Tunnel boring machines in hard rock mining
From the proceedings of the MVSSA 2013 conference: The Art of Precision
Historically, mine planners have recognised the benefits of using Tunnel Boring Machine [TBM] technology to facilitate rapid access to ore reserves in hard rock mines. Recently methods that utilise TBM technology to extract ore from narrow reef ore bodies have been researched. Operating TBMs in a deep hard rock environment presents many new complications, not least the greater heat loads and ventilation and refrigeration systems required to mitigate them. Heat is added by the drive motor, exposed rock, rock handling and by ancillary equipment required by the TBM including conveyors and Load Haul Dumpers [LHDs] and it is important that the ventilation and cooling system are compatible with other service requirements [power, water and compressed air]. This paper discusses the heat load and energy balance estimates and practical integration of the TBM systems with environmental considerations including air temperatures and quantities, distance of the advancing face from the through ventilation position, high face advance rates and suppression of dust, noise and other pollutants.
Tunnel Boring Machines [TBMs] are used for large bore, rapid tunnel excavation. TBMs typically have a circular cross section between 1 m and 8 m and can bore through a variety of soil and rock strata.
The cutter head typically rotates at between 1 and 15 rpm, depending on the TBM diameter and the rock type. The advance rate of TBMs can vary between 10 m/day and 60 m/day, depending on the ground conditions.
Mine planners have recognised the benefits of using TBM technology to facilitate rapid access to ore reserves in hard rock mines and the earliest application of TBMs in mining dates back as early as the late 1950s. Although the penetration rates achieved at the time exceeded those of drill and blast operations, the capital and operational costs could not be justified.
More recently methods that utilise TBM technology to extract ore from narrow reef ore bodies have been researched. Meaningful work has been conducted to investigate the application of TBMs in hard rock mines by AngloGold Ashanti. The intent is to utilise TBMs for development as well as ore extraction, Beukes & Labuschagne (2013). References to similar work include papers by Cigla et al. (2001), Allum & Van Der Pas (1995), etc.
Operating TBMs in a deep hard rock environment presents many new challenges with regards to heat loads and the ventilation and refrigeration systems required to mitigate them. There are multiple heat sources in a TBM drive and it is important that the ventilation and cooling system are compatible with other mine service requirements.
Note: The paper refers to TBM drives, which can either be a development end or a production zone.
The first TBM ever reported was built by the Belgian Engineer Henri-Joseph Maus. In 1845 he obtained approval to construct the first railroad connection between France and Italy and started designing the ‘Mountain Slicer’ that was to dig the Fréjus Rail Tunnel through the Alps. Maus completed building the ‘Mountain Slicer’ by 1846 in an arms factory near Turin. It consisted of a locomotive-sized machine, with 100 percussion drills mounted in the front, mechanically power-driven from the entrance of the tunnel. However, it was never used and the tunnel was finally built using conventional drill and blast methods.
The first “successful” TBM manufactured and operated was in the United States in 1853, during the construction of the Hoosac Tunnel. Unfortunately, it drilled only 3 m into the rock before breaking down. One hundred years later, James S. Robbins manufactured a machine that was able to cut through a very difficult shale rock formation, at an advance rate of 49 m/day, which at the time was ten times faster than any other advance rate.
The breakthrough that made TBMs more efficient was the invention of the rotating head and the use of cutting discs/wheels instead of rotating spikes. The consequence was that the frequency of replacement of the wearing parts could be reduced. TBMs typically consist of one or two shields to protect the TBM and personnel (depending on the integrity of the rock) and trailing support mechanisms.
At the front of the shield is a rotating cutter head and behind the cutter head is a chamber. Behind the chamber is a set of hydraulic jacks, which is used to push the TBM forward. The cutter head cuts the rock face into chips or excavating soil (called muck). Depending on the type of TBM, the muck will fall onto a conveyor belt system or into skips and be carried out of the tunnel. Figure 1 shows a typical TBM. Surface boring machines with total lengths of up to150 m (machine and train) are not uncommon. The train contains the operator’s cabin, hydraulic equipment, cooling equipment, ventilation equipment, hose reels, spares, consumables, etc.
