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Wednesday, 09 March 2011 20:07

Health Risks of Underground Construction Work

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Hazards

Underground construction work includes tunnelling for roads, highways and railroads and laying pipelines for sewers, hot water, steam, electrical conduits, telephone lines. Hazards in this work include hard physical labour, crystalline silica dust, cement dust, noise, vibration, diesel engine exhaust, chemical vapours, radon and oxygen-deficient atmospheres. Occasionally this work must be done in a pressurized environment. Underground workers are at risk for serious and often fatal injuries. Some hazards are the same as those of construction on the surface, but they are amplified by working in a confined environment. Other hazards are unique to underground work. These include being struck by specialized machinery or being electrocuted, being buried by roof falls or cave-ins and being asphyxiated or injured by fires or explosions. Tunnelling operations may encounter unexpected impoundments of water, resulting in floods and drowning.

The construction of tunnels requires a great deal of physical effort. Energy expenditure during manual work is usually from 200 to 350 W, with a great part of static load of the muscles. Heart rate during work with compressed-air drills and pneumatic hammers reaches 150 to 160 per minute. Work is often done in unfavourable cold and humid microclimatic conditions, sometimes in cumbersome work postures. It is usually combined with exposure to other risk factors which depend on the local geological conditions and on the type of technology used. This heavy workload can be an important contribution to heat stress.

The need for heavy manual labour can be reduced by mechanization. But mechanization brings its own hazards. Large and powerful mobile machines in a confined environment introduce risks of serious injury to persons working nearby, who may be struck or crushed. Underground machinery also may generate dust, noise, vibration and diesel exhaust. Mechanization also results in fewer jobs, which reduces the number of persons exposed but at the expense of unemployment and all of its attendant problems.

Crystalline silica (also known as free silica and quartz) occurs naturally in many different types of rock. Sandstone is practically pure silica; granite may contain 75%; shale, 30%; and slate, 10%. Limestone, marble and salt are, for practical purposes, completely free of silica. Considering that silica is ubiquitous in the earth’s crust, dust samples should be taken and analysed at least at the start of an underground job and whenever the type of rock changes as work progresses through it.

Respirable silica dust is generated whenever silica-bearing rock is crushed, drilled, ground or otherwise pulverized. The main sources of airborne silica dust are compressed-air drills and pneumatic hammers. Work with these tools most often occurs in the fore part of the tunnel and, therefore, workers in these areas are the most heavily exposed. Dust suppression technology should be applied in all instances.

Blasting generates not only flying debris, but also dust and nitrogen oxides. To prevent excessive exposure, the customary procedure is to prevent re-entry to the affected area until the dust and gases have cleared. A common procedure is to blast at the end of the last work shift of the day and to clear out debris during the next shift.

Cement dust is generated when cement is mixed. This dust is a respiratory and mucous membrane irritant in high concentrations, but chronic effects have not been observed. When it settles on skin and mixes with sweat, however, cement dust can cause dermatoses. When wet concrete is sprayed in place, it too can cause dermatoses.

Noise can be significant in underground construction work. Principal sources include pneumatic drills and hammers, diesel engines and fans. Since the underground work environment is confined, there is also considerable reverberant noise. Peak noise levels can exceed 115 dBA, with time-weighted average noise exposure equivalent to 105 dBA. Noise-reducing technology is available for most equipment and should be applied.

Underground construction workers can also be exposed to whole-body vibration from mobile machinery and to hand-arm vibration from pneumatic drills and hammers. The levels of acceleration transmitted to the hands from pneumatic tools can reach about 150 dB (comparable to 10 m/s2). Harmful effects of hand-arm vibration can be aggravated by a cold and damp working environment.

If soil is highly saturated with water or if construction is conducted under water, the work environment may have to be pressurized to keep water out. For underwater work, caissons are used. When workers in such a hyperbaric environment make too rapid a transition to normal air pressure, they risk decompression sickness and related disorders. Since the absorption of most toxic gases and vapours depends on their partial pressure, more may be absorbed at higher pressure. Ten ppm of carbon monoxide (CO) at 2 atmospheres of pressure, for example, will have the effect of 20 ppm CO at 1 atmosphere.

Chemicals are used in underground construction in a variety of ways. For example, insufficiently coherent layers of rock may be stabilized with an infusion of urea formaldehyde resin, polyurethane foam or mixtures of sodium water glass with formamide or with ethyl and butyl acetate. Consequently, vapours of formaldehyde, ammonia, ethyl or butyl alcohol or di-isocyanates may be found in the tunnel atmosphere during application. Following application, these contaminants may escape into the tunnel from the surrounding walls, and it may therefore be difficult to fully control their concentrations, even with intensive mechanical ventilation.

