ABSTRACT:
Tunnel boring machines (TBMs) have been used for the construction of long tunnels for more than 70 years and their use has included a number of long tunnels for hydropower and other civil infrastructure projects.
Most of the major mountain ranges – including the Alps, Andes, Caucasus, Himalayas, and Rockies, where several hydropower projects have been built and continue to be planned – are associated with high overburden and challenging geotechnical conditions for the construction of long tunnels. The historical use of TBMs in the Himalayas was hampered with challenges, some due to specific aspects not directly related to geotechnical risks but rather the inappropriate type of TBM for the prevailing geological conditions.
Two TBM-driven tunnels for hydropower/multipurpose water projects have been recently completely successfully, and ahead of schedule, in Nepal, with progress rates of more than 20m/day; this has spurned renewed interest in their use for future projects. Two TBMs also successfully completed 20km of the central section of the 28.5km-long headrace tunnel – also the deepest tunnel in the Himalayas – at the Neelum Jhelum project, in Pakistan. TBMs are also in current use on two major hydro projects in India that have indicated positive progress to date with up to 43m of advance in a single day.
Past, recent, and present projects for TBMs for hydro projects in the Himalayas are presented and discussed, along with the key risks and logistical challenges, and the technical evaluations necessary in early studies.
1 INTRODUCTION
TBMs have been used in construction of long tunnels, including for hydropower and civil infrastructure projects, respectively, for decades. In most major mountain ranges these long tunnel projects have high overburden and are excavated in challenging geotechnical conditions.
Goel (2014, 2016) presented several challenges and lessons learned associated with the first series of TBMs that were attempted to be used in the Himalayas from the late 1980’s to the late 2000’s. Given that there have been some very positive results within the past decade and in particular, most recently with some ongoing projects, it is warranted to document a fresh perspective for the use and applicability of TBMs in the Himalayas.
With improved technical evaluations and risk management practices having provided success, the tendency in the past to shy away from application of TBMs in major mountains due to actual problems encountered, and/or perceived risks, should now be challenged.
2 KEY RISKS AND CHALLENGES FOR TBMS IN THE HIMALAYAS
2.1 Geological Conditions:
Faults/Abrasivity/Inflows
The Himalayas are the most geologically complex mountain range in the world where hydrostatic upward/ uplift movement continues due to the ongoing collision of the Indian and Eurasian tectonic plates, which has resulted in some areas of highly disturbed geological formations.
These mountains can be described with general geological regions of Sub-Himalayas, Lesser, and Higher/Greater Himalayas that are separated by the major regional thrust faults of the Main Boundary Thrust (MBT) and Main Central Thrust (MCT).
The Sub-Himalayas and Lesser Himalaya regions are generally associated with sedimentary rock formations with limited disturbance; the Higher/Greater Himalaya region is generally associated with crystalline and metamorphic bedrock (granites, schists, phyllites, quartzites) with extensive disturbance with folding/ tilting of the main rock formations, extensive faulting and fracture zones and high abrasivity. Undisturbed zones also exist within the Higher/Greater Himalaya. There exist prominent bedding of the metamorphic rock units often representing zones of disturbance as geological faults and significant fracture zones (see Figure 1).
While the various rock formations can be present in massive zones of great thicknesses, thin low strength zones of schists and phyllites can be present throughout the Higher/Greater Himalaya. With the prominent bedding of the various rock formation there is the presence of frequent geological faults and fracture zones, which can vary in thickness by up tens of metres and are commonly associated with very large groundwater inflows/inrushes with fines. Mauriya et al. (2010) present a useful discussion on the challenges and strategies for tunnelling projects in the Himalayas based on commonly recognised risks.
2.2 Geotechnical Conditions:
In-Situ Stresses
The Himalayas are the highest mountain range in the world and thus there exists the highest overburden with high in-situ stresses. Hydropower tunnel alignments in the Himalayas, therefore, may be subjected to elevated in-situ stresses resulting in overstressing, including rockbursting as well as squeezing where weak zones are present since they are either sub-parallel to valleys or pass below major mountain ridges with deep cover. Given the varied geological conditions, it can be expected that the in-situ stresses will vary significantly and that site-specific stress testing should be performed.
In-situ stress testing has been completed for many hydro projects, comprising both hydraulic fracturing and overcoring, typically in the areas of the pressure shafts, powerhouses, and valley crossings as presented by Kumar et al. (2004). Low in-situ stresses may also be apparent below low topographic ridges, which may influence the stability of large span powerhouses as experienced in Bhutan (Dorji et al., 2024).
