| Underground construction
Figure 9 – Sunkoshi-Marin headrace tunnel profile SW
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Main BoundaryThrust (MBT)fault NE
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presented as a result of sub-vertical faults (Brandl et al., 2010 and Millen and Brandl, 2011) The TBM has remained entrapped since the third entrapment in late 2012 with multiple unsuccessful attempts to re-start the project.
Kishanganga (2011) – India The 330MW Kishanganga hydropower project includes a 23.7km headrace tunnel and was the fourth project in the Himalayas where a TBM was used for a 14.75km portion of the headrace tunnel. Construction of the headrace tunnel commenced in April 2011 and was completed in June 2014 (38 months) and typically achieved an overall average of 12.5m/day or about 400m per month but with a maximum monthly progress of 812m using a 6.2m diameter double shield universal (DSU) TBM with 483mm (19”) cutters and a 350mm thick pre-cast concrete segmental tunnel lining (PCTL). The TBM was specifically designed by the TBM tunnel contractor (SELI) based on extensive experience in difficult tunnelling conditions and including key design features to allow for de-risking and continued advance through challenging conditions. Figure 7 presents the longitudinal tunnel profile where the maximum cover was 1400m and therefore there was a recognised risk of squeezing of the PCTL. Mixed geology was present along the headrace tunnel alignment comprising andesites, phyllitic quartzite and meta-siltstones and sandstones of variable rock quality and strength. The success of the timely completion of the construction of the Kishanganga headrace tunnel is fully attributed to the experience and competence of the TBM tunnelling contractor.
Recent TBM hydropower tunnels in the Himalayas
Bheri Babai multi-purpose project – Nepal The 47MW Bheri-Babai Multi-purpose project was the first use of a TBM in Nepal for the construction of the entire 12.2km headrace tunnel. Construction of the headrace tunnel commenced in October 2017 and was completed in April 2019 (18 months – 12 months ahead of schedule) and typically achieved an overall average of 24m/day or about 712m per month but with a maximum monthly progress of 1202m using a Robbins 5m diameter double shield TBM with 483mm (19”) cutters and a 300mm thick pre-cast concrete segmental tunnel lining (PCTL). The TBM headrace tunnel was completed one year ahead of schedule and notably passed without any delay through the Main Boundary Thrust fault that was the recognised key risk for the project. A single delay of five days was
experienced due to uncemented sedimentary bedrock that required the construction of a bypass tunnel to the front of the TBM for liberation. Figure 8 presents the space required for the assembly and launch of the TBM at the Bheri Babai headrace tunnel.
Sunkoshi-Marin multi-purpose project – Nepal The 39MW Sunkoshi-Marin Multi-purpose project was the second use of a TBM in Nepal for the construction of the entire 13.3km headrace tunnel. Construction of the headrace tunnel commenced in October 2022 and was completed in May 2024 (19 months) and typically achieved an overall average of 25m/day (maximum 67 m/day) or about 750 m per month but with a maximum monthly progress of 1224m using the same Robbins TBM from the Bheri-Babai project with an enlargement of the cutterhead to 6.2m diameter for the double shield TBM with 483mm (19”) cutters and a 300mm thick pre-cast concrete segmental tunnel lining (PCTL). Figure 9 presents the longitudinal tunnel profile where the maximum cover was 1250m and therefore there was a recognised risk of squeezing of the PCTL. The TBM headrace tunnel also notably passed without any delay through the Main Boundary Thrust fault that was the recognized key risk for the project. A single delay of 21 days was experienced due to a weak phyllite zone that required the construction of a bypass tunnel to the front of the TBM for liberation. (Home and Shrestha, 2023).
Neelum Jhelum hydropower project – Pakistan The 969MW Neelum Jhelum hydropower project was the first use of TBMs in Pakistan for the construction of the twin 10km sections of central portion of the 28.5km headrace tunnel with a maximum cover of 1900m being the deepest tunnel in the Himalayas. Construction of the TBM sections of the headrace tunnel commenced in March and April 2013 and were completed in October 2016 and May 2017 respectively. Geology along the central section of the headrace tunnel comprised the Murree Formation of intermixed sandstones, siltstones and mudstones with poor quality and durability of the mudstone zones. A major rockburst was experienced in one of the TBM drives upon the intersection of a massive sandstone zone located before the area of maximum cover but with a cover of 1300m where the in situ stress ratio was measured at k=2.9 resulting in a delay of six months since severe damage occurred to the TBM. The TBMs were Herrenknecht 8.5m diameter open gripper type whereby the headrace tunnel was designed and constructed as a one-pass
Above: Figure 10 – TBM tunnel with final shotcrete lining
Below: Figure 11 – TBM portal with partially consolidated river deposits
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