Advantages and risks of the application of TBMs for mining projects

Despite the advent of tunnel boring machines (TBMs) more than 70 years ago in the early 1950s, their use for mining infrastructure has been limited to a few access, exploration, drainage and water supply tunnels (Brox, 2013). TBMs have successfully been used for the construction of the majority of tunnels generally longer than 4 km (2.5 miles) for civil, hydropower and oil and gas infrastructure projects with more than 50,000 km (31,000 miles) of tunnels, and more than 10,000 TBMs have been manufactured (Zheng et al., 2014). TBMs offer many advantages over conventional excavation methods including improved safety and working environment, reduced ground support, reduced overbreak, higher advance rate and overall lower cost. Today, TBM suppliers can more easily design TBMs for the anticipated geotechnical conditions and to specifically design for any identified adverse conditions to derisk the TBM from possible entrapment. Figure 1 TBM assembly/launch cavern inside an underground mine. TBMs have successfully completed infrastructure projects, and they continue to be used for the construction of the majority of infrastructure tunnels given the recognized benefits of lower total project costs and shorter duration (earlier completion) of the construction schedule. Given that time is money for most infrastructure projects, especially those involving the generation of revenue upon completion as for energy, water supply and traffic, the importance of the successful application of low-risk, rapid tunnel construction technologies cannot be overemphasized. In particular, the hydropower industry has reaped and continues to reap the benefits of the application of TBMs in challenging mountainous regions including those of the Andes and Himalayas for the early completion of long and deep tunnels as part of major revenue-generating energy projects for nearly 1,000 km (621 miles) of tunnels. A secondary benefit that has been recognized with the use of TBMs for these projects has been where limited access and/ or environmental restrictions may preclude the use of multiple intermediate access adits, thus significantly reducing the construction impact footprint of projects on local communities and environmental resources, including ground water. The international mining industry is developing an increasing number of ore reserves that are located in ever more remote project sites where long and sometimes deep access tunnels are required. Accordingly, the application of TBMs is being increasingly considered. However, there appears to be a reluctance among many mining developers to fully embrace the use of TBMs for the following key reasons: Limited historical use of TBMs for mining projects (limited track record). A small number of unsuccessful uses where TBMs should not have been trialed due to unidentified adverse conditions. Misconception or misunderstanding that TBMs are prone to be entrapped in difficult ground conditions without mitigation solutions to limit overall delays. Undesirable curved invert/floor for mining operations. Overall, there appears to be an ongoing reluctance and negative perception by mining professionals to accept the use of TBMs for mining projects. This continues even where comprehensive technical assessments have concluded TBMs to be of low risk. Notwithstanding that, more than 95 percent of tunnels more than 4 km (2.5 miles) in length have been successfully constructed for civil and hydropower infrastructure and continue to be planned to be constructed using TBMs. Figure 1 presents the TBM assembly and launch cavern used for the 11-km (6.8-mile) TBM-excavated water supply tunnel at the El Teniente Mine in Chile. Are mining projects unique for TBM use? A commonly recognized perception in the international tunneling industry is that mining projects are considered to be unique in comparison to other major infrastructure projects as a reason for not using TBMs. However, this perception is actually a misconception for the following reasons: Mining projects are no more time and cost sensitive than other major infrastructure projects, especially those that are revenue generating such as hydropower projects, where early completion can mean improved economics and earlier-than-expected payoff and profit-making of the project. While it is recognized that most mining projects are located in extremely remote locations, including challenging mountainous terrain locations for access, numerous major tunnels have been constructed using TBMs in similarly remote and high-elevation locations, including the Himalayas, Caucasus and Andes Mountains. While mining projects require the early procurement of major equipment for operations (such as large shovels, haul trucks and crushers) due to long lead times, the significant costs associated with this early procurement is not more onerous than those for the early procurement of a TBM to take advantage of an early start of a major tunnel. While most tunnels for a mining project are desired to have a flat floor for traffic during operations and/or for the