Swannell et al. (2016) present a useful summary of past in-situ stress testing in the Himalayas (mainly in India) that was considered as part of the loading conditions for the pre-cast concrete segmental lining for the Kishanganga project’s headrace tunnel.
Brox and Piaggio (2025) present the results of deep in-situ stress testing from both past and recent projects whereby higher than expected in-situ stresses were measured, in particular in the regions of nearby plate tectonics, and which therefore should be considered for similarly sited deep headrace tunnels.
Finally, Panthi (2012) reviewed the probable insitu stresses along the TBM-excavated section of the headrace tunnel at the Parbati II hydro project with a maximum cover of 1500m, concluding that elevated in-situ stresses were likely present and to have been a contributary cause of rockbursting.
2.3 Logistics: Access/Power/Spoil
Other important risks are related to TBM logistics, including access for mobilisation, power supply and spoil disposal. Most hydro projects are located in remote mountainous areas where existing roads may be of limited quality and perhaps bridges having limited load and size capacity; significant upgrades may be required for access and to enable transport to and mobilisation of a TBM on site.
The power demand for TBMs can be appreciable, possibly up to 10MW for very large diameter machines.
Therefore, adequate power supply to the site and underground must be established and maintained, either by a connection to an existing powerline, for example from an existing nearby power station, or by the use of generators with diesel consumption. Major cost savings can be realised with a connection to an existing powerline that requires the early installation of a transformer, which was done at the Pakal Dul hydro project (see Figure 2).
Finally, there may exist strict environmental restrictions within a project region for the safe disposal of spoil. TBM spoil from competent bedrock comprises variable size chips ranging from a few centimetres to 15cm in length, which can be effectively utilised for road sub-base material or well-drained backfilling for alternative construction purposes.
3 HISTORICAL TBM TUNNELS IN THE HIMALAYAS
3.1 Dul Hasti, India
The 390MW Dul Hasti hydro project includes a 10.6kmlong headrace tunnel with a maximum cover of 1250m. It was the first project in the Himalayas where a TBM was used. Construction commenced in 1989 and the power plant was expected to be commissioned in 1995 but was delayed to 2007 due to various contractual challenges.
A 6.75km-long upstream section of the headrace was planned to be excavated using a TBM due to the lack of a practical, intermediate access adit that could allow Drill & Blast excavation. However, only a total of approximately 2.9km of the tunnel was bored using an M/S Robbins 270 series open face hard rock TBM of 8.3m diameter, equipped with 432mm (17”) cutters. The actual ground conditions encountered were reported to be much more adverse than expected. There were blows out of probe holes, resulting in inflows of up to 1100 l/s with appreciable sand and silt. The shield experienced higher than expected cutter consumption in the quartzites. Figure 3 presents the longitudinal profile of the tunnel.
Overall progress of only 86m/month was achieved by the original contractor, that was removed. The project client took over and use of the TBM was abandoned. Tunnelling was then performed by Drill & Blast with the progress rate nearly double.
3.2 Parbati-II, India
The 800MW Parbati-II hydropower project includes a 31.5km-long headrace with a maximum cover of 1600m and was the second project in the Himalayas where a TBM was used, for a 9km-long central section of the tunnel.
Construction of NHPC’s project commenced in 2002 and faced several challenges, including cloudbursts and flash floods above ground, and rockbursts underground. Commissioning of the power plant’s four generation units (4 x 200MW) was finally performed in 2025.
A total of approximately 2km of the central 9kmlong section of headrace was completed using an Atlas Copco Robbins MK-27 open face hard rock TBM of 6.8m diameter with 432mm (17”) cutters. Geology in this portion comprised mainly granitic gneiss. Significant overbreak occurred that could not be supported with traditional pattern bolts and instead required the installation of continuous ring beams, which appreciably reduced progress.
After two years of limited progress, the TBM manufacturer was called in and progress improved, with up to 250m/month achieved.
However, upon the intersection of the massive quartzite there were severe rockbursts, at a cover depth of 1100m, with loss of life. In 2007 the TBM encountered a water bearing zone within the quartzite under 900m cover, which saw inflows of up to 120 l/s with sand and silt. The inflows buried the TBM. Three years passed, during which time was needed to control the inflows and the TBM was abandoned. The remainder of the headrace was excavated by Drill & Blast Method.
