POWER, WATER & STORAGE | SECTOR


Majority non-durable rock (typically young volcanic and sedimentary rocks)


Significant squeezing conditions (Several major faults)


Non-squeezing conditions (Limited major faults)


Majority durable rock (typically granitic*


metamorphic rocks)


Note: *Hydrothermally altered granitic rocks are typically non-durable


Highly permeable/toxic/gas rock


(Lahar/highly fractured sedimentary rocks/gases/asbestos)


Significant overstressing conditions (Low strength rock with deep cover)


and


(High/medium strength rock/limited cover) Li itd


Limited or no significanigifi overstressing conditions


t


Inflow/leakage risk conditions (High groundwater pressures, gas emissions/exposure)


Open gripper TBM with rock support


Single shield TBM with precast lining


(Face pressure closed mode required


Above, figure 14: TBM Evaluation and Selection Criteria (Brox, 2021) SOURCE: DEAN BROX


Single shield TBM with precast lining


Double shield TBM with precast lining


Double shield TBM with rock support


Final lining as shotcrete


as required


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;


October 2025 | 27


Evaluate distribution of ground and confirm poor quality <30%


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