Although the principle methods of tunnel mucking out and transport have changed little over the last 20 years, with the exception of the growth in the use of belt conveyors, the issues associated with their use have. Factors such as safety, efficiency, productivity and downtime, summing up to the use and/or modification of existing methods have demanded more attention from both contractors and manufacturers.
The use of belt conveyors for tunnelling projects has been steadily growing over the last few years, especially for long drives; but despite advances in curved drives and horizontal lifts, mobile transport is still the popular choice where flexibility is a key issue. It is, however, the high-powered mobility of large numbers of plant that can pose inherent safety problems that are less associated with static equipment such as piped and belt-conveyed mucking out.
Reflecting the numerous factors that must be considered in the choice and design of mobile transport (railbound and trackless), this article concentrates on this option.
Ventilation and emissions
One of the leading current concerns of regulatory authorities and contractors, and hence specialist manufacturers, is the effect of mobile plant on working environment. The contaminants requiring attention to maintain acceptable ventilation levels include carbon particles (from diesel exhausts), products of combustion, or incomplete combustion (carbon monoxide – CO, carbon dioxide – CO2, nitrogen oxides – NOx), and generated heat (engines, brakes, transmission, etc). In addition movements of, mainly trackless, vehicles can easily add to the dust lifted into the ventilation flow.
The leading legal controls on emissions from new engines are from the EPA in the US and the European Union Machinery Directive. The latter, however, forbids member countries from imposing type-approval requirements on engines other than those of the Directive. Since underground conditions are usually more severe than surface working, the more stringent demands on ventilation are likely to require special measures to reduce emissions so that toxins in the air are below acceptable exposure levels. In the EU these are covered by the Chemical Agents Directive under which a consolidated list of Indicative Occupational Exposure Limit Values (IOLEV) has been established, although member states set their own limits according to the stated ‘indicative’ limits. Although a limit for nitrous oxide (NO) of 0.2ppm was envisaged by the EU, in the UK the Health & Safety Executive (HSE) set a limit of 1ppm in the working atmosphere, but recognised that this might not be practical for underground work.
The HSE’s Donald Lamont explained that the British Tunnelling Society (BTS) commissioned a report from the Institute of Occupational Health that supported a proposal for an NO limit of 5ppm over an 8h shift (up to 15ppm for short periods) that would not give rise to an unreasonable health risk. The BTS has produced interim best practice guidance on reducing exposure to NO.
In drill and blast tunnelling the toxic gases generated by blasting, although not directly caused by mucking out, can have an effect on the ventilation problems. The commencement of mucking out can or should be delayed by poor ventilation, and fumes and toxic gases (NOx, CO & CO2) can be released from the muck-pile during loading out. However, modern gel explosives greatly reduce fume generation.
Donald Lamont also points out that there is more to atmospheric contaminants than engine emissions. “The problem is that ambient air contains NO and NO2. If you are seeking to dilute to 1ppm and ambient air contains 0.25ppm, the ventilation flow at discharge from the duct is already contaminated to 25% of the control level. This introduces an inefficiency that is not there for, say, SO2 (sulphur dioxide) or methane. The higher the ambient concentration, the less efficient the system. [For example] ambient levels from aircraft and traffic pollution during the summer around [London] Heathrow are given on the Internet as 0.8ppm.”
It is best to minimise emissions at source which, these days, means a combination of new-generation ‘clean burn’ engines and exhaust filters including catalytic converters.
New generation engines
Manufacturers of diesel engines have been designing new models to keep up with, or stay ahead of, the demands of various nations’ for improved exhaust emissions, applicable chiefly to public highway transport. Often these developments go hand-in-hand with improving fuel efficiency and overall running costs.
One of the leading diesel engine manufacturers, Cummins, has made the design and production of engines for underground applications a major priority. One of its latest introductions is a 130kW, 5.9 litre turbo diesel developed for Dodge Ram pick-up trucks often used underground in North America for personnel transport, servicing and utility needs. The Ram has been approved by the Mine Safety & Health Administration (MSHA) for mining use. It is claimed that the reduced-emissions truck can save $16-18 000 per year per vehicle in ventilation costs. This is based on an estimate of $4.00-4.50 ventilation costs for every ft3/min of gaseous exhaust emissions. Cummins utilised the duty cycle feedback from the Ram trucks operating underground to electronically remap and recalibrate the 242kW standard engine to achieve a low-emission profile whilst still retaining a power delivery suited to mine speed limits and truck applications. The in-cylinder combustion formula of the engine could be modified due to the injection flexibility of the high-pressure common rail fuel system and the precision control of the electronic management system.