Figure 1: Tunnel boring machine
TBMs are generally very expensive due to special engineering and non-mass production and can be difficult to transport. However, it is becoming more feasible to make use of the TBM method than conventional drill and blast methods, due to the high advance rates of these machines.
A TBM with a diameter of 14.4 m was used for the Niagara Tunnel Project to bore a hydroelectric tunnel beneath the Niagara Falls. The TBM was manufactured by the Robbins Company and is to date the largest hard rock TBM. The largest TBM ever built (to bore through soft ground) for the Orlovski Tunnel, was built by Herrenknecht AG and had a diameter of 19.25 m.
The Gotthard Base Tunnel (railway tunnel) beneath the Swiss Alps is expected to open in 2016. The tunnel is bored with a TBM and will have a total route length of 57 km, which gives it the title of the world longest rail tunnel.
Conventional hard rock mining
The primary method of production and rock breaking in the South African hard rock mining industry is conventional drilling and blasting. The critical path for the drill and blast cycle includes (Stewart et al. 2006):
- Charging and blasting
- Ventilation (clearing blasting fumes)
- Preliminary ground control
Because of this time consuming process, tunnelling advance rates in fully mechanised drill and blast hard rock mines are limited to approximately 5 m/day per single end, applying multi-blasting. Due to the high advance rates of TBMs, the mining industry has been investigating the utilisation of TBMs for rapid access to ore reserves in hard rock mines. Although the earliest application of TBMs in mining dates back as early as the late 1950s, methods that utilize TBM technology to extract ore from narrow reef ore bodies have recently been researched.
Drill and blast is a well-developed technology in the hard rock mining industry and the choice of the method of excavation becomes a matter of economics.
One of the key issues when considering the two methods is the haulage (tunnel or development) length. TBMs have high capital costs, but due to higher advance rates, unit cost per meter of tunnel is lower (beyond a certain tunnel length). One of the downfalls when considering the use of a TBM in mining is the turning radius of the machines and the length of the train behind the cutting head. A fit-for-purpose underground TBM with a diameter of 5 m can be designed with a machine length of about 15 m, a train length of approximately 30 m and a turning radius in the order of 75 m.
A rule-of-thumb factor, as documented in Nord (2006), indicates when a TBM application might be financially suitable. It takes into account the tunnel length, tunnel diameter and the rock strength. Typically, if the factor is higher than three, it might be economical to use a TBM but if less than one, drill and blast is the preferred option. The rule, however, does not take into account ground conditions and the rock abrasiveness.
Due to the size of TBMs, most machines would have to be dismantled for transport underground and then reassembled. It is therefore necessary to prepare surface and underground assembly sites.
An area large enough to host the TBM must be constructed with a framework and hoists that can support the maximum weight of the components that need to be lifted. Additional services required to do tests runs of the machine must also be installed.
A launching chamber must be prepared at the start of the tunnel to initiate TBM mining. All haulages along the route to the launching chamber must be large enough to ensure a clear passage of the TBM. The launching chamber is developed by conventional methods. Other preparation work includes provision of services such as power, water and compressed air. Figure 2 shows a typical layout of a TBM.
Figure 2: Typical Layout of a TBM
The TBM consists of a tapered steel structure, which is protected by a shield (1). The machine extends and drives forward in the tunnel and in order to do this, it is supported by hydraulic thrust cylinders (2), installed on the last segment ring (3). The cutter head wheel (4) is fitted with hard rock cutting disks, which roll across the tunnel face, cutting the rock face into chips or excavating soil (muck).
Muck bucket lips (5), which are positioned behind the disks, carry the extracted rock behind the cutting wheel. The muck then falls onto a conveyer belt (6) and is carried out of the tunnel.