TBM tunnelling was employed again at the Parbati II project, successfully completing construction of the twin, 1.5km-long, inclined pressure shafts. The shafts were bored with a 4.88m-diameter double shield TBM through granites and erecting precast concrete segmental lining (see Figure 4).
3.3 Tapovan Vishnugad, India
The 520MW Tapovan Vishnugad hydro project being developed by NTPC includes a 12.1km-long headrace tunnel and was the third project in the Himalayas to use a TBM, on an 8.6km-long portion of the tunnel.
Construction of the headrace began in late 2008 and typically achieved advance rates of 500m per month for the first year, using a 6.5m-diameter double shield TBM. The shield erected 300mm-thick precast concrete segmental lining with an internal diameter of 5.6m.
Geology along the headrace includes the Central Himalayan Crystalline series with mainly quartzites, gneisses, augen gneisses and mica-schists. The alignment passes through multiple small and large shear zones, and faults with geothermal groundwater and the alteration of the schists and gneisses to clays.
Figure 5 presents the longitudinal profile of the headrace tunnel showing the complex geology.
Difficult tunnelling conditions were encountered after about one year of TBM excavation. At about Ch. 9000m there was instability of a wedge block at the face of the TBM, which resulted stoppage of the machine. There was inrush of groundwater and fines up to rates of 800 l/s, and failure of a portion of the erected precast concrete tunnel ring.
A 180m-long bypass tunnel was constructed, by drilling & blasting methods, around the TBM to reach the front of the shield and rehabilitate the area. After a total downtime of about eight months, TBM boring continued.
Second and third entrapments of the TBM occurred, in early and then in late 2012, after another 3000m of excavation, and prior to Ch. 6000m.
All of these significant TBM stoppages appear to have occurred near the intersection of inferred fault/fracture zones with very acute angles to the tunnel axis. The entrapment mechanism has been presented as a result of sub-vertical faults (Brandl et al., 2010 and Millen and Brandl, 2011). The TBM has remained trapped since 2012.
3.4 Kishanganga, India
The 330MW Kishanganga hydro project includes a 23.7km-long headrace tunnel and was the fourth such tunnelling project in the Himalayas to use a TBM. The shield was used on a 14.75km-long portion of the headrace.
Construction of the headrace tunnel commenced in April 2011 and was successfully completed in June 2014 (after 38 months) – the first fully successful use of a TBM on a Himalayan hydro project, in entirely completing the planned portion of headrace, and therefore marking a turning point in major mountain range tunnelling.
Mixed geology was present along the headrace tunnel alignment comprising andesites, phyllitic quartzite and meta-siltstones and sandstones of variable rock quality and strength.
The 6.2m-diameter double shield universal (DSU) TBM was fitted with 483mm (19”) cutters and erected a 350mm-thick precast concrete segmental tunnel lining.
The success and timely completion of Kishanganga headrace tunnel is fully attributed to the experience and competence of the TBM tunneling contractor.
The TBM typically achieved an overall average progress rate of 12.5m/day or about 400m per month, with a maximum monthly progress of 816m.
The shield was specifically designed by the TBM tunnel contractor (Seli) based on extensive experience in difficult tunnelling conditions and including key design features to allow for de-risking and continued advance through challenging conditions.
Figure 6 presents the longitudinal tunnel profile where the maximum cover was 1400m and therefore there was a recognised risk of squeezing ground.
4 RECENT TBM HYDROPOWER TUNNELS IN THE HIMALAYAS
4.1 Bheri Babai Multi-Purpose Project, Nepal
The 47MW Bheri Babai Multi-Purpose project was the first use of a TBM in Nepal. It was used for the construction of the entire 12.2km-long headrace tunnel.
Construction of the headrace commenced in April 2017 and was completed in April 2019 (18 months – 12 months ahead of project schedule) and typically achieved an overall average of 24m/day or about 712m per month but with a maximum monthly progress of 1202m using a 5.0m-diameter Robbins double shield TBM with 483mm (19”) cutters and erecting a 300 mmthick precast concrete lining.
The TBM-driven tunnel was completed one full year ahead of schedule and, notably, the excavation passed without any delay through the Main Boundary Thrust fault, recognised to be the key construction risk for the project. A single delay of five days only was experienced due to uncemented sedimentary bedrock, requiring construction of a bypass tunnel to free the front of the TBM.
Figure 7 presents the space required for the assembly and launch of the TBM at the Bheri Babai headrace tunnel.
