A 6.4m diameter Robbins Double Shield TBM overcome challenging geology to cross the finish line 11 months ahead of schedule in May 2024.
At 13.3 km in length, Nepal’s Sunkoshi Marin Diversion Multipurpose Project (SMDMP) is one of the longest TBM-driven tunnels through the Himalayan mountain range and is only the second such tunnel to be completed in the country.
Liu Fengfan, SMDMP Project Manager for contractor B-2/COVEC, said the TBM’s design and performance meant it had adapted well to the Himalayan geological conditions.
Ground conditions included two major fault zones with challenging and varied geology ranging from highly jointed mudstone, sandstone and conglomerate to quartzite, granite, and more. The tunnel was bored through mostly igneous rock with maximum overburden of 1320m.
The TBM and its experienced crew were able to persevere through multiple instances of the shield becoming trapped with bypass tunnels required to free it, including one instance 4km into tunneling that required 27 days’ worth of work to free the machine from the collapsing and squeezing ground.
TBM has swift advance rates
Despite these challenges, the crew was able to more than make up the time with swift advance rates including 1503.3m in one month and 72.6m in one day, both project records set in March 2024.
The refurbished TBM was originally used at a smaller diameter on Nepal’s first TBM-driven tunnel, the Bheri Babai Diversion Multipurpose Project. The design incorporates many of the same components of that original machine, and the process involved final assembly on location at the jobsite.
“One of the highlights is that this machine integrates the overall structure of the 5.06m diameter TBM from the Bheri Babai project. After upgrading to 6.4 m diameter, all new front shield components were not assembled in the factory. Instead, we adopted onsite first time assembly This was very effective, and I find it particularly noteworthy,” said Jin Haikuan, Deputy Project Manager & TBM Production Manager for B-2/COVEC.
With tunnelling complete, the work of providing both power and water can begin. The tunnel is connected to a new 28.6MW surface powerhouse on the Marin River to alleviate the shortage of power supply in the area. It will also be used for farmland irrigation in the districts of Dhanusha, Mahottari, Sarlahi, Rauthat and Bara in the Terai Plain. This will divert water from the Sunkoshi River to the Marin River for the project owner, the Department of Water Resources and Irrigation under Nepal’s Ministry of Energy, Water Resources and Irrigation.
The project’s successful completion will not only benefit local communities but also the future of tunneling in Nepal.
“B-2/COVEC and Robbins have successfully introduced and spread TBM technology in Nepal and shown the advantages of it. This country needs more tunnels,” said Robbins President Lok Home at the breakthrough ceremony. “To have these good performances on these first two projects is really a good thing for the country and our industry.”