Another recent application for Cummins engines is Atlas Copco’s new Scooptram ST1030 10-t load-haul-dump vehicle. The Cummins QSL9 uses the high-pressure common rail system to meet the US EPA Tier 3 emission requirements. A trial application of two units at the Jacobina Mine in Brazil demonstrates more power, lower fuel consumption, less noise and emissions and high availability. Cummins is to provide ‘Limited Repairer’ level training to Atlas Copco personnel to work on the engine.
At the other end of the scale in Cummins/Atlas Copco combinations is the top range 50-t MineTruck MT5010, equipped with the 485-kW (650-HP) QSK19 engine.
As well as the introduction of engines for underground applications, Cummins launched a low NOx rebuild programme for engines built from 1996 to 1998, which allows for low emission calibrations to be included at the time of overhaul and/or rebuild.
Caterpillar’s approach to cleaner engines is its ACERT Technology, which reduces emissions from the point of combustion without sacrificing engine performance. It relies on advanced technologies for improved air management, precision combustion technology, advanced engine electronics and a simple ‘after-treatment process’. The C-12 engine with ACERT Technology has just gained Caterpillar a sixth award in the J D Power & Associates 2006 Heavy-Duty Truck Engine and Transmission Customer Satisfaction Survey in the ‘Vocational’ category, which includes construction applications.
Various devices for reducing the effect of diesel engine exhausts have been necessary for a long time, particularly filters to remove particles, chemical catalysts to remove carbon monoxide and other noxious gases, and flame-trap to prevent dangerous contact with any ‘gaseous’ (explosive mixes of methane) atmospheres. Current devices have similar purposes but the necessary performances are more demanding. A major manufacturer of diesel locomotives for tunnelling, Brookville Equipment, offers an exhaust catalyst for reduction of carbon monoxide (CO) and hydrocarbons (HC). There is also a ceramic exhaust filter for CO, HC and DPM (particles) reduction of up to 85%. For an even cleaner exhaust emission a DST dry filtering system is offered for up to 95% reduction in CO, HC and DPM.
Other approaches to cleaner exhaust and more efficient engines are still under development or are only just entering the market. Earlier this year Hy-Drive Technologies sold an initial order of 20 of its on-board hydrogen generating systems (HGS) to Canada-based Mining Technologies International (MTI). The recipient manufactures trucks, loaders and utility vehicles amongst other underground equipment, and MTI will be adapting the HGS for use on selected diesel equipment. The HGS is said to reduce exhaust emissions whilst enhancing fuel efficiency. President and ceo of MTI, Robert Lipic, said, “We turned to Hy-Drive because they have an innovative solution for reducing the harmful exhaust emissions that all mining vehicles inherently produce. Their simple retrofit package reduces the emissions as part of the natural combustion process, instead of just filtering the exhaust.”
Electric alternatives
Mobile plant driven by electric motors is an alternative to diesel engines that can eliminate all emission problems. However they can have some serious limitations in power-to-weight ratios and the need for additional plant, especially where trolley power transmission is considered. In considering battery-powered versions, one has to have a charging station, which is usually sited on the surface to avoid additional ventilation requirements. The weight of battery-powered plant is less of a problem with railbound methods (and may be an advantage for traction), and many battery locos have been employed in tunnels, especially in smaller sections. Larger battery vehicles have suffered from heat generation, life expectancy or weight problems depending on their size and duty.
Despite the drawbacks of traditional drawbacks of batteries and the promised technical developments, they remain a favourite with many tunnellers. Larry Conrad, vice-president of Brookville Equipment Corp, a major user of traction batteries, reports, “We look forward to what the future holds with battery improvements, however flooded lead-acid batteries remain the users choice for power-high current applications. The most important (improvement) to date is the energy conservation offered by modern microprocessor IGBT (Insulated Gate Bipolar Transistor) controls.”
Brookville offers battery locos in the 4-20t range with motors of 22-75kW. The batteries can be charged on board or lifted out on battery trays. The Company’s diesel loco range is rated at 4-45t with power of 49-298kW. Safety and efficiency features include remote uncoupling, liquid-cooled wet-disc brakes with dynamic braking, and rubber-bonded, v-spring suspension.