installation of key equipment (conveyor), the curved invert/floor excavated by a TBM can be easily prepared to meet these requirements by either the backfilling of tunnel spoil as a single one-pass operation behind a TBM as it advances or by the concurrent placement of a precast concrete segmental floor (Fig. 2) as the TBM advances to provide an immediate operating floor after breakthrough of the tunnel. Figure 2 Precast concrete floor decking for a TBM tunnel. One aspect where mining projects for major tunnels may in fact be recognized as unique is where there exists adverse geotechnical conditions as a result of major alteration of the prevailing bedrock conditions as well as additional disturbance of the bedrock with additional geological faults. Alteration of the host or site bedrock is quite common in the vicinity of major orebodies and alteration is typically associated with reduced rock strength and quality as well as acidic ground water. Similarly, the presence of additional geological faults associated with major orebodies is also typically associated with reduced rock strength and quality. Accordingly, the alignment of any proposed tunnel in close proximity to a major orebody should be carefully planned and designed to avoid the presence of any associated adverse geotechnical conditions as much as possible. Brief history, types and applications of TBMs History of TBMs. The first use of a fully mechanized TBM was for the construction of a tunnel in lowstrength bedrock at the Oahe Dam project in South Dakota in 1952. The success of the construction of this tunnel resulted in the manufacturing of additional TBMs for the construction of long tunnels in stronger bedrock. The early TBMs developed up to the 1980s were of limited power capacity, which resulted in their removals from some projects with extremely strong and abrasive quartzite rock such as in the Himalayas. A key technological advancement with TBMs is that the power capacity and ability to excavate extremely strong and abrasive rock has greatly increased in the last 20 years. This includes the use of larger-sized cutting discs measuring 483 and 508 mm (19 and 20 in.) that allows for increased thrust, resulting in improved rates of advance. TBMs have been used on about 75 mining projects and 220 km (136 miles) of tunnels to date (Brox, 2014; Zheng et al., 2014), including the completion of dozens of kilometers of tunnels in coal mines in Germany in the 1960s to 1970s. Other early uses include trials in extremely strong and abrasive rock in the deep gold mines in South Africa that were unsuccessful due to the limited power capacity of the TBMs for these extreme conditions. The longest continued and successful use of TBMs for mining applications has been at the Stillwater Mine in Montana, since the 1980s. Types of TBMs and most suitable applications. There are essentially three main types of TBMs used for the construction of tunnels in rock: (1) open or gripper, (2) single shield and (3) double shield (Fig. 3). Figure 3 Main types of TBMs for mining. Open/gripper TBMs have been fabricated up to nearly 15 m (50 ft) in diameter and include a fingershield extending behind the cutterhead over the forward part of the TBM that provides limited protection of workers but allows for the installation of rock support as soon as the rock becomes exposed before the position of the grippers. These TBMs are most suited to homogeneous and goodquality rock conditions and where a limited number of geological faults are expected. They allow for the easy completion of probe drilling behind the cutterhead that may impact the advancement of the TBM. Open/gripper TBMs are ideally suited for proposed tunnels in rock that has been characterized with a significant portion of goodquality rock conditions and limited amounts of geological faults. In comparison, double-shield TBMs include a three-component structural shield around the TBM that typically extends up to three times the diameter back from the cutterhead as well as grippers and rear thrusters. The multicomponent shield allows for the advancement of the forward shield of the TBM while the rear shield remains fixed for gripping, and is capable of achieving high rates of advance in challenging and varying rock conditions. The distinct advantage of double-shield TBMs is the flexibility to install a variety of tunnel support including standard rock support consisting of rock bolts, mesh and shotcrete as well as precast concrete segments or steel ribs. The TBMs’ effective thrust and overall advance may be significantly impacted by squeezing or high-deformation rock conditions that may cause entrapment of the TBM shield. The use of a double-shield TBM for tunnel excavation through major geological fault zones should therefore be thoroughly evaluated with strong consideration of the implementation of mitigation measures prior to any such excavation. Under such difficult tunneling conditions, double-shield TBMs can, however, utilize the rear thrusters to push off of precast segments or specially designed steel ribs installed as part of the rock support that is typically