TBMs use spray water to allay dust and cool heat generated in the cutting discs by friction during face cutting. This may either be delivered in a separate water pipe or alternatively spray water can be tapped off from the chilled water pipes, which provide chilled water to the TBM for removal of motor heat, etc.
TBM drive heat loads
It is important to control the temperature and air conditions at all points in the tunnel, air duct and the face zone (referred to as the TBM zone) to protect operators and equipment. It is therefore necessary to take into account all possible sources of heat in the TBM drive. From previous work (von Glehn & Bluhm 1995) that included monitoring and simulation of TBM drives; it was evident that the heat loads could be divided into two groups; heat loads from the tunnel zone and heat loads from the TBM zone.
The following have to be taken into account when calculating the heat load in the tunnel zone:
- Heat flow from the surrounding rock (including moisture effects)
- Effect of chilled water and spray water pipes in the tunnel (including condensation)
- Effect of diesel vehicles operating in the tunnel (including moisture)
- Effect of ground water and drain water
- Heat transfer between air in the tunnel and air in the duct
- Thermal effect of the fans in the tunnel
- Thermal effect of lights, cables, transformers, electric motors, etc.
- Heat transfer from the muck (broken rock and water mixture)
The following have to be taken into account when calculating the heat load in the TBM zone:
- Heat generated by the TBM (electric motors, hydraulic oil pumps, fans, etc.)
- Effect of machine cooling water
- Effect of spray water
- Effect of airflow through the TBM cutter head zone
- Effect of airflow through the dust scrubber
- Heat transfer from the muck to the air
- Heat flow from the surrounding rock in the TBM zone (including moisture effects)
- Thermal effect of additional fans in the TBM zone
- Thermal effect of diesel vehicles operating in the TBM zone (including moisture)
The heat generated by the TBM motors is the most substantial heat load in a TBM zone. All the electrical power used by the TBM (not the motor rated power), will essentially manifest itself as heat; either into the broken rock (muck), cooling water or directly into the air. The TBM rated motor power may include the cutter head drive motors, hydraulic pump motor, lube pump motor, conveyor drive motor, auxiliary fans in the TBM zone, scrubber fans, probe drill and backup hydraulics.
From previous work carried out (von Glehn & Bluhm 1995, 1997), it has been found that thermal storage takes place in the muck, surrounding rock and in the cutter head parts in the TBM face zone.
Due to the thermal capacity of the components, a damping effect results with a time-constant of approximately two hours. Measurements indicated that the maximum average heat from the TBM is between 40% and 55% of the rated power (hereunder referred to as the TBM Factor). This, however, will vary from case to case.
The heat generated by the TBM that is not removed by the cooling water is split up, with approximately 70% being transferred to the muck and 30% transferred directly into the air. Consequently, heat is transferred from the warm muck into the air in both the TBM zone and the tunnel zone.
Another significant thermal input into the air is the heat from the surrounding rock, especially in the TBM zone where the hot rock is newly exposed.
Due to the high-speed development of TBM drives, this heat flow must be taken into account in both the Tunnel zone and the TBM zone.
Thermal energy balance and cooling requirements
A typical energy balance for a TBM drive is shown in Figure 3. The TBM drive is divided into the Tunnel zone and the TBM zone.
Figure 3. Energy Balance in the TBM Drive
In the Tunnel zone, cool air enters the ventilation duct and is forced to the TBM zone. As the air flows through the duct, the air temperature will increase due to heat transfer from the warmer return air to the cooler intake air. The mechanism of heat transfer from the return air to the intake air is a combination of both convection and conduction and is a function of the duct material properties and the temperature difference.
Similarly, the return air will gain heat from sources such as surrounding rock, vehicles, broken rock on the conveyor, lights, open drains and chilled water pipes. All these heat sources and sinks influence the heat transfer between the return air and the intake air in the duct and thus the temperature of the air being delivered to the TBM zone. Calculating the temperature of the air delivered to the TBM zone is therefore an iterative process.
