SECTOR | POWER, WATER & STORAGE
(SW)
2000 1500 1000 500 0m
-500
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0 km
Above, figure 8: Sunkoshi-Marin headrace tunnel profile SOURCE: ROBBINS Undersluice Section
Approach canal Desanding Basin
Headpond Power Conduit Transition from Rectangular to Circular Profile
Slope 1 in 300 Length of HRT = 13100m Headrace Tunnel (HRT)
Tunnel Portal Surge Shaft
Powerhouse Penstock
Tailrace
(SW) 2000
1500 1000 500 0m
-500
Figure 8 presents the longitudinal tunnel profile
where the maximum cover was 1250m and squeezing was a risk. The TBM boring for this headrace tunnel also passed
through the Main Boundary Thrust Fault without any delay, having been recognised as a key project risk. A single delay lasting 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).
4.3 Neelum Jhelum Hydropower Project, Pakistan The 969MW Neelum Jhelum hydro project was the first use of TBMs in Pakistan for the construction of the twin 10km-long sections in the central portion of the 28.5km- long headrace tunnel, under a maximum cover of 1900m. It was the deepest tunnel in the Himalayas. TBM boring on the headrace commenced in March and April 2013 and the drives were completed in October 2016 and May 2017, respectively. Geology along the central section of the headrace
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. With a cover of 1300m, the
in-situ stress ratio was measured at k=2.9. The rockburst caused severe damage to the TBM and led to a delay of six months. TBM progress was hampered by elevated in-situ
stresses that resulted in frequent overstressing, including rockbursts. Shield advance averaged about 8m-10m/day. The TBMs were Herrenknecht 8.5m-diameter open
gripper type. The headrace tunnel was designed and constructed as a one-pass approach with the initial support installed within the L1 section ahead of the grippers and the final shotcrete lining was constructed within the L2 section, some 65m behind the face using shotcrete robots. The approach had some shortcomings on the project, in terms of shotcrete quality and required significant remedial works (Peach et al., 2019). Figure 9 presents the final shotcrete lining of headrace tunnel.
5 CURRENT TBM HYDROPOWER TUNNELS IN THE HIMALAYAS
5.1 Vishnugad Pipalkoti, India The 444MW Vishnugad Pipalkoti hydro project is currently in construction and includes a 12.3km- long headrace tunnel, main excavations for which commenced in late 2016 with work on the TBM starting portal. However, it immediately became apparent that bedrock was not present at the designated area but rather partially consolidated river deposits, that required a specially designed and built launch cavern with heavy
Above left, figure 9: TBM tunnel with final shotcrete lining PHOTO CREDIT: DEAN BROX Above right, figure 10: TBM portal with partially consolidated river deposits PHOTO CREDIT: DEAN BROX 24 | October 2025
Tunnel Inlet Portal Settling Basin Inlet
Settling Basin Outlet
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