designed for such conditions. Double-shield TBMs allow for probe drilling through the rear shield with no impact on TBM advancement. Double-shield TBMs are ideally suited for proposed tunnels in rock that had been characterized with a significant portion of fair-quality rock conditions with substantial amounts of geological faults or potentially unstable rock conditions. Single-shield TBMs are used much less often for rock tunnels due to their actual lower advance rates in comparison to double-shield TBMs. Single-shield TBMs have typically been used in conjunction with precast concrete segmental lining for the construction of water conveyance tunnels in nondurable rock conditions. The main advantage of singleshield TBMs for such tunnels is the shorter construction schedule in comparison to the use of an open/gripper TBM followed by a cast-inplace concrete lining. The distinct disadvantage of single-shield TBMs for rock tunnels is that they are limited to thrusting off of the segmental lining only, as they do not include grippers. As such, single-shield TBMs are more difficult to get freed when entrapped within a geological fault zone. The evaluation of the most suitable type of TBM for any particular project should primarily be based on a sound understanding of the anticipated total distribution of the geotechnical conditions and, in particular, the quantity of adverse geotechnical conditions. Brox (2021) presents a simple graphic to assist in the technical evaluation of the most suitable type of TBM for any project (Fig. 4) for which the quantity of adverse geotechnical conditions should not be more than 30 percent. As included in Fig. 4, TBMs can also be used for early access to underground mining reserves overlain by saturated overburden with the use of face-pressurized TBMs with the concurrent installation of precast concrete segmental linings as per the typical construction approach for metro tunnels in urban areas. For these applications, TBM excavation should continue until “socketed” into low-permeability bedrock with no further elevated ground-water pressures and then should also be considered for the use of predevelopment, such as a ring tunnel configuration around the mining reserves to provide multiple locations for further development of the mining layout by drill-andblast methods. Figure 4 Technical evaluation of suitable types of TBM. Key advantages of TBMs Improved safety and environment for workers. Each of the main types of TBMs provide different levels of safety for workers subject to the exposed geotechnical conditions during tunnel construction. For open/gripper TBMs, the rock conditions become exposed immediately behind the TBM cutterhead and fingershield, thereby requiring the rapid installation of ground support so workers do not become exposed to any unsupported ground. In comparison, for shielded TBMs, the workers are fully protected inside the shield of the TBM, which prevents any loosened rock from falling into the working area but requires the installation of ground support or precast concrete segmental lining at the end of the shield to stabilize the exposed ground. Environmental conditions for workers can be greatly improved with the use of TBMs because blasting, with its associated fumes, is not required, though adequate and often enhanced ventilation is required for TBMs as their operation results in elevated temperatures at the advancing tunnel face. If gaseous conditions are or may be present, a TBM can be operated in a “closed mode,” whereby the face area is entirely contained within a front chamber of the TBM such that workers will not be exposed to gases such as methane, or the adverse environmental conditions of asbestos. Reduced excavation overbreak. The limited vibrations associated with TBM excavation prevents damage to and the loosening and dislodgement of potentially unstable rock from a tunnel profile, thereby maintaining stability and preventing overbreak, thus reducing the total volume of excavated material for transport and disposal. The excavation of tunnels by TBMs to the theoretical diameter without any fallout of ground can actually provide a false sense of stability, and all exposed ground should be supported to minimum standards for safety. Reduced ground support. TBMs cause vibrations but do not impart any form of damage to the surrounding rock other than the redistribution of stresses, and therefore minimizes but does not prevent instabilities from occurring as rock conditions are exposed and prior to the installation of support to maintain stability. Accordingly, ground support quantities in terms of rock bolts, mesh and shotcrete can be reduced as much as 30 percent compared to drill-and-blast excavation. Higher advance rates. The greatest advantage of TBMs is the ability in fair- to good-quality ground to achieve and sustain high rates of advance that are a minimum of three to four times that of drill-and-blast excavation and a minimum of two times that for excavation using roadheaders, which are restricted for long tunnels due to enhanced ventilation requirements. Figure 5 presents typical TBM