Underground construction |
Engineering Himalayan headrace tunnels with TBMs
By analysing past and ongoing hydropower tunnelling, Dean Brox details the technical, logistical, and operational challenges faced, and the lessons learned for successful TBM deployment in extreme mountainous terrain.
Author details
Dean Brox of Dean Brox Consulting Ltd. Vancouver, Canada
TUNNEL BORING MACHINES (TBMS) have been used for the construction of long tunnels for more than 70 years and their use has included several long tunnels for hydropower and civil infrastructure projects. Most of the major mountain ranges in the world 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 of which were due to specific aspects that were not directly related to geotechnical risks but rather the inappropriate type of TBM for the prevailing geological 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
Above: Figure 1 – Prominent dipping bedrock geology in the Himalayas
Below: Figure 2 – Early installation of transformer for TBM power
results within the past decade and in particular, most recently with some ongoing projects, it is considered to be warranted to document a fresh perspective for the use and applicability of TBMs in the Himalayas since improved technical evaluations and risk management practices have provided success. The past tendency to shy away from the application of TBMs due to actual problems encountered and/or perceived risks should now be challenged with the optimism of the success realised in recently completed and currently ongoing projects.
Key risks and challenges for TBMs in the Himalayas
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 and over past time has resulted in some areas of highly disturbed geological formations. The Himalayas 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
26 | November 2025 |
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with limited disturbance whereas 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 as presented in 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 that can vary in thickness up 10’s of meters and are commonly associated with very large 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.
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 whereby the hydropower tunnel alignments 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 of the Himalayas, it can be expected that the in situ stresses will vary significantly and therefore site-specific stress testing should be performed. In situ stress testing has been completed for many past projects comprising both hydraulic fracturing and overcoring typically in the area 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 segmental lining for the Kishanganga headrace tunnel. Brox and
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