MECHANISED TUNNELLING | TECHNICAL
On the Himalayan mountains, Nepal is developing a multipurpose project that is to serve community and economic needs for food and energy in the form of the Sunkoshi Marin Diversion Multipurpose Project (SMDMP). SMDMP is gathering water to use for irrigation as
well as hydropower generation. Water will flow to the surface power station and also crop fields in the districts of Dhanusha, Mahottari, Sarlahi, Rauthat and Bara in the Terai Plains. The multipurpose development has been declared a
National Pride project by the Nepalese Government. To convey the water, a key part of the project
infrastructure is a headrace supply tunnel, which is 13.1km long. It will convey water from the Sunkoshi River via
hydraulic structures, including a concrete diversion weir and also a desander to remove silt, to the 28.6MW power plant on Marin River. A joint venture Chinese contractor is undertaking the
works. The JV of China Overseas Engineering Co and China Railway No2 Engineering Group Co was awarded the contract in early 2021. A Robbins 6.4m-diameter double-shield TBM –
previously on the Bheri Babai Diversion Multipurpose Project – was selected to bore the headrace tunnel. Main tunnel works on the project began in late 2022 with the launch of the machine in late 2022. During the early months of tunnelling, the TBM
achieved a best week of 224.6m advance and the best day saw 36.6m of progress. But, in subsequent months, there came challenges later as well as good progress in the varied Himalayan geology under maximum overburden of 1320m. It was recognised that geological condition could
include squeezing ground, fault zones and groundwater inflows. The TBM design has a tapered shield, enclosed cutterhead, overcut capability and high thrust to reduce risk from squeezing ground. Robbins says challenging and varied conditions
with geology ranged from highly jointed mudstone, sandstone and conglomerate to quartzite, granite, and more. Two major fault zones were encountered and crossed early on. A further challenge also came early on, about 2150m
into the drive, when jointed sedimentary rock halted the machine. A bypass tunnel was mined and the TBM was freed after nine days. Slightly more than that distance farther on – and
after passing through dolomite, slate, schist, quartzite and phyllite – the double-shield became stuck again, at almost 4670m into the drive. Collapsing and squeezing ground held the machine and this time it took three times as long – 27 days – to be free. But despite the challenges, the contractor has made
otherwise good progress. By mid Q3-’23 the TBM was more than 8km into the drive and had achieved a best month, in June, of 1224m advance – beating the Bheri Babai best month of 1202m. Robbins believes it is a record for the regional geology and TBM record on the Indian subcontinent.
January 2024 | 19 Left:
Robbins double-shield TBM is progressing well in Nepal
Far left:
TBM preparing to get underway more than a year ago
IMAGES COURTESY OF ROBBINS
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