The most significant heat load in a hard rock Tunnel zone is typically from surrounding rock. The estimation of the heat transfer from the surrounding rock, for rock surfaces that have been exposed for long times, is well documented (Goch & Patterson 1940, Jaeger & Chamalaun 1966, Hemp 1985). Due to the complexity of the fundamental equations, calculation methods rely on interpolated approximations.
For TBM drives with newly exposed rock, approximations to the Jaeger and Chamalaun tables (Jaeger & Chamalaun 1966) were used as they are valid for newly exposed excavations.
In the TBM zone, the heat generated in the TBM cutter head is the most significant heat load and is electrical energy converted to friction heat Factors that affect heat flow in the TBM zone are spray water flow, spray water temperature, TBM advance rate and Virgin Rock Temperature (VRT).
The enthalpy of the air in the TBM zone and Tunnel zone is then used in conjunction with psychrometric properties to calculate the air condition. By taking into account all heat sources, an energy balance is carried out to calculate required air cooling.
Ventilation system and integration
Figure 4 shows the typical secondary ventilation of a TBM drive.
Figure 4: Typical Ventilation of a TBM Drive
The secondary ventilation design for a TBM drive must first consider the type of ventilation system to be implemented (force or exhaust). Typically a forced ventilation system would be the option of choice; as this allows for more flexibility when considering the type of ventilation duct and duct installation.
Some of the advantages of a forcing system that are applicable to conventional mining as well as TBM mining are:
- Good quality air is delivered to the face at a high velocity
- The fan and motor are always in fresh air
- Leakage is from the column and is thus easily detected
The only disadvantage of the forced system in TBM mining is:
- People travelling and working in the TBM drive do so in return air
Two types of duct can be considered: steel ducting and flexible ducting. In general, flexible ducting has a lower friction factor (k-factor) than steel ducting and because of long tunnel lengths achieved with TBMs and significant air flow requirements this becomes one of the major considerations. The k-factor for a steel duct is typically in the order of 0.004 Ns 2/m4 and for flexible ducting it is typically less than 0.002 Ns 2/m4. Flexible ducting is light and is typically supplied in 100 m lengths, utilizing a cassette fitted to the TBM feeding the ducting and a suspension wire as the TBM moves forward. The 100 m lengths are then coupled together with methods such as zippers, Velcro or interlocking rings and clamps, which in turn reduce labour intensity. Although flexible ducts are not as strong as steel ducts, a typical 1 000 mm diameter flexible duct, with a safety factor of 10 can withstand a positive pressure of in excess of 10 kPa.
The leakage factor is specified as a function of the pressure, with most flexible duct suppliers specifying a leakage factor lower than 0.055 m3/s per 100 m at 1 kPa. However, the most important issue with leakage in actual mining situations is the maintenance of the duct material and proper installation of couplings. Taking this into account, it would be safe to assume a maximum leakage factor of 0.05 m 3/s per 100 m per 1 kPa.
Long tunnel lengths also have a major effect on the fan selection and heat loads. Firstly, the longer the tunnel (or ventilation duct) the more pressure is required from the fan to deliver air to the face (friction).
The higher the pressure and the longer the duct; the more leakage there is from the duct. In order to deliver the required quantity to the face, a high pressure fan station is required, which could consist of a number of axial flow fans in series or the utilisation of a single centrifugal fan.
To reduce the pressure load on the ventilation fans, larger ducts sizes can be installed. Flexible ducting is also available in flat “oval” shapes, which yields larger cross-sectional areas with better utilisation of the available space as shown in Figure 5.
Figure 5: Flexible Ducting with an Oval Shape
Air cooling might be required in the tunnel zone as well as in the TBM zone if sufficient cooling cannot be provided by the ventilation air alone. If air coolers are to be installed in-line with the ventilation ducting, provision must be made for cooling coil/car cubbies
When air coolers are considered, the pressure drop over these coolers has to be taken into account when the fan selection is carried out. The air pressure drop across these coolers can vary between 1 000 Pa and 2 000 Pa.
