| Underground construction


Figure 14 – Longitudinal profile for Melamchi Phase 2 tunnel N 80 E


Water level = 1474.80m


Invert level = 1473.30m


EAST 3500m 3000m 2500m 2000m 1500m


Datum EL. 1300m Chainage (m)


S 80 W


Inlet portal


Yangri tunnel outlet


3500m 3000m 2500m 2000m 1500m


Datum


Water level invert level


the 8.9km Yangri Tunnel with a maximum cover of 1700m which has been evaluated for the application of TBM construction given that there is no possibility for an intermediate access adit and with difficult portal locations for access (Brox, 2022). Figure 14 presents the longitudinal profile for the proposed Yangri tunnel.


Other relevant TBM projects in India


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 of 10.5km and 10.3km. 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 pre-cast concrete segmental lining (PCTL). The TBMs were specifically designed with accessories for overcoming the risks of squeezing conditions and include a cutterhead torque box, high thrust rams, and shield void measurement system. Progress achieved an average of 630m per month with a maximum of 555m with sustained daily progress of 12 to 18m and a maximum daily progress of 39m (Cooper, 2025).


AMR and Veligonda Water Transfer Projects The Alimineti Madhava Reddy (AMR) water transfer and supply project comprises a single, 10m diameter, 46km tunnel located in southeast India in the state of Andhra Pradesh that transfers 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 7.9m and 10m diameter, 19km tunnels located immediately south of the AMR project. Both projects have used Robbins double shield TBMs as well as a double shield Herrenknecht TBM in conjunction with pre-cast concrete segmental linings (PCTL). The geology along these tunnel alignments has comprised very strong (250-450 MPa) and abrasive quartzites that has hampered 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 back up of the TBM which may not be salvageable.


TBM types and risk mitigation requirements


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 pre-cast concrete segmental lining;


Double Shield with traditional rock support, and; Double Shield with pre-cast concrete segmental lining.


However, it should be noted that geotechnical conditions often associated with geological faults comprising highly fractured and/or soft clay gouge with elevated groundwater pressures or within unique geological formations such as highly permeable lahar warrant the use the face-pressurized or hybrid types of TBMs operated in close mode for such limited sections when encountered along a long and deep hydropower tunnel.


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


Significant squeezing conditions


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


(several major faults)


Non-squeezing conditions


(limited major faults)


Significant overstressing conditions


(typically granitic* and metamorphic rocks) Majority durable rock (low strength rock with deep cover)


Limited or no significant overstressing conditions (high/medium strength rock/ limited cover)


Highly permeable/toxic/gas rock Lahar/highly fractured sedimentary rocks/ gasses/asbestos)


Inflow/leakage risk conditions


(high groundwater pressures, gas emissions/exposure)


Note * Hydrothermally altered granitic rocks are typically non-durable


Below: Figure 15 – TBM evaluation and selection criteria (Brox, 2021)


Single shield TBM with pre-cast lining


Double shield TBM with pre-cast lining


Double shield TBM with rock support


Open gripper TBM with rock support


Final lining as shotcrete as required


Single shield TBM with pre-cast lining


(face pressure closed mode required)


www.waterpowermagazine.com | November 2025 | 31


0+000 0+330 0+650 1+060 1+365 1+930 2+730 3+220 3+600


3+910 4+070


4+310 4+730 5+160


5+570 5+750


5+980 6+600 7+000 7+700 8+000 8+430 8+630 8+900


Yangri River


Evaluate distribution of ground and confirm poor quality <30%


Melamchi River


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