advance rates in relation to diameter for various projects, including a limited number of historical mining projects. Maximum daily TBM advance rates in excess of 60 m (200 ft) have been achieved in good-quality rock conditions with an open/gripper TBM with a large diameter (9 m, or 30 ft), operating at great depth, and maximum monthly TBM advance rates in excess of 1,200 m (4,000 ft) have been achieved for moderate diameters (6 m, or 20 ft). Lower linear and total tunnel costs. Given the reduced overbreak and ground support requirements along with the higher rates of advance associated with TBMs with less labor, the higher costs of a new or previously used TBM are appreciably offset, resulting in much lower linear cost as well as total tunnel cost by as much as 25 percent subject to consideration of all cost factors. Reduced environmental footprint. TBMs are normally used for the construction of a long tunnel from portal to portal with the majority of construction activities at a single portal site. Accordingly, the use of TBMs removes the need for intermediate access adits as is typically required for drill-and-blast methods, which are associated with greater impacts at the project site with multiple access roads, camps and ongoing supplies during construction, particularly for projects located in close proximity to communities or existing important infrastructure. Figure 5 TBM advance rates in relation to diameter. Reduced energy impact/footprint. TBMs require an appreciable amount of energy for construction, including more than 10 MW for TBMs with diameters greater than 10 m (32 ft). While TBMs can be operated with diesel generators, there are major cost savings associated with the use of local grid power. Even at remote project sites, it is often planned to install a local substation as part of any early works advanced contract to take advantage of the lower total costs of energy and environmental savings of the use of high volumes of diesel along with the requirements for the supply and transport of diesel to the project site for the total duration of construction. Possible reuse of tunnel spoil. Unlike the spoil that is generated from drill-and-blast methods, which is contaminated with blasting residues, TBM spoil is relatively clean, assuming it is transported directly from the advancing face via an initial conveyor system to either the railcars typically used for small-diameter tunnels or to a larger main conveyor for largerdiameter tunnels and does not come into contact with hydraulic oils and other contaminated products or chemicals used for TBM tunneling. Accordingly, TBM spoil, with its useful gradation of size of chips, can be used for a variety of purposes within a project site, including the backfilling of areas and as roadbase material, or provided often without charge to the local communities that can often find uses for it. Key disadvantages of TBMs The key disadvantages associated with the use of TBMs are as follows: Long procurement time, minimum of 12 months. Possible access challenges for mobilization. Minimum large alignment radii dependent on diameter. Circular or curved invert/floor. Failure of main components including main bearing. Key skilled labor required. High power demand (offset by earlier completion). Deferred overstress behind tunnel face (TBM backup). Not applicable for ancillary excavations. Limited flexibility for unexpected adverse conditions. These disadvantages are not typically applicable to every project, and the risks associated with them can usually be reasonably managed to prevent major delays. In particular, and as with major mining projects, many large infrastructure projects require the early procurement of major equipment of significantly higher costs versus that of a TBM. For remote projects, it is prudent to require spare parts of all major components to be available near the project site in order to limit appreciable delays. With the increasing global demand for the use of TBMs, there is global demand for the required skilled labor, and project clients must be mindful to accept costs that can afford to attract toplevel labor (A-Team) in order to complete their projects in a timely manner. Finally, one of the main disadvantages that is often misunderstood is that TBMs do not have acceptable flexibility to advance through unexpected conditions. It should, however, be noted that TBMs can be required to include special de-risking measures and components such as grouting ports in the TBM cutterhead or shield to facilitate advancing through such adverse conditions. In addition, a commonly adopted and successful de-risking solution for the entrapment of TBMs is the construction of small bypass tunnels around the TBM to remove any unstable materials. Many bypass tunnels have been successfully constructed on several projects to limit delays to less than one week before resuming TBM excavation. Table 1 Statistics and entrapment rates of TBM manufacturers. Risk of TBM entrapment and industry statistics TBM entrapment risk. TBMs can and have become trapped upon the intersection of unexpected adverse geotechnical