| Underground construction


Figure 3 – Longitudinal profile of Dul Hasti hydropower tunnel


Quartzite Phyllite


Quartizite with phyllite itercalalation


F


2500 2000 1500 1000


T B M excavated section Mica schist/gneiss


Schist/gneiss with calcareous band


Graphitic schist


Air vent shaft


Kishtwar fault


Shalimar nala


F F


Tunnel day lighted (Ch. 4230m from U/S)


0.0415% Slope 0.585% F T B M excavated section P.H. Fault


Major problematic zones (flowing conditions)


2500 2000 1500 1000


Figure 4 – Longitudinal profile for Parbati hydropower tunnel N010°


3600 3500 3400 3300 3200 3100 2900 2800 2700 2600 2500 2400 2300 2200 2100 2000 1900


Remaining length of tunnel - 4852m


0 100 300 500(m) N190°


Piaggio (2025) present the results of deep in situ stress testing from both past and recent projects whereby higher than expected in situ stresses were measured, in particular in regions of nearby plate tectonics, and which therefore should be considered for similarly sited deep headrace tunnels for hydropower projects. Finally, Panthi (2012) reviewed the probable in situ stresses along the TBM excavated section of the headrace tunnel at the Parbati II hydropower project with a maximum cover of 1500m and concluded that elevated in situ stresses were likely present to have contributed to the cause of rockbursting.


Logistics – access/power/spoil Other important risks are related to TBM logistics including access for mobilisation, power supply and spoil disposal. Most hydropower projects are located in remote mountainous areas where existing roads may be of limited quality including bridges of limited capacity and therefore significant upgrades may be required for the mobilisation of a TBM. The power demand for TBMs can be appreciable and vary up to 10MW for very large TBM diameters and therefore an adequate power supply must be established and maintained either by a connection to an existing powerline for example from an existing nearby power station, or by the use of generators with diesel consumption. Major cost savings can be realised with a connection to an existing powerline that requires the early installation of a transformer as was done at the Pakal Dul hydropower project as presented in Figure 2. 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 centimeters up to 15cm in length which can be effectively utilised for road sub-base material or well-drained backfilling for alternative construction purposes.


Historical TBM tunnels in the Himalayas


Dul Hasti (1989) – India The 390MW Dul-Hasti hydropower project included a 10.6km headrace tunnel with a maximum cover of 1250m and was the first project in the Himalayas where a TBM was used. Construction commenced in 1989 and was expected to be commissioned in 1995 but was delayed to 2007 due to various contractual challenges. A 6.75km upstream section of the headrace tunnel was planned to be excavated using a TBM due to the lack of a practical access adit. However, only approximately a total of 2.9km of the upstream 6.75km section of the headrace tunnel was completed using an M/S Robbins 270 series open face hard rock TBM of 8.3m diameter with 432mm (17”) cutters. The actual conditions encountered were reported to be much more adverse than expected and there was the occurrence of the blow out of probe holes that resulted in inflows up to 1100 l/s with appreciable sand and silt as well as higher than expected cutter consumption in the quartzites. Figure 3 presents the longitudinal profile of the tunnel. An overall progress of only 86m/month was


achieved by the original contractor which reduced upon removal of the contractor and taking over


www.waterpowermagazine.com | November 2025 | 27


Below: Figure 5 – Inclined pressure shaft by TBM


Elevations in metre Chenab River


Chenab River


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