By Rose Bell
Considering that the 6.4m diameter Robbins double shield TBM broke through the challenging geology on May 8 in Nepal’s Sindhuli district, this machine on the Sunkoshi Marin Diversion Multipurpose Project in Nepal has started its drive 11 months ahead of schedule.
As one of the longest TBM-driven tunnels through the Himalayan mountain range, this TBM is 13.3km long and is only the second bored tunnel to be completed in the country.
While the ground conditions included two major fault zones with challenging and varied geology ranging from highly jointed mudstone, sandstone and conglomerate to quartzite and granite, the tunnel was bored through mostly igneous rock with a maximum overburden of 1,320m.
Due to the geology of the path, there were multiple instances of the shield being trapped over the tunneling process that required the construction of bypass tunnels to free it, so that one incident 4km into tunneling took 27 days to free the machine from collapsing and squeezing ground.
Despite the challenges, the crew made up the time with swift advance rates, including a record 1,503.3m in one month and 72.6m in one day in March 2024.
The distinguished design of this machine is TBM’s success reason, whereas the machine was originally used with a 5.06m diameter on Nepal’s first TBM-driven tunnel, the Bheri Babai Diversion Multipurpose Project, and incorporated many of the components. It was upgraded to the larger diameter and new front shield components were assembled on site.
According to Robbins president Lok Home: “Contractor B-2/COVEC and Robbins had successfully introduced TBM technology in Nepal. This country needs more tunnels. To have these good performances on these first two projects is really a good thing for the country and our industry.”
While this tunnel links to a new 28.6MW surface powerhouse on the Marin River, it is due to alleviate electricity shortages in the area and provide farmland irrigation in the districts of Dhanusha, Mahottari, Sarlahi, Rauthat and Bara in the Terai Plain by diverting water from the Sunkoshi River to the Marin River.