conditions, typically consisting of large geological faults that are often associated with weak/soft claylike conditions that result in squeezing upon intersection during TBM excavation. In some cases, highly fractured conditions may be present that result in the collapse of large volumes onto a TBM, causing entrapment. The delays caused by such entrapments can vary from a few hours to multiple months, depending on the nature of the geotechnical conditions and the manner in which the TBM became entrapped. The typical reason why unexpected geological faults or other adverse geotechnical conditions may be intersected during the construction of a long tunnel with a TBM is that only limited preconstruction geotechnical investigations were performed and/or limited or inadequate interpretation and analysis of the geotechnical information were undertaken, which may be due to limited available time and/or project budget constraints or due to limited experience of the project designers. Industry statistics for TBM entrapments. The occurrence of TBM entrapment in rock tunnels has been evaluated based on what is believed to be the first-ever survey of this critical information from the majority of TBM manufacturers in the industry as of 2024. Table 1 presents the statistical data provided by the majority of the international TBM manufacturers in terms of numbers of TBMs manufactured, their use in terms of total tunnel length, and the total number of entrapments. The majority of TBMs were manufactured by a limited number of companies, and therefore these TBMs have been used the most in the industry, resulting in a greater number of total entrapments. However, the important aspect and metric is that the rate of entrapment, defined in terms of the total number of entrapments per kilometer of tunnel length, is less than 1 percent, which represents an extremely low rate of failure when considering other sophisticated machinery and equipment including those used in the mining industry. From this information, it can be postulated that the probability of TBM entrapment for any given long tunnel project, assuming limited adverse geotechnical conditions and that the application of a TBM is considered to be viable, can be expected to be extremely low. The low probability of entrapment is, however, not applicable for those projects with long tunnels where there is a large proportion of adverse geotechnical conditions and therefore where a TBM should not be considered to be viable, as discussed earlier. Key requirements and mitigation solutions Technical design features – Entrapment de-risking. The manufacturing of TBMs has advanced appreciably over the past 30 years to include mitigation solutions as key design features to avoid and reduce the delays associated with TBM entrapment, which can be required to be included for any project by way of technical specifications. In particular, it has been recognized that double-shield TBMs, once thought of as being associated with higher risk of entrapment versus single-shield or gripper TBMs, offer a better ability for the de-risking of entrapments. The following risk mitigation measures can be incorporated into the design of a TBM for the de-risking of TBM entrapment (Grandori, 2016): Conical (tapered/stepped) shield designs. Retractable shield capability. Enhanced steering control for mixedface conditions. Extreme overcut capacity (typically 50 cm on diameter). High thrust, power and torque (similar to that for earth-pressure-balance (EPB) TBMs). Enhanced ground treatment capability including multiple ports through the cutterhead, frontshield, tailshield and bulkhead for probe drilling, drainage and grouting. TBM spoil control including flood doors to regulate the flow of spoil and water to the conveyor, and to facilitate similar/ quasi-EPB closed mode of operation to limit face instability and inrushes/ flowing ground. All of the above de-risking measures can be easily equipped and included in the design of a TBM at the start of a project and the costs associated with these special measures, while not appreciable, still far outweigh the risks associated with the delays of a TBM being entrapped. Geotechnical investigations during construction. The most important risk mitigation solution to attempt to avoid and/or prevent the entrapment of a TBM is to regularly perform forward investigations ahead of an advancing TBM, consisting of the drilling of long probe holes to detect water and weak/ fractured ground. They can be easily managed and performed during maintenance without disruption to normal production. In addition, the TBM tunneling industry has had available a geophysical technique referred to as tunnel seismic prediction, or TSP (Hecht-Mendez and Dickmann, 2016) to explore ahead of an advancing TBM to detect zones of low seismic velocity that typically represent weak and/ or fracture zones. The application of probe drilling in conjunction with forward geophysical techniques has slowly become standard international good practice for risk management for the construction of long tunnels using TBMs. TBM use for major infrastructure projects