Underground construction |
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 mobilization, 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 15.
TBM risk mitigation The key risk mitigation requirements related to the use
Above: Figure 16 – Pre-cast concrete segmental lining for Bheri Babai
Below: Figure 17 – Very large inflows causing subsidence at surface
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 recognized for mountainous regions, alternative methods of investigations including geophysical investigations and detailed surface mapping with representative rock block testing for strength, petrology and abrasivity since 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. 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 including geophysical investigations and detailed surface mapping with representative rock block testing for strength, petrology and abrasivity since 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 pre-cast concrete segmental linings (PCTL) that have been used and successfully completed for more than 925km of hydropower tunnels (Brox and Grandori, 2023). PCTL 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 confirms this low-risk construction approach. Figure 16 presents the pre-cast concrete segmental lining installed at the Bheri Babai headrace tunnel. Finally, the potential risk for overstressing including
References
Available online at www.
waterpowermagazine.com - search Dean Brox
the prediction of the expected spatial occurrence of rockbursting can be evaluated using the method presented by Brox (2012, 2013) that has been 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 and contract documents as part of full and total disclosure of project risks for tender.
32 | November 2025 |
www.waterpowermagazine.com 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, including the 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 by blasting that may induce vibrations to overlying structures resulting in damage as well as causing stress relaxion inside the tunnels resulting in the opening of major fractures and geological faults that may significantly reduce the original groundwater table during construction which may not fully be re-established after construction and during future pressurised operations. TBMs offer a more environmentally acceptable solution without vibrations and also less relaxation around a headrace tunnel to limit the impact to the groundwater table. Figure 17 presents the very large inflows that occurred during the construction of the 15.3km headrace tunnel at the Uma Oya Multi-Purpose project in Sri Lanka along with the reduction of groundwater table that resulted in subsidence and damages to houses at surface.
Lessons learned for future TBM use The following conclusions and lessons are considered
to have been learned based on historical and recent TBM hydropower tunnel projects in the Himalayas: Long hydropower tunnels are finally getting completed using TBMs in the Himalayas at some very remote project sites after some good planning, lessons learned, and improved technology; 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 a 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 pre-cast 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 Good planning and risk management practices have proven to be effective in contributing to the success of the use of TBMs in the Himalayas.
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