SECTOR | POWER, WATER & STORAGE
Right, figure 2:
Early installation of transformer for TBM power
PHOTO CREDIT: VOITH
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
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.
20 | October 2025
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 in-
situ 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).
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