will not break when rolled into threads 50 mm long and 3 mm in diameter and held by one end. With cohesive soils, trench walls will stand vertically for short periods of time. These soils are responsible for as many cave-in deaths as any other soil, because the soil appears stable and precautions often are not taken.
Granular soils consist of silt, sand, gravel or larger material. These soils exhibit apparent cohesion when wet (the sand-castle effect); the finer the particle, the greater the apparent cohesion. When submerged or dry, however, the coarser granular soils will immediately collapse to a stable angle, 30 to 45°, depending on their particle angularity or roundness.
Sloping prevents trench failure by removing the weight (of the soil) that can lead to trench instability. Sloping, including benching (sloping done in a series of steps), requires a wide opening at the top of a trench. The angle of a slope depends on the soil and environment, but slopes range from 0.75 horizontal: 1 vertical to 1.5 horizontal: 1 vertical. The slope of 1.5 horizontal: 1 vertical is set back 1.5 m on each side at the top for each meter of depth. Even the slightest slope is beneficial. However, the width requirements of slopes often make this approach impracticable on construction sites.
Shoring can be used for all conditions. A shore consists of an upright on each side of a trench, with braces in between (see figure 1). Shores help prevent trench wall collapse by exerting outward forces on a trench wall. Skip shores consist of vertical uprights and cross braces with soil arching between; they are used in clays, the most cohesive soils. Shores must be no more than 2 m apart from each other. Greater distances between cross braces can be achieved by using wales (or walings) to hold the uprights in place (see figure 2). Close sheeting is used in granular and weaker cohesive soils; the trench walls are covered entirely with sheeting (see figure 3). Sheeting can be made of wood, metal or fibreglass; steel trench sheets are common. Tight sheeting is used when flowing or seeping water is encountered. Tight sheeting prevents water from eroding and bringing soil particles into a trench. A shoring system must always be kept tight against the soil to prevent collapse. Braces can be of wood or of screw, hydraulic or pneumatic jacks. Wales can be of wood or metal.
Figure 1. Shores consist of uprights on each side of a trench with cross braces in between
Figure 2. Wales hold uprights in place, allowing greater distance between cross braces
Figure 3. Close sheeting is used in granular soils
Shields, or trench boxes, are large personal protective devices; they do not prevent trench wall collapse but protect workers who are inside. Shields are generally made of steel or aluminium and their size commonly ranges from approximately 1 m to 3 m high and 2 to 7 m long; many other sizes are available. Shields may be stacked on top of each other (figure 4). Guard systems must be in place against hazardous movements of shields in the event of a trench wall collapse. One way is to backfill on both sides of a shield.
Figure 4. Shields protect workers from trench wall collapse
New products are available that combine the qualities of a shore and a shield; some devices are useable in particularly hazardous ground. Shield-shore units can be used as static shields or can act as a shore by hydraulically or mechanically exerting forces on the trench wall. The smaller units are particularly useful when repairing breaks in utility pipes in city streets. Massive units with shield panels can be forced into the ground by mechanical or hydraulic means. Soil is then excavated from inside the shield.
Several steps are recommended to prevent engulfment by water or sewage in a trench. First, known utilities should be contacted before digging to learn where water (and other) pipes are located. Second, water valves that feed pipes into the trench should be closed. Cave-ins that break water mains or cause accumulations of water or sewage must be avoided. All utility pipes and other utility equipment need to be supported.
Deadly Gases and Fumes and Insufficient Oxygen
Harmful atmospheres can lead to worker death or injury resulting from a lack of oxygen, fire or explosion or toxic exposures. All trench atmospheres where abnormal conditions are present or suspected should be tested. This is especially true around buried garbage, vaults, fuel tanks, manholes, swamps, chemical processors and other facilities that can release deadly gases or fumes or deplete oxygen in the air. Construction equipment exhausts must be dispersed.
Air quality should be determined with instruments from outside the trench. This can be done by lowering a meter or its probe into the trench. The air in trenches should be tested in the following order. First, oxygen must be 19.5 to 23.5%. Second, flammability or explosibility must be no higher than 10% of the lower flammable or explosive limits (LFLs or LELs). Third, levels of potentially toxic substances—such as hydrogen sulphide —should be compared with published information. (In the US, one source is the National Institute for Occupational Safety and Health Pocket Guide to Chemical Hazards, which gives, permissible exposure limits (PELs)). If the atmosphere is normal, workers may enter. Ventilation may correct an abnormal atmosphere, but monitoring must continue. Sewers and similar spaces where the air is constantly changing usually require (or should require) a permit-entry procedure. Permit-entry procedures require full equipment and a three-person team: a supervisor, an attendant and an entrant.
Falls and Other Hazards
Falls into and within trenches can be prevented by providing safe and frequent means for entering and exiting a trench, safe walkways or bridges where workers or equipment are permitted or required to cross over trenches and barriers adequate to stop other workers or bystanders or equipment from approaching a trench.
Falling equipment or materials can cause death or injury through blows to the head and body, crushing and suffocation. The spoil pile should be kept at least 0.6 m from the edge of a trench, a barrier should be provided that will prevent soil and rock material from rolling into the trench. All other materials, such as pipes, must also be prevented from falling or rolling into a trench. Workers must not be permitted to work under suspended loads or loads handled by digging equipment.
All utilities should be marked prior to digging in order to prevent electrocution or severe burns caused by contact with live power lines. Equipment booms must not be operated near overhead power lines; if necessary, overhead lines must be grounded out or removed.
Often, one death or severe injury in a trench is compounded by a poorly thought-out rescue attempt. The victim and rescuers may become trapped and overcome by deadly gases, fumes or lack of oxygen; drowned; or mutilated by machines or rescue ropes. These compounded tragedies can be prevented by following a safety and health plan. Equipment such as air testing meters, water pumps and ventilators should be well-maintained, properly assembled and available on the job. Management should train and require workers to follow safe work practices and wear all necessary personal protective equipment.