Since the early 1990s, there has been an increased use of TBMs and the successful completion of long tunnels as part of the construction of major civil and hydropower projects worldwide. The civil and hydropower industries have recognized the application and have successfully managed the risks associated with TBMs for their projects. In fact, the majority of long tunnels (longer than 4 km, or 2.5 miles) sited in bedrock, as well as the majority of short tunnels (shorter than 4 km, or 2.5 miles) sited in overburden, continue to be constructed using TBMs as the lowestrisk approach of construction, especially for tunnels in urban areas where the disturbance of overlying existing infrastructure must be prevented or limited. For hydropower tunnels, there is an increasing use of precast concrete segmental linings as a low-risk and economic construction approach for long tunnels (Brox and Grandori, 2023) with nearly 1,000 km (621 miles) of tunnels constructed using this approach. In particular, TBMs have been successfully used, and continue to be used, for some of the largest infrastructure projects with challenging geotechnical conditions including: Gotthard Base Rail Tunnel — 52-km (32-mile) deep tunnel. Kargi Hydropower Tunnel — multiple fault zones requiring bypasses that were faster than drill-and-blast methods. Olmos Water Supply Tunnel — deep tunnel subjected to regular rockbursting. Nepal Hydropower Tunnels — through major geological faults. Lake Mead Water Supply Tunnel — high ground-water pressures. India Water Supply Tunnels — abrasive quartzite. Brenner Base Rail Tunnel — 64-km (40- mile) deep tunnel. TBM use in recent and current mining projects The most recent uses of TBMs in the last decade for mining projects include for 7-km (4.3-mile) access tunnels at the Stillwater Mine, an 8-km (5-mile) exploration tunnel in Chile, and twin 1-km (0.6-mile) decline access tunnels in Australia. The most noted current use of a TBM for a mining project is for the construction of a 36-km (22-mile) conveyor tunnel as part of the Woodsmith polyhalite mine in the northeast of the United Kingdom. A single-shield TBM with diameter of 4.5 m (15 ft) is being used in conjunction with precast concrete segments. TBM tunneling commenced in mid-2019, and a total of 18,000 m (59,000 ft) were completed from a single portal in 1,000 days to present a world record for sustained production. The best daily production was 53 m (173 ft), and the best monthly production was 1,020 m (3,340 ft). The conveyor tunnel is being excavated through the Redcar mudstone formation that includes thin layers of chert where ground stability has been reported as good with no major delays at the tunnel face. As of October 2023, tunnel advance reached 26 km (16 miles) following a four-month stoppage where appreciable maintenance was completed on the TBM to prepare it to complete the final 18 km (11 miles). At the time of writing, a double-shield TBM with diameter of 8 m (26 ft) was about to be launched for the construction of a 10.5-km (6.5-mile) decline access tunnel at the Xianshan iron ore mine in China (Fig. 6). Over the past decade and most recently, the use of TBMs has been increasingly considered as part of the construction of major access and drainage tunnels for the following mining projects: Soto Norte, Colombia. Quebradona, Colombia. Olympiad, Russia. Tintaya Antapaccay, Peru. Escondida, Chile. Hu’u Onto, Indonesia. Figure 6 TBM launching for mine decline tunnel in China. Reasons for TBMs not being used in key projects There exist a few major long tunnels where TBMs were not used, and it is interesting to recognize the key reasons for this. TBMs were not used for the following key major tunnel projects: Lotschberg Base Rail Tunnel — 2 × 32 km (20 miles). El Teniente New Mine Level Access Tunnels — 2 × 9 km (5.6 miles). Ceneri Base Rail Tunnels — 2 × 15 km (10 miles). NEOM Line Project — 2 × 33 km (20.5 miles). The postulated reason why TBMs were not used at each of these major projects include abundant inferred adverse geotechnical conditions before the start of construction by the project designers. Squeezing conditions were anticipated at multiple locations for both the Lotschberg and Ceneri Base Rail tunnels located in Switzerland based on previous major tunnels in those areas; however, these adverse conditions were fortunately not realized during construction. For the new mine access tunnels at the El Teniente Mine in Chile, the most attractive offer for the project client from a total of seven design-build proposals was based on a drill-and-blast approach with multiple intermediate access adits that assumed no interference from the existing mining operations. The second and third most attractive bids were based on the use of a TBM. Unfortunately, the assumed production of 4 m/d at four working faces of the most attractive bid was never achieved during construction due to site interference and other reasons of design changes. The original bid schedule was four years; however, the twin tunnels are now finally nearly complete after 14 years, having faced major delays associated with