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).
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.