Occupational health and safety protection in tunnels is regulated in Germany by national and European directives. These aim to ensure the highest possible protection for workers from harmful dust, exhaust gases and hazardous substances during the rehabilitation and construction of tunnels.
While EU directives are formulated quite abstractly at national levels, in Germany these specifications are applied to practice through technical rules. For measures against substances hazardous to health, the Technical Rules on Hazardous Substances (TRGS) should be mentioned here, which reflect the current state of technology, occupational medicine and other scientific findings. Below, and explained in greater detail, are the possible hazards for employees during tunnel rehabilitation in Germany, as well as procedures for avoiding and combating pollutants, all explained with reference to the Mannheim-Stuttgart high-speed line (SFS 4080) which deployed a new system of dust removal from two ballast-cleaning machines.
HAZARDS FROM WORK INVOLVING EMISSIONS
In Germany, before starting work with potential hazardous substances, the employer must first carry out a risk assessment. This will meet the requirements of TRGS 400, point 4, which determines protective measures for the health and safety of those working in the area affected by the hazardous substances.1 For example, the use of diesel-powered construction machinery in tunnels during track construction or associated refurbishment releases hazardous substances that pose a health risk to workers. Exhaust gases from diesel engines contain hazardous substances, including nitrogen monoxide (NO), nitrogen dioxide (NO2), carbon monoxide (CO) and carbon dioxide (CO2), as well as potentially toxic particulates – the so-called diesel soot particles.2
In addition to diesel-engine exhaust gases, particulates in dust from mechanical work processes pose a risk to occupational health and safety in tunnels. When working in railway tunnels, dust is mainly generated while working with ballast, grinding work on rails, or rehabilitation of the tunnel structure.
ACHIEVING REQUIRED PROTECTION LEVELS
When planning protective measures, a check must first be made as to whether it is possible to eliminate, or at least minimise, any hazardous substances by substituting any construction equipment to be used. For example, when selecting construction equipment, checks must be made on whether suitable battery-powered machines can be substituted to carry out the work. Since current technology is unable to offer battery-powered machines for much track construction work, the required occupational health and safety must be achieved via further protective measures.
When using diesel-powered machinery, technical optimisation measures on combustion engines (exhaust gas after-treatment systems, particle filters and/ or DeNOx systems) are supplemented by ventilation measures. The aim of technical ventilation is a controlled and assured dilution of – or removal of – gaseous and particulate pollutants. In particular, the most recent reductions in occupational exposure limits for nitrogen monoxide and nitrogen dioxide have led to an increase in occupational health and safety measures.
Unlike tunnels to be newly excavated, tunnels requiring rehabilitation are already excavated and are therefore not a closed system that can be supplied by artificial fresh air supply according to proven procedures. The open system is influenced by several additional parameters. For example, the different temperatures and air densities at the portals and in the rock create natural thermal air movements. Wind creates a uniform pressure distribution over the cross-section and consequently a moving piston of air through the tunnel. These influences vary depending on the season and can occur in opposing directions. This makes planning the ventilation much more complex, as all influences must be fully accounted for.
Typical of track construction sites are the short time frames for assembly and disassembly of ventilation systems. This requires advance detailed planning in which project-specific logistical concepts for assembly and disassembly phases of systems are developed in addition to the ventilation planning. Furthermore, the planning phase must ensure that ventilation equipment installed in the tunnel does not have a negative influence on construction work logistics, and that the required working areas are not restricted. In addition to operational needs, important influencing factors in the creation of a ventilation concept are the fire requirements, particularly those for escape.
For ventilation system design, wind influences on the portal where air exhaust occurs must be taken into account, as well as thermal conditions in the tunnel that could counteract the ventilation. The definition of concrete load cases, which is practically always carried out in engineering, has no significance in the design of a ventilation system, since the maximum case can occur at any time during construction. Planning should result in a ventilation system that counteracts the natural uplift and wind loads, and sufficiently dilutes any released pollutants in the tunnel.
Organisational measures supplement protection methods. The planning of ventilation is closely linked to the planning of the construction process. In order to ensure protection to workers’ health, no additional workplaces may be set up in the emission-source ventilation area. Therefore, determining the ventilation and working direction is an essential part of the protection concept. This fact also shows that natural air movement in a tunnel without the use of mechanical ventilation does not guarantee sufficient occupational safety, as it cannot be ensured that there is a continuous air flow. Furthermore, the direction of natural air flow could change at any time, which means that work areas could lie in the downdraft of emission sources.
Personal protective measures – such as particle-filtering masks – should be applied within a hierarchy of measures only after prior examination of possible process substitution, and after the implementation of every possible technical and organisational measure. A suitable level of protection can possibly only be achieved by combining several protective measures.
SUPPLEMENTARY MEASURES FOR DUST HAZARDS
In addition to protective measures against pollutants from diesel engine emissions, which consist of measures such as particle filters and tunnel ventilation for the removal of pollutants, dust-generating work requires further occupational safety measures. During most work undertaken in tunnels, dust is initially generated and released in the working area. In principle, it can be said that the most efficient form of pollutant reduction is the collection or targeted extraction of dust generated at the point of origin.
By using highly efficient de-dusting systems, the extracted air can be cleaned and returned to the tunnel environment. However, due to technological limitations, this form of dust collection is not feasible for all work processes. Mechanical ventilation measures must therefore be supplemented by prior wetting of the ballast, for example, when ballast work is undertaken with a road-rail excavator.