significant overstress including regular rockbursting that slowed production to 0.5 m/d. The encountered conditions over the majority of these tunnels were considered to be conducive for the use of TBMs as recognized by many professionals involved during the construction of the project. For the NEOM line project, the tunnel designers did not have extensive TBM tunnel design experience even though the majority of the encountered rock conditions were of good quality. Key aspects for TBM applicability and success Technical viability. It is of vital importance to thoroughly investigate the geology of a project to identify the potential challenges and to study the applicability of a TBM as the TBM should be compatible with the anticipated ground conditions. Brox (2021) presents a criterion that less than 30 percent adverse geotechnical conditions — including geological faults, abrasive ground and extremely high ground-water pressures — should be anticipated for the acceptance of the use of a TBM. This criterion is fundamentally based on the fact that TBMs can achieve production rates of typically about a minimum of three times that of drill-and-blast excavations and therefore any delays due to geotechnical conditions should be limited to the proportion of the advance rates of the different methods. While TBMs are ideal for homogeneous ground conditions, it is recognized that special design features are available for the de-risking of entrapment, which are increasingly specified for many projects with adverse geotechnical conditions. In addition, high-powered TBMs were not previously available in the industry and were the main reason for some historical failures such as high quartzite content at the Parabati II hydropower project in India. The most important aspect to be evaluated is the technical viability, which should only be performed by an experienced professional without assuming an overly optimistic prediction of the geotechnical conditions. Figure 7 Flat-floor TBM tunnel trial at Fresnillo Mine, Mexico. Correct type of TBM. As there are multiple types of TBMs available in the industry, it is critical to evaluate each type for each project based on the best judgment and interpretation of the anticipated geotechnical conditions. The assessment of the most suitable type of TBM for a particular project should importantly consider historical practice and lessons learned from past projects to avoid being overly optimistic and running into similar shortcomings. Industry experience. There exist several internationally recognized TBM tunneling contractors that have a wealth of project experience to competently evaluate the most suitable type of TBM for a given project and execute the project with good quality and safety. In addition, there are several TBM manufacturers with competent technical professional staff with a wide spectrum of experience on how their TBMs have performed in various geotechnical conditions. Project clients should always carry out a prequalification process to confirm the experience of all contractors and manufacturers to be considered for bidding. Fair contracting practice and risk allocation. Project clients should recognize the importance of fair contracting practices with appropriate pricing items of lump sums for well-defined scopes of work and remeasured quantities for uncertain scopes of work such as ground support as well as fair risk allocation with the execution of a geotechnical baseline report (GBR) to provide compensation for more adverse conditions if encountered during construction that have been demonstrated to have impacted the productivity of the contractor. The practice of GBRs is rapidly increasing throughout the world and has become well established in many countries, with some tunneling contractors refusing to bid if the practice is not included. Accessibility for mobilization/support. The safe and timely mobilization of a TBM to a project site is fundamental in order to avoid any major delays to start of tunnel construction. Project clients should perform access road surveys and inspections of all key existing infrastructure, such as bridges, well prior to bidding during early design to confirm that there are no impediments for TBM mobilization. Access to any project site must also be maintained at all times during construction for the transport of important consumables used for TBM tunnel construction. Supplier support. The post-delivery support required to be offered and executed by a TBM manufacturer cannot be overemphasized. Such important aspects include the timely provision of consumables (TBM cutters) and spare parts, TBM maintenance as well as possible training, and the provision of a TBM operator for an initial trial period or length of the tunnel. While these services appear to be fully recognized, the realization of sound support services during construction is never guaranteed with an everincreasing tunneling industry that depends on the use of so many TBMs. Technological advances for mining projects At the time of writing, the Robbins MDM5000 flat-floor TBM mining machine is planned for a second trial at a