Cable-reel power supply has also been used, especially for LHD vehicles, but the extent of operation is limited by the length of the cable; normally not more than 200m either side of the switchgear. This limitation tends to be more serious in the more linear operation of civil tunnelling than in mining.
Although possible, trolley electric power is rarely used for civils tunnelling, due to the size of the temporary investment required, although it is common in large mines.
Ergonomics
When an operator may need to spend the whole shift in one position it is important for safety and comfort that the seating and cab is well designed. Considerations include the viewing range, seat suspension including vibration dampening, sufficient space for ancillary equipment such as helmet and cap-lamp battery, ease of repositioning for reversing operations etc.
During the tunnel loading cycle, space is often at a premium, making face working are inherently more hazardous unless pedestrians are banned altogether. Even in larger tunnel sections it is tempting to use the maximum capacity for both loading out and transport, necessitating the most efficient use of space. The pattern of vehicle movements is also important, with all unnecessary movements eliminated if possible. Generally the most efficient systems have the means of loading out either adjacent to or in front of the means of transport. In the former, a side tipping bucket such as the Gjerstad will be used on a wheel or tracked loader. In the latter the loader (e.g. gathering arm type) will usually have a short conveyor to carry the spoil over the loader and onto the truck or conveyor being used for transport. In the same orientation are the Eimco or similar overhead loaders, and the Haggloader and Shuttletrain from GIA. A third option is to use a load-haul-dump (LHD) vehicle which, although economical only over relatively short transport distances, can service several faces to carry spoil to a central tip for further transport.
The efficient use of space at the face has been tackled by Caterpillar with the development of compact hydraulic excavators. The smaller Cat 321 CCR, now available in face shovel configuration, has buckets of 0.8-1.8m3 capacity. Its tail-swing radius is only 1.68m. It has maximum forward reach of 6.35m and a maximum dump height of 7.73m.
Wherever possible pedestrians should be kept away from powered mobile equipment, especially if the operator’s field of view is limited. This is easier with railbound transport and larger sections in which separate walkways can be constructed. It is also promoted by the use of personnel transport, trackless or railbound.
A possible drawback of railbound transport is the need to plan train movements for longer tunnels, and a likely need for fixed by-pass locations, perhaps with additional excavation. Trackless, or ‘free-steered’, vehicles offer more flexibility in operation, especially if some form of cassette system is offered as by GIA’s Kirunatruck Combi system and Normet utility vehicles. In tunnelling there seems to be a perception that trackless vehicles are not so robust, and this is clearly true as regards the tyres, if nothing else. As a supplier of most major forms, Brookville’s Larry Conrad said, “Brookville has mainly supplied rail type equipment for tunnelling, but looks forward to implementing new types of rubber-tyred equipment to help increase the production of moving people and material.”
Additional precautions for pedestrian safety around vehicles can include automatic reversing audible warning and, especially with larger vehicles such as dump trucks, CCTV cameras for rear visibility.
Noise and vibration
Excessive noise generation and vibration can be both dangerous in operation and hazardous to long-term health. Modern engines generate lower noise levels but the noise can be confined within the tunnel. It is still important, therefore, for operators to have well insulated, closed cabs, where possible, and wear ear defenders. Seat and vehicle suspension systems combat vibration being transmitted to the operator’s body. Even with the generally smoother operation of railbound transport, adequate suspension is still necessary.
Improved performance
One customer application for Cummins engines is a retrofit to Bell articulated dump trucks (ADTs). In a pilot programme a Cummins QSM-11 was installed into one of eleven Bell ADTs operating in a copper mine in Namibia. Results demonstrate substantial savings in performance, fuel economy and overall costs. The mine had a choice of rebuilding its old engines at a costs of over R250,000, or purchasing new engines at R275,000. However, the rebuilt engines had a forecast life of 9000h compared to 20 000h for the new Cummins engines.
The improvements were achieved with the aid of the increasingly common procedure of electronic engine control.
A Cat 325CCR compact excavator loads a 30t GHH truck Cat 325CCR One of four 30t GHH articulated trucks working in the Wienerwald Tunnel 30t GHH articulated trucks Attacking the muck pile can release fumes, gases and dust into the atmosphere Attacking the muck pile A second-hand Kiruna articulated dump-truck used by Skanska on the Vuosaari Tunnel Kiruna articulated dump-truck
