Underground construction |


Figure 6 – Complex geology of Tapovan headrace tunnel


SE


2900 2700 2500 2300 2100 1900 1700 1500


(Joints shown are schematic) Legend


A - Metabasics and schists with gneissic bands B - Metabasics and schists with augen gneissic bands C - Augen gniesses with schists and quartizite bands


D - Coarse grained garnet-biotite-kyanite gneisses


E - Banded and augen gneisses with quartzites, schists and foliated gneissic bands


Headrace Tunnel


NW


Chainage (m)


Quartzites Metabasics


Geological contact


Figure 7 – Kishanganga headrace tunnel profile Drill and blast tunnel


4000m 3500m 3000m 2500m


Dam volcanics F F Panjal


Razdhan formation


F TBM tunnel F F


Hasthoji formation


F F


Hafkhalan formation


Hanti Granitoid


TBM portal


by the project client. The use of the TBM was abandoned since the achieved progress by drill and blast was nearly double.


Below: Figure 8 – Portal for assembly and launch at Bheri Babai


Parbati (2002) – India The 800MW Parbati-II hydropower project included a 31.5km headrace tunnel with a maximum cover of 1600m and was the second project in the Himalayas where a TBM was used for a 9km central section of the headrace tunnel. Construction commenced in 2002 and faced several construction challenges, including cloudbursts, flash floods, and rockbursts. Commissioning was finally performed in 2025. Approximately a total of 2km of the central 9km section of the headrace tunnel was completed using an Atlas Copco Robbins MK-27 open face hard rock TBM of 6.8m diameter with 432mm (17”) cutters. The portion that was completed comprised mainly granitic gneiss where significant overbreak occurred that could not be supported with traditional pattern bolts and rather required the installation of continuous ring beams that appreciably reduced progress. After two years of limited progress the TBM manufacturer was called in which improved progress to as much as 250m/month. Upon the intersection of the massive quartzite there was the occurrence of severe rockbursting at a depth of 1100m with the loss of life, and in 2007 the TBM encountered a water bearing zone within the quartzite under a cover of 900m with 120 l/s (1700 gpm) of inflows including sand and silt that buried the TBM. Three years passed during which time it was needed to control the inflows but overall resulted in the abandonment of the TBM. Figure 4 presents the longitudinal profile for the Parbati hydropower tunnel (Panthi, 2012, Clark and Chorley, 2014). TBMs were also used at the Parabati II project and successfully completed the construction of the twin, 1.5km long, inclined pressure shafts with a 4.88m


28 | November 2025 | www.waterpowermagazine.com


diameter double shield TBM through the prevailing granites with pre-cast concrete segmental linings as shown in Figure 5.


Tapovan (2008) – India The 520MW Tapovan hydropower project includes a 12.1km headrace tunnel and was the third project in the Himalayas where a TBM was used for an 8.6km portion of the headrace tunnel. Construction of the headrace tunnel commenced in late 2008 and typically achieved 500m per month for the first year using a 6.5m diameter double shield TBM with a 300mm thick pre-cast concrete segmental tunnel lining (PCTL) with an internal diameter of 5.6m. The geology along the headrace tunnel includes the Central Himalayan Crystalline series with mainly quartzites, gneisses, augen gneisses and mica-schists with multiple small and large shear zones and faults with geothermal groundwater and the alteration of the schists and gneisses to clays. Figure 6 presents the longitudinal profile of the headrace tunnel showing the complex geology.


Difficult conditions were encountered after about


one year of TBM excavation at about chainage 9000m with the instability of wedge block at the face of the TBM that resulted in the stoppage of the TBM along with the inrush of water and fines up to 800 l/s along with the failure of a portion of the PCTL ring. A 180m long bypass tunnel was constructed around the front of the TBM to rehabilitate the area and allow for continued TBM excavation and the total downtime for this was about eight months. Second and third TBM entrapments occurred in early and late 2012 after another 3000m of excavation at prior to chainage 6000m. All of these significant TBM stoppages appear to have occurred near the intersection of inferred fault/ fracture zones with very acute angles to the tunnel axis despite the entrapment mechanism, this has been


Altitude (m) 0.0 Barrage site


1000


Duli Gad bend


2000


3000


Chormi adit


4000


5000


6000


7000


8000


9000


10000


11000 Surge shaft 11850


Fault zone


Fault zone


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