Following various pilot tests, it has been proved that a combination of wetting and dedusting/ventilation contributes significantly to a reduction of pollutant loads from alveolar dusts (A dusts), inhalable dusts (E dusts) and quartz, thus leading to compliance with German occupational exposure limits.
MEASUREMENT MONITORING OF THE WORK
According to German TRGS 402 requirements, project-specific measurement concepts must be prepared within the planning framework in addition to the ventilation concept. These must be drawn up on the basis of the equipment to be used, the procedures, the work performance and organisation, the emission locations or work areas, the planned ventilation as a mechanical protection system, and the spatial conditions of the tunnel structure.3 In addition to the use of personal air-quality monitoring devices for any hazardous substances determined in the risk assessment, stationary metrological area monitoring (for example at ventilation portals) must also be considered as part of the measurement concept. In addition to any hazardous substances, air velocity in the tunnel must also be measured to provide information about the air exchange and thus, the removal of any pollutants.
The operation of the ventilation system is continuously determined by natural influences, such as wind pressure at the portal, or thermal conditions within the tunnel, as well as by the construction process itself. Ingoing and outgoing work trains lead to a change in the cross-section of the tunnel, and thus to a need to adjust the required fan output. Due to such influencing factors, the control of the ventilation system on the basis of the measured air quality must be permanently applied during the works by professionals responsible specifically for this task.
DUST EXTRACTION DURING TUNNEL REHABILITATION
Implementing a concept for occupational health and safety is illustrated using the example of a machine for cleaning the track bed as part of a tunnel rehabilitation project in Germany. The decisive factor in developing an innovative dust extraction system for a ballast cleaning machine (BRM) was that ballast cleaning also had to be carried out mechanically inside the tunnel structures. Until now, the ballast bed in tunnels was mainly cleaned conventionally. The necessary occupational health and safety measures therefore relate on the one hand to pollution caused by diesel exhaust gases from the machines – countered by ventilation and particle filters – and on the other hand to the pollution arising from dust produced during work processes. Sometimes, the use of a ballast-cleaning machine results in considerable dust creation (see figure 1). This occurs equally during mechanical clearing of old ballast, the screening process, the transport on conveyor belts, and the placing of the cleaned ballast.
Efficient dust reduction is therefore necessary to protect personnel. The use inside tunnels of ballast-cleaning machines without de-dusting technology would lead to the permissible limit values for A-dust and E-dust being exceeded despite the ventilation, resulting in a health hazard. This is especially due to the number and location of the various work areas in the vicinity of the ballast-cleaning machine. Decisive for evaluating the situation were, among others, the German technical rules for hazardous substances (TRGS): TRGS 554 ‘Exhaust gases from diesel engines’, TRGS 559 ‘Mineral dust’ and TRGS 900 ‘Workplace concentration’.
Since extraction with subsequent de-dusting is the only method for reducing air-borne dust, a solution for dust extraction had to be found that could be implemented in compliance with all legal and technical requirements. Its development was preceded by several pilot tests to determine the basics as well as the innovative work. During the pilot construction sites, the focus was on identifying potential dust sources on the complete ballast-cleaning machine, as well as further preventive measures for dust reduction.
In order to limit the air volume extracted, the system also had to be sealed off against diffuse airflows from outside. For this purpose, a construction was developed in the area of the clearing chain that is stable above the upper edge of the rail and flexible below it in order to adapt to the ballast surface. It was important here that the machine operator’s view of the clearing chain area was not restricted.
Two dry de-dusting systems were used to remove dust from the extracted air. A special feature is the compact design of these de-dusting systems, which allows them to be integrated into the machine network (see figure 2).
Dust was collected in ‘big bags’ and, if necessary, could be disposed of after leaving the tunnel or, if already in the tunnel, with a road-rail excavator. The filters used were suitable for the filtration of all common mineral dusts. With a surface fabric weight of 260g/ m2 and an air permeability of 65L/dm2, the pressure loss across the filter was kept as low as possible in order to minimise the required power consumption of the axial fans used to generate negative pressure.
All extraction points of the de-dusting system were encapsulated with tarpaulins in addition to extraction hoods (see figure 3), to guarantee optimal dust collection, and prevent dust particles from being carried away by the ventilation system. For dust collection, it was important to ensure that not only the extraction points themselves, but also the dust-emitting points where for structural reasons extraction was not feasible, were encased in advance. This should prevent dust escaping from the machine. These points include a large part of the conveyor systems installed on the machine as well as the upstream MFS carriages (material conveying and silo unit) of the ballast cleaning machine.
Operating in parallel to the dust extraction measures on ballast cleaning machines, a tunnel ventilation system was required to discharge emissions away from the diesel engines. During the conversion work in the tunnel, pairs of fans were set up opposite each other on the edge paths. This form of installation eliminates dead spaces between working machines and the adjacent tunnel wall.
The innovative extraction system (see figure 4) was used for the first time during the 205-day refurbishment of 31km of tunnel on the 4080 high-speed line between Mannheim and Stuttgart.
CONCLUSION OF PREVIOUS DUST EXTRACTION TESTS AND IMPLEMENTED MEASURES
Integrating de-dusting systems into ballast-cleaning machines can protect personnel from dust hazards. Highly efficient de-dusting systems make it possible to comply with workplace limits even when using ballast-cleaning machines in tunnels.
The combination of ongoing construction progress, site logistics and project-specific ventilation systems make the ventilation of a tunnel a complex issue. Therefore, early planning is indispensable and, in addition to a smooth project progress, can also lead to cost-efficient project solutions.