4.2 Sunkoshi-Marin Multi-Purpose Project, Nepal
The 39MW Sunkoshi-Marin Multi-Purpose project was the second use of a TBM in Nepal for the construction of the entire 13.3km-long headrace tunnel.
Construction of the headrace tunnel started in October 2022 and was completed in May 2024 (19 months) and typically achieved an overall average of 25m/day (maximum 67m/day) and about 750m per month with maximum progress of 1224m. The same Robbins TBM from the Bheri Babai project was used, with the cutterhead enlarged to 6.2m-diameter and equipped with 483mm (19”) cutters. The precast concrete segmental lining had a thickness of 300mm.
Figure 8 presents the longitudinal tunnel profile where the maximum cover was 1250m and squeezing was a risk.
The TBM boring for this headrace tunnel also passed through the Main Boundary Thrust Fault without any delay, having been recognised as a key project risk. A single delay lasting 21 days was experienced due to a weak phyllite zone that required the construction of a bypass tunnel to the front of the TBM for liberation (Home and Shrestha, 2023).
4.3 Neelum Jhelum Hydropower Project, Pakistan
The 969MW Neelum Jhelum hydro project was the first use of TBMs in Pakistan for the construction of the twin 10km-long sections in the central portion of the 28.5km long headrace tunnel, under a maximum cover of 1900m. It was the deepest tunnel in the Himalayas. TBM boring on the headrace commenced in March and April 2013 and the drives were completed in October 2016 and May 2017, respectively.
Geology along the central section of the headrace comprised the Murree Formation of intermixed sandstones, siltstones and mudstones with poor quality and durability of the mudstone zones. A major rockburst was experienced in one of the TBM drives upon the intersection of a massive sandstone zone, located before the area of maximum cover. With a cover of 1300m, the in-situ stress ratio was measured at k=2.9. The rockburst caused severe damage to the TBM and led to a delay of six months.
TBM progress was hampered by elevated in-situ stresses that resulted in frequent overstressing, including rockbursts. Shield advance averaged about 8m-10m/day.
The TBMs were Herrenknecht 8.5m-diameter open gripper type. The headrace tunnel was designed and constructed as a one-pass approach with the initial support installed within the L1 section ahead of the grippers and the final shotcrete lining was constructed within the L2 section, some 65m behind the face using shotcrete robots. The approach had some shortcomings on the project, in terms of shotcrete quality and required significant remedial works (Peach et al., 2019). Figure 9 presents the final shotcrete lining of headrace tunnel.
5 CURRENT TBM HYDROPOWER TUNNELS IN THE HIMALAYAS
5.1 Vishnugad Pipalkoti, India
The 444MW Vishnugad Pipalkoti hydro project is currently in construction and includes a 12.3kmlong headrace tunnel, main excavations for which commenced in late 2016 with work on the TBM starting portal. However, it immediately became apparent that bedrock was not present at the designated area but rather partially consolidated river deposits, that required a specially designed and built launch cavern with heavy grouting. Unfortunately, the initial launch cavern was not effective to allow for the launching of the TBM and a total delay of six years was realised before the TBM broke into bedrock, after 200m (Kahli and Potnis, 2023). Figure 10 presents the TBM portal area during the initial excavation in 2016, when partially consolidated river deposits with boulders were discovered without bedrock.
The TBM comprises a specially designed Terratec 9.0m-diameter double shield universal (DSU) TBM, and the tunnel lining is formed of 350mm-thick precast concrete segmental rings. The shield design was targeted to improve the TBM’s capability to advance through squeezing/converging rock formations under high cover. It includes a telescopic joint design (allowing the machine to operate in double shield mode in very weak rock), and capability to investigate and treat the ground around and ahead of the face (Grandori, 2016). Geology along the headrace tunnel alignment comprises predominantly slates with dolomitic limestone. TBM excavation finally commenced in July 2023 and since April 2024 has progressed consistently at about 11m/day or 365m per month.
5.2 Pakul Dul, India
The 1000MW Pakal Dul hydro project is under construction and includes twin, 7.5km-long headrace tunnels. Construction of the headrace started in November 2023. Two Herrenknecht 7.2m-diameter, single shield TBMs are being used for the excavations. Tunnel lining is precast concrete segmental rings.
Geology along the headrace tunnel alignment comprises mixed quartzites, phyllites, schists, and gneissic granites. Figure 11 presents the TBM at the starting portal platform. TBM launching and assembly took place in a limited area.
