POWER, WATER & STORAGE | SECTOR


Finally, there may exist strict environmental


restrictions within a project region for the safe disposal of spoil. TBM spoil from competent bedrock comprises variable size chips ranging from a few centimetres to 15cm in length, which can be effectively utilised for road sub-base material or well-drained backfilling for alternative construction purposes.


3 HISTORICAL TBM TUNNELS IN THE HIMALAYAS


3.1 Dul Hasti, India The 390MW Dul Hasti hydro project includes a 10.6km- long headrace tunnel with a maximum cover of 1250m. It was the first project in the Himalayas where a TBM was used. Construction commenced in 1989 and the power plant was expected to be commissioned in 1995 but was delayed to 2007 due to various contractual challenges. A 6.75km-long upstream section of the headrace


was planned to be excavated using a TBM due to the lack of a practical, intermediate access adit that could allow Drill & Blast excavation. However, only a total of approximately 2.9km of the tunnel was bored using an M/S Robbins 270 series open face hard rock TBM of 8.3m diameter, equipped with 432mm (17”) cutters. The actual ground conditions encountered were reported to be much more adverse than expected. There were blows out of probe holes, resulting in inflows of up to 1100 l/s with appreciable sand and silt. The shield experienced higher than expected cutter consumption in the quartzites. Figure 3 presents the longitudinal profile of the tunnel. Overall progress of only 86m/month was achieved by


the original contractor, that was removed. The project client took over and use of the TBM was abandoned. Tunnelling was then performed by Drill & Blast with the progress rate nearly double.


3.2 Parbati-II, India The 800MW Parbati-II hydropower project includes a 31.5km-long headrace with a maximum cover of 1600m and was the second project in the Himalayas where a TBM was used, for a 9km-long central section of the tunnel. Construction of NHPC’s project commenced in 2002


and faced several challenges, including cloudbursts and flash floods above ground, and rockbursts underground. Commissioning of the power plant’s four generation


units (4 x 200MW) was finally performed in 2025. A total of approximately 2km of the central 9km-


long section of headrace was completed using an Atlas Copco Robbins MK-27 open face hard rock TBM of 6.8m diameter with 432mm (17”) cutters. Geology in this portion comprised mainly granitic gneiss. Significant overbreak occurred that could not be supported with traditional pattern bolts and instead required the installation of continuous ring beams, which appreciably reduced progress. After two years of limited progress, the TBM


manufacturer was called in and progress improved, with up to 250m/month achieved. However, upon the intersection of the massive


quartzite there were severe rockbursts, at a cover depth of 1100m, with loss of life. In 2007 the TBM encountered a water bearing zone within the quartzite under 900m cover, which saw inflows of up to 120 l/s with sand and silt. The inflows buried the TBM. Three years passed, during which time was needed to control the inflows and the TBM was abandoned. The remainder of the headrace was excavated by Drill & Blast Method. TBM tunnelling was employed again at the Parbati


II project, successfully completing construction of the twin, 1.5km-long, inclined pressure shafts. The shafts were bored with a 4.88m-diameter double shield TBM through granites and erecting precast concrete segmental lining (see Figure 4).


3.3 Tapovan Vishnugad, India The 520MW Tapovan Vishnugad hydro project being developed by NTPC includes a 12.1km-long headrace tunnel and was the third project in the Himalayas to use a TBM, on an 8.6km-long portion of the tunnel. Construction of the headrace began in late 2008 and typically achieved advance rates of 500m per month for the first year, using a 6.5m-diameter double shield TBM. The shield erected 300mm-thick precast concrete segmental lining with an internal diameter of 5.6m. Geology along the headrace includes the Central


Himalayan Crystalline series with mainly quartzites, gneisses, augen gneisses and mica-schists. The alignment passes through multiple small and large shear zones, and faults with geothermal groundwater and the alteration of the schists and gneisses to clays. Figure 5 presents the longitudinal profile of the headrace tunnel showing the complex geology.


Above, figure 3: Longitudinal profile of Dul Hasti hydropower tunnel SOURCE: IAEG PROCEEDINGS 2023 October 2025 | 21


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