depth of 700 m (2,297 ft) at the Fresnillo Mine in Mexico to excavate an approximately 2-km (1.2-mile) drive for exploration purposes at the mine. Figure 7 shows the flat-floor tunnel excavated from the first trial where an average rate of progress of 8 m/d was achieved in sedimentary bedrock of 60 to 90 MPa. The mine has performed some geotechnical investigations of long horizontal drillholes to allow for the characterization of the anticipated conditions in close proximity and subparallel to the silver-bearing vein deposits that include a limited number but highly adverse geological faults of crushed material with major ground-water inflows. In addition, Herrenknecht has successfully completed trials of a prototype small-diameter (3 m, or 10 ft)-shaft TBM for the vertical excavation of strong rock conditions at its manufacturing facility in Germany. Finally, Komatsu has developed a new mining TBM machine for site trials at the Chuquicamata underground mine in Chile to evaluate the performance of the machine to be able to excavate tight radii excavations as required for block cave mining methods and install early ground support. Each of these examples represents exciting technological advances for the mining industry that will hopefully become standard technology in the future for improved rates of excavation for tunnels and shafts. Conclusions and recommendations The following key conclusions and recommendations are presented regarding the applications of TBMs for mining projects: Mining projects have similar characteristics to other infrastructure projects in terms of time and cost sensitivity, remoteness of site, ability for early procurement of key equipment due to long lead times, and desire to have a flat tunnel floor for operations. There exist multiple types of TBMs, and the viability of a TBM for a given project must be evaluated in terms of the anticipated geotechnical conditions with no more than about 30 percent of adverse conditions, and previous experience in the project region. Key advantages for TBMs are improved safety and environment for workers, reduced overbreak and ground support, higher advance rates, lower linear and total costs, reduced environmental footprint, reduced energy consumption for grid connection, and possibly reuse of spoil. Key disadvantages for TBMs are long procurement times of minimally 12 months, possible access challenges for mobilization, minimum large alignment radii dependent on diameter, circular or curved invert/floor, failure of main components including main bearing, key skilled labor required, high power demand, not applicable for ancillary excavations, and limited flexibility for unexpected adverse conditions. A key risk with the use of TBM is entrapment; however, based on an industry survey of TBM manufacturers the rate of entrapment is less than 1 percent in terms of tunnel lengths. The new generation of TBMs can be and should be specified to include key mitigation solutions to prevent entrapment including conical shield designs, retractable shield capability, enhanced steering control for mixed face conditions, extreme overcut capacity, high thrust, power and torque, enhanced ground treatment capabilities, and TBM spoil control. The best mitigation against entrapment is the performance of proactive investigations during TBM tunneling including forward probe drilling and geophysical survey techniques. The key aspects for TBM applicability and success are a comprehensive technical viability assessment, the correct selection of type of TBM, industry experience of TBM manufacturers and TBM contractors, fair contracting practice and risk allocation, accessibility for timely mobilization, and ongoing support from the TBM supplier during construction. Technological advances continue to be made for the use of TBMs in increasingly challenging site conditions. References Zheng, Y., Gong, Q., Li, L., and Zhao, J. 2014. A Review of the Applications of Tunnel Boring Machines in Mines. 15th Australasian Tunneling Conference, Sydney. Grandori, R. 2016. DSU TBM for Vishnugad Pipalkoti Project – TBM Design Development for Large Diameter Rock Tunnels Under the high Covers of the Himalaya, World Tunnel Congress, San Francisco Brox, D. 2013. Technical Considerations for TBM Tunneling for Mining Projects. Society of Mining Transactions. Zheng, Y., Zhang, Q.B., and Zhao, J. 2016. Challenges and opportunities of using tunnel boring machines in mining, Tunneling and Underground Space Technology 57 (287-299). Foulstone, G. 2017. Experiences and Challenges for the use of TBM Technology in Mining Operations, Linkedin Article. Hecht-Mendez, J., and Dickmann, T. 2016. 3D-TSP – Advanced geological prediction during tunneling projects in the Andes. 15th Columbian Geotechnical Congress and 2nd International Specialized Conference on Soft Rocks, Cartagena. Brox, D., and Grandori, R. 2023. Pre-Cast Linings for Hydropower Tunnels: Advantages and Successes. World Tunnel Congress, Athens, Greece

Advantages and risks of the application of TBMs for mining projects

Authored by:
Dean Brox

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