Underground construction | Setting records in Nepal
Deep within the southern Himalayan mountains of Nepal, a Robbins Double Shield TBM sets a new TBM record within the Indian Subcontinent for the Sunkoshi Marin Diversion Multipurpose Project
Right: The Robbins TBM is boring the 13.3 km headrace tunnel for Nepal’s Sunkoshi Marin Diversion Multipurpose Project
Below: At the end of June 2023, a 6.4m diameter Robbins Double Shield TBM achieved an impressive advance rate of 1,224.13m in one month
IN THE SOUTHERN HIMALAYAN mountains of Nepal, a Robbins Double Shield TBM has crossed into new territory. At the end of June 2023, the machine turned in an impressive advance rate of 1,224.13m in one month – a record for Himalayan geology and believed to be a TBM record within the Indian Subcontinent. Multiple factors played into the successful result.
“Proper TBM selection for the existing subsurface is the key reason for success. The Double Shield is the most logical TBM type and has played a major role in this successful journey. Operation, maintenance, and an experienced, dedicated team are additional factors,” said Mitra Baral, Project Director of SMDMP under the Department of Water Resources and Irrigation, part of Nepal’s Ministry of Energy, Water Resources & Irrigation. Contractor Covec/B-2 echoed those sentiments:
Below: The Robbins TBM, now more than 8.3km into its tunnel, achieved a record for tunneling in Himalayan geology, as well as the Indian subcontinent
“The designed adaptation of the TBM & backup provided by Robbins is verified to be scientific and good under the geology, which are Siwalik stratum and the lower Himalayan zone--both quite complex and variable. The performance of the equipment is generally stable,” said Covec/B-2’s Project Manager, Liu Feng Fan.
Headrace tunnel Launched in October 2022, the 6.4 m diameter
machine has been boring the 13.3 km headrace tunnel for the Sunkoshi Marin Diversion Multipurpose Project (SMDMP). Contractor B-2/COVEC (China Overseas Engineering Group Co. Ltd and China Railway No. 2 Engineering Group Co Ltd) have navigated the Robbins TBM through two major fault zones with challenging and varied conditions ranging from highly jointed mudstone, sandstone and conglomerate to quartzite, granite, and more.
18 | October 2023 |
www.waterpowermagazine.com
The record result is all the more impressive considering the adversity the machine has overcome to date. Approximately 2,156m into the excavation the machine became stuck in jointed sedimentary rock, requiring nine days of intensive work and a bypass tunnel to free it. As the machine progressed to more than 4km in, the geology changed to dolomite, slate, schist, quartzite and phyllite. The machine became stuck a second time 4,669m in due to collapsing and squeezing ground, requiring a further 27 days of work to release it. Despite the challenges early on, the machine and
its experienced crew have persevered. Currently the TBM is more than 8.3km into the tunnel and boring in high strength granite, having passed through the two major fault zones predicted on the tunnel alignment. “Timely support from Robbins has made for a good relationship and teamwork. It has been quite important for the success of our project,” said Liu. Prajwal Man Shrestha, General Manager of Robbins Nepal, emphasized that the successful results thus far are a milestone for the use of TBM tunneling in the country: “There are now private sector hydropower developers considering the use of a TBM for their project instead of the conventional D&B method. Very soon we may see that most tunnels longer than 6 or 7 km will be exploring the possibility of using TBMs on their project.” As for the future of tunneling in Nepal, Baral is
positive it will continue to grow and evolve: “Many TBM projects could be planned for many different areas. Taking into consideration that Nepal is a mountainous country with complex geology, tunnels are indispensable not only for irrigation and hydropower but also roads, railways, water supply, and even metro lines in the cities.”
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