TBM progress has been exceptional. Recent progress, to early 2025, was of 46.6m on the best day and 630m in the best month. The success of the technical evaluation to use a TBM for the Pakal Dul project, along with the TBM procurement, has been attributed to comprehensive risk management practices with all stakeholders (Armetti and Panciera, 2023).
6 PROPOSED TBM HYDROPOWER TUNNELS IN THE HIMALAYAS
A TBM is to be used on a new major project including the 285MW Upper Tamor Hydropower Project, in eastern Nepal. Upper Tamor includes an 8.7km-long, 7.2m-diameter headrace tunnel.
The headrace tunnel bore would be the third time use of the same TBM in Nepal, for a total of 32km, and also the first use of a TBM for a private hydropower project in the country. The formal contract for the works was signed in mid-2025 with Robbins and also the contractor who constructed both the headrace tunnels at Bheri Babai and Sunkoshi-Marin Multiple-Purpose projects in Nepal. Figure 12 presents the longitudinal profile for the proposed Upper Tamor headrace tunnel. In addition, the Melamchi Water Supply Project – Phase 2, in Nepal (that includes a mini-hydro station) has the 8.9km-long Yangri Tunnel under maximum cover of 1700m. The tunnel has been evaluated for TBM bored tunnelling, given that there is no possibility for an intermediate access adit and with difficult portal locations for access (Brox, 2022). Figure 13 presents the longitudinal profile for the proposed Yangri Tunnel.
7 OTHER RELEVANT TBM PROJECTS IN INDIA
7.1 Rishikesh-Karanprayag Rail Tunnel Project
The Rishikesh-Karanprayag rail project has been in construction in the Himalayan foothills since late 2022 and includes two tunnels (upline and downline) of lengths 10.5km and 10.3km, respectively.
The area is dominated by metamorphic rocks, including schists, gneisses, and quartzites. Twin, 9.1m-diameter single shield TBMs have been used in conjunction with precast concrete segmental lining to construct the tunnel. The TBMs were specificallydesigned with accessories for overcoming the risks of squeezing conditions and include a cutterhead torque box, high thrust rams, and shield void measurement system. Progress to date has achieved an average of 630m per month with a maximum of 555m, with sustained daily progress from 12m-18m (Cooper, 2025).
7.2 AMR and Veligonda Water Transfer Projects
The Alimineti Madhava Reddy (AMR) water transfer and supply project comprises a single, 10m-diameter, 46km-long tunnel located in southeast India, in the state of Andhra Pradesh. It is to transfer water below a Tiger Reserve from the Srisailam Reservoir to an area of farmland to the north. In addition, the Veligonda water transfer and supply project comprises 19km-long tunnels of 7.9m and 10m diameter, respectively, located immediately south of the AMR project The projects have used Robbins double shield TBMs as well as a double shield Herrenknecht TBM. Tunnel lining is precast concrete segmental rings. The geology along these tunnel alignments has comprised very strong (250MPa-450 MPa) and abrasive quartzites, hampering TBM progress to about 250m per month (Harding, 2010). The north TBM drive of the AMR project recently experienced a major inrush of weak and soft material that resulted in multiple fatalities and the rotation of the TBM backup, which may not be salvageable.
8 TBM TYPES AND RISK MITIGATION REQUIREMENTS
8.1 TBM Types
Hydropower tunnels are generally sited in mixed and competent bedrock and face-pressurised TBMs operated in close mode are generally not required. The typical types of TBMs used for hydropower tunnels are:
- Open gripper with traditional rock support;
- Single Shield with precast concrete segmental lining;
- Double Shield with traditional rock support; and
- Double Shield with precast concrete segmental lining.
However, it should be noted that geotechnical conditions often associated with geological faults can warrant the use the face-pressurised or hybrid types of TBMs operated in close mode for limited sections when encountered along a long and deep hydropower tunnel.
Such fault zones can comprise highly fractured and/or soft clay gouge with elevated groundwater pressures or be within unique geological formations, such as highly permeable lahar.
8.2 TBM Evaluation and Selection Criteria
A comprehensive technical evaluation must be undertaken for the selection of the most appropriate type of TBM to be used for the construction of headrace tunnels for hydropower projects given the severe impacts that may arise from the various prevailing geological and geotechnical risks. Grandori et al., (2018) and Brox (2020) present and discuss the various geotechnical and logistical aspects that should be considered as part of a TBM evaluation and selection process, including the following:
- Rock Types and Distribution;
- Geological Faults and Weak Zones;
- Geological Synclines and Anticlines/Folding;
- Durability of Rock and Final Lining Requirements;
- Squeezing Potential;
- Overstressing Potential, including Rockbursting; and,
- High Groundwater Inflows, Pressures and Temperatures.