Through-ventilation distance is another issue that has to be considered. At a certain face advance distance, it becomes impractical to ventilate the face with a forced system alone and connections between TBM drives must then be made to keep the throughventilation distances economically viable.
From previous studies conducted by AGA, (Beukes & Labuschagne 2013), it has been concluded that the amount of ventilating air required for TBM mining is higher than for conventional methods.
This is mainly due to more heat being generated in the face zone by the TBM thus the implementation of TBM mining requires non-conventional ventilation and cooling methods.
Occupational hygiene and health and safety
Where personnel have to operate the TBM and work in the Tunnel zone, wet-bulb temperatures have to be kept below a certain design limit, typically below 29°C.
The next major pollutant is dust. This is managed at the TBM zone via a scrubber system integrated into the TBM. Dust will also be liberated from the conveyor belt. This dust is managed by controlling the air velocity over the conveyor and by wetting the rock. It might be required in long drives to re-wet the air as the rock dries out. Dust masks can also be used where dust is a problem.
The next pollutant to deal with is noise and this is firstly managed by addressing it at source and this is done by TBM manufacturers to client specifications. In addition, hearing protection must be used by personnel. TBM drives in hard rock mines introduce a risk of fire due to the presence of electricity, oil, diesel and conveyor belts and personnel should be provided with self-contained self-rescuers. In addition an early warning system should be installed and refuge bays need to be provided as per legal requirements.
Implementing TBM technology in hard rock mining is challenging from a ventilation and cooling perspective and requires non-conventional ventilation and cooling methods.
The amount of ventilating air required for TBM mining is higher than for conventional methods and requires high pressure fans and duct systems. Heat load calculation methods exist to predict TBM zone and Tunnel zone air conditions and allow design of appropriate ventilation and cooling systems.
Due to the typically high airflow and cooling requirements for TBM mining integration with existing mining systems needs to be carefully considered where TBM mining is implemented in an existing conventional mine.
W. Marx & J. Viljoen BBE Consulting D.O. Del Castillo Hatch
Beukes, M.G. and Labuschagne, J.A. The effect of automated mining on the occupational environment, Proceedings MVS 2013 Annual Conference, Johannesburg, February 2013.
Cigla, M., Yagis, S. and Ozdemir, L. Application of Tunnel Boring Machines in Underground Mine Development. Excavation Engineering and Earth Mechanics Institute, Department of Mining Engineering, Colorado School of Mines, Golden, Colorado, USA, 2001
Allum, R. and VanDerPas, E. TBM Technology in a Deep Underground Copper Mine. Rapid Excavation and Tunnelling Conference: pp. 129-143, 1995.
Stewart, P., Ramezanzadeh, A. and Knights, P. Benchmark Drill and Blast and Mechanical Excavation Advance Rates for Underground Hard-Rock Mining Development, Australian Mining Technology Conference, 26 – 27 September AIMM, Melbourne, 2006.
Nord, G. TBM versus drill and blast, the choice of tunnelling method. International Conference and Exhibition on Tunnelling and Trenchless Technology, 2006.
von Glehn, F.H. and Bluhm, S.J. Ventilation and cooling of TBM Drives in high temperature conditions. Journal of the Mine Ventilation Society of South Africa, vol. 48, no. 5. pp. 146-158, 1995.
von Glehn, F.H. and Bluhm, S.J. Heat generation and climatic control in the operation of tunnel boring machines. Proceedings of the Sixth International Mine Ventilation Congress, Pittsburgh, PA, SME, 1997.
Goch, D.C. and Patterson, H.S. The heat flow into tunnels. Journals of the Chemical, Metallurgical and Mining Society of South Africa, vol. 41, 1940.
Jaeger, J.C. and Chamalaun, T. Heat flow in an infinite region bounded internally by circular cylinder with forced convection at the surface. Australian Journal of Physics, vol. 19, 1966.
Hemp, R. Air temperature increases in airways. Journal of the Mine Ventilation Society of South Africa, vol. 38, nos. 1 & 2, 1985.