Underground construction |
The TBM comprises a specially designed Terratec 9m diameter double shield universal (DSU) TBM (similar to Kishanganga TBM) in conjunction with a 350mm thick pre-cast concrete segmental tunnel lining (PCTL). This new TBM design was targeted to improve the TBM capability to advance through squeezing/ converging rock formations under high cover, the telescopic joint design (allowing by such to operate the machine in double shield mode in very weak rock), and the capability of the TBM to investigate and treat the ground around and ahead of the tunnel face (Grandori, 2016). Geology along the headrace tunnel alignment comprises predominantly slates with dolomitic limestone. TBM excavation finally commenced in July 2023 and since April 2024 has progressed consistently at about 11m/day or 365m per month.
Above: Figure 12 –TBM assembly and launching at starting portal platform
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 65 behind the face using shotcrete robots. TBM progress was hampered by elevated in situ stresses that resulted in frequent overstressing including rockbursts and averaged about 8-10m/ day. The one-pass design and construction approach was associated with some shortcomings in terms of the quality of the final shotcrete lining and required significant remedial works (Peach et al., 2019). Figure 10 presents the final shotcrete lining of headrace tunnel.
Current TBM hydropower tunnels in the Himalayas
Vishnugad Pipalkoti – India The 444MW Vishnugad Pipalkoti hydropower project is currently in construction and includes a 12.3km headrace tunnel. Construction of the headrace tunnel commenced originally in late 2016 with the excavation of the TBM starting portal. However, it immediately became apparent that bedrock was not present at the designated TBM portal area but rather partially consolidated river deposits that required a specially designed and built launch cavern with heavy grouting. Unfortunately, the initial launch cavern was not effective to allow for the launching of the TBM and a total delay of six years was realised before the TBM broke into bedrock after 200 m (Kahli and Potnis, 2023). Figure 11 presents the TBM portal area during the initial excavation in 2016 when partially consolidated river deposits with boulders were discovered without bedrock.
Figure 13 – Longitudinal profile for Upper Tamor Hydropower
2500 2300 2100 1900 1700 1500 1300 1100 900
Chainage (M) Q value
RMR value
Description Support type
0+000 to 0+402 10 to 47
1.13 to 19.25
0+402 to 0+881 3.43 to 27.9
51 to 67
The rock mass is good to very good banded gneiss, with occasional shear zone
Type I: 5%, Type II: 10% Type III: 35%, Type IV: 35%, Type V: 15%
0+881 to 1+783 3.43 to 27.9
51 to 67 The rock mass is good to very good banded gneiss
Type I: 15%, Type II: 35%, Type III: 35%, Type IV: 15%
49 to xx
1+783 to 3+122 63 to 82
Banded gneiss 3.19 to 27.26
15.75 to 25.5
HIGH OVERBURDEN Quartzite
Type I: 25%, Type II: 30%, Type III: 20%, Type IV: 20%, Type V: 5%
Overburden greater than 900m HIGH STRESS CONDITION
Surge shaft Powerhouse
Tailrace tunnel alignment
54 to 72
Banded gneiss 15.51 to 24.67
Additional support depending on the site condition, Steel ribs – the spacing of 1 to 3m which is covered by 50 cm thick fiber reinforced shotcrete
Type I: 40%, Type II: 30%,Type III: 20%, Type IV: 10%, Type V: 5%
3+122 to 6+095 461 to 79
15.75 o 25.77 The rock mass is good banded gneiss Type I: 35%, Type II: 30%, Type III: 20%, Type IV: 10%, Type V: 5%
MCT zone
Type V
6+220 to 8+713 43 to 71
4.73 to 17.95 The rock mass is fair to good Feldspathic schist Type I: 25%, Type II: 30%, Type III: 25%, Type IV: 10%, Type V: 10%
275 43 to 71
4.73 to 17.85 15.75 to 25.77 55 to 80
The rock mass is good to very good augen gneiss Type I: 30%, Type II: 40%, Type III: 20%, Type IV: 10%
Open canal
1700 1500 1300 1100 900 700
Pakul Dul – India The 1000MW Pakal Dul hydropower project is currently under construction and includes twin, 7.5km headrace tunnels. Construction of the headrace tunnels started in November 2023. Two, Herrenknecht 7.2m diameter single shield TBMs are being used in conjunction with pre-cast concrete segmental linings (PCTL). Geology along the headrace tunnel alignment comprises mixed quartzites, phyllites, schists, and gneissic granites. Figure 12 presents the TBM at the starting portal platform that was assembled and launched in a very limited area which has been successfully managed for TBM logistics during construction. TBM progress has been exceptional with the recent
progress in early 2025 of 630m in a single month and 46.6m in a single day. The success of the technical evaluation to use a TBM for the Pakal Dul project along with the TBM procurement has been attributed to comprehensive risk management practices with all stakeholders (Armetti and Panciera, 2023).
Proposed TBM hydropower tunnels in
the Himalayas A TBM is also planned to be used on the 285MW Upper Tomar Hydropower Project in Eastern Nepal that includes an 8.7km, 7.2m diameter headrace tunnel. This constitutes the first TBM to be used for a private hydropower project in Nepal and the formal contract was signed for this project in mid-2025 to use for the third time the same Robbins double shield TBM with the same tunnel contractor who constructed both the headrace tunnels of the Bheri Babai and Sunkoshi- Marin Multiple-purpose projects. Figure 13 presents the longitudinal profile for the proposed headrace tunnel. In addition, the Melamchi Water Supply Project - Phase 2 in Nepal (that includes for a min-hydro station) includes
30 | November 2025 |
www.waterpowermagazine.com
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