Radon occurs naturally in some rock and may leak into the work environment, where it will decay into other radioactive isotopes. Some of these are alpha emitters that may be inhaled and increase the risk of lung cancer.

Tunnels constructed in inhabited areas can also be contaminated with substances from surrounding pipes. Water, heating and cooking gas, fuel oil, petrol and so on may leak into a tunnel or, if pipes carrying these substances are broken during excavation, they may escape into the work environment.

The construction of vertical shafts using mining technology poses similar health problems to those of tunnelling. In terrain where organic substances are present, products of microbiological decomposition may be expected.

Maintenance work in tunnels used for traffic differs from similar work on the surface mainly in the difficulty of installing safety and control equipment, for example, ventilation for electric arc welding; this may influence the quality of safety measures. Work in tunnels in which pipelines for hot water or steam are present is associated with great heat load, demanding a special regime of work and breaks.

Oxygen deficiency may occur in tunnels either because oxygen is displaced by other gases or because it is consumed by microbes or by the oxidation of pyrites. Microbes may also release methane or ethane, which not only displace oxygen but, in sufficient concentration, may create the risk of explosion. Carbon dioxide (commonly called blackdamp in Europe) is also generated by microbial contamination. The atmospheres in spaces which have been closed for a long time may contain mostly nitrogen, practically no oxygen and 5 to 15% carbon dioxide.

Blackdamp penetrates into the shaft from the surrounding terrain due to changes in the atmospheric pressure. The composition of the air in the shaft may change very quickly—it may be normal in the morning, but be deficient in oxygen by the afternoon.

Prevention

Prevention of exposure to dust should in the first place be implemented by technical means, such as wet drilling (and/or drilling with LEV), wetting of the material before it is pulled down and loaded to the transport, LEV of mining machines and mechanical ventilation of tunnels. Technical control measures may not be sufficient to lower the concentration of respirable dust to an acceptable level in some technological operations (e.g., during drilling and sometimes also in the case of wet drilling), and therefore it may be necessary to supplement the protection of the workers engaged in such operations by the use of respirators.

The efficiency of technical control measures must be checked by monitoring the concentration of airborne dust. In the case of fibrogenic dust, it is necessary to arrange the programme of monitoring in such a way that it allows the registration of the exposure of individual workers. The individual exposure data, in connection with data about each worker’s health, are necessary for the assessment of the risk of pneumoconiosis in particular work conditions, as well as for the assessment of the efficiency of control measures in the long-run. Last but not least, the individual registration of exposure is necessary for evaluating the ability of individual workers to continue in their jobs.

Due to the nature of underground work, protection against noise depends mostly on the personal protection of hearing. Effective protection against vibrations, on the other hand, can be achieved only by eliminating or decreasing the vibration by mechanization of risky operations. PPE is not effective. Similarly, the risk of diseases due to physical overload of the upper extremities can be lowered only by mechanization.

Exposure to chemical substances can be influenced by the selection of appropriate technology (e.g., the use of formaldehyde resins and formamide should be eliminated), by good maintenance (e.g., of diesel engines) and by adequate ventilation. Organization and work regime precautions are sometimes very effective, especially in the case of the prevention of dermatoses.

Work in underground spaces in which the composition of the air is not known demands strict adherence to safety rules. Entering such spaces without isolating breathing apparatuses must not be allowed. The work should be done only by a group of at least three people—one worker in the underground space, with breathing apparatus and safety harness, the others outside with a rope to secure the inside worker. In case of accident it is necessary to act quickly. Many lives have been lost in efforts to save the victim of an accident when the safety of the rescuer was disregarded.

Pre-placement, periodic and post-employment preventive medical examinations are a necessary part of the health and safety precautions for workers in tunnels. The frequency of periodic examinations and the type and scope of special examinations (x ray, lung functions, audiometry and so on) should be individually determined for each workplace and for each job according to the working conditions.

Prior to groundbreaking for underground work, the site should be inspected and soil samples should be taken in order to plan the excavation. Once work is underway, the work site should be inspected daily to prevent roof falls or cave-ins. The workplace of solitary workers should be inspected at least twice each shift. Fire suppression equipment should be strategically placed throughout the underground work site.

 

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Read 6886 times Last modified on Thursday, 19 May 2011 21:44