In addition to the important geotechnical aspects there exist multiple logistical and other aspects that require a careful evaluation, including access for mobilisation, portal space availability, power availability, environmental spoil disposal requirements, and contractor experience. Brox (2021) presents a TBM selection criteria logic chart of the key technical aspects to highlight the typical selection process, as presented in Figure 14.
8.3 TBM Risk Mitigation
The key risk mitigation requirements related to the use of TBMs for the construction of headrace tunnels for hydropower projects are that adequate geotechnical investigations and planning are performed, well in advance, during the early study stages of a project.
While the challenges of high elevation and deep drilling are recognised for mountainous regions, alternative methods of investigations are critically important to attempt, including geophysical investigations and detailed surface mapping with representative rock block testing for strength, petrology and abrasivity; many of the rock units within the Himalayas contain a high content of quartz, which can have a significant impact on TBM progress and operating costs.
An important risk mitigation approach for TBM hydropower tunnels is the adoption of the well-recognised approach of one-pass precast concrete segmental linings, which have been used and successfully completed more than 925km of hydropower tunnels (Brox and Grandori, 2023).
One-pass lining of precast concrete segmental rings offers greater safety to workers during construction with protection from the impacts of high in-situ stresses (e.g., rockbursting) but not from sudden inrushes through the face. The current TBM hydropower tunnels in the Himalayas further confirm this low-risk construction approach. Figure 15 presents the precast concrete segmental lining installed at the Bheri Babai headrace tunnel.
Finally, the potential risk for overstressing, including the prediction of the expected spatial occurrence of rockbursting, can be evaluated using the method presented by Brox (2012, 2013) based on and verified by numerous case projects of deep tunnels where significant overstressing and rockbursting was realised. Such an evaluation should be a fundamental part of a due diligence technical assessment to safety-related construction risks to be presented in a Geotechnical Baseline Report (GBR) and contract documents as part of full and total disclosure of project risks for tender.
9 ENVIRONMENTAL/SOCIAL ADVANTAGES FOR TBMS
Many of the hydropower projects that previously have been and continue to be constructed across the Himalayas are sited within valleys where there exist well established communities both at upper elevations as well as along lower elevations adjacent to rivers.
Accordingly, these communities, and their associated infrastructure of houses, schools, clinics, and other important infrastructure, including water wells and springs, may be at risk during the construction of headrace tunnels.
Blasting may induce vibrations to overlying structures, resulting in damage as well as causing stress relaxion inside the tunnels, and possibly the opening of major fractures and geological faults that could significantly reduce the original groundwater table. These levels might not be fully re-established after construction and afterward, during future pressurised operations of tunnels.
TBMs offer a more environmentally acceptable solution without vibrations and also less risk of relaxation around a headrace tunnel which should limit the impact to the groundwater table.
10 CONCLUSIONS/LESSONS LEARNED FOR FUTURE TBM USE
The following conclusions and lessons are considered to as learned based on historical and recent TBM hydropower tunnel projects in the Himalayas:
- Long hydro tunnels are finally getting completed in the Himalayas at some very remote project sites after some good lessons learned, improved technology and good planning;
- The sedimentary geology of the Lesser Himalaya is less disturbed and has allowed for greater than expected TBM progress for the early completion of multiple projects, notably in Nepal to date;
- A well-experienced and competent TBM tunnel contractor and TBM labour crews are required for the successful timely completion of any headrace tunnel associated with challenging geotechnical conditions whereby special experience is typically called upon for unique solutions;
- The geotechnical conditions at the TBM portal must be confirmed with comprehensive geotechnical investigations to avoid adverse conditions for the timely launching of the TBM;
- TBMs are capable of safely constructing very deep and long tunnels with the installation of conventional rock support but with impacts of elevated in situ stresses;
- The most prudent approach for the construction of hydropower tunnels using TBMs in the Himalayas is in conjunction with precast concrete segmental lining which provides safety to workers from high in-situ stresses;
- Special design features/components, including high torque and thrust capacity, are required for TBMs to be successfully used in the Himalayas to cope with the challenges of the geotechnical conditions; and,
- Good planning and risk management practices have proven to be effective in contributing to the success of the use of TBMs in the Himalayas.
