Work on the first tunnel boring machine (TBM) driven tunnel in Nepal is progressing well with the Bheri Babai Diversion Multipurpose Project reporting an advance of over 1km in a month.
China Overseas Engineering Group Co (Covec) is using a Robbins Double Shield TBM to drive the water tunnel through the Himalayas for the Government of Nepal’s Department of Irrigation.
The TBM was launched in summer last year and has averaged an advance rate of 800m before setting the new record.
The use of a TBM is a departure from the normal drill and blast techniques used in Nepal but early studies identified that it would take 12 years to drive the 12.2km tunnel using conventional approaches.
The tunnel is located in the Siwalik Range, which is part of the Southern Himalayan Mountains, where geology consists of mainly sandstone, mudstone, and conglomerate.
Covec general manager Wang Wu Shui said there are several factors that have contributed to the good progress on the project. “In China, there is a proverb about TBM construction: ‘geology is the premise, equipment is the foundation, and talents are the key’,” he said. “The great advance rates achieved at present mainly lie in preliminary planning, process control, and professional construction personnel.”
According to Wu Shui, machine maintenance occurs daily at a fixed time to ensure the best TBM performance and to prevent downtime. He said that geological engineers are sent to analyse the ground conditions twice daily so that construction personnel can adjust the tunneling parameters and prepare for auxiliary measures if geological changes are predicted.
The ground conditions during the record-setting month consisted mainly of sandstone and mudstone, but that is set to change. At about the 5.8 km mark, the machine will encounter a major fault zone known as the Bheri Thrust. Clay and water ingress are expected throughout the fault, which is about 400m to 600m wide.
Once completed the new tunnel is expected to provide water to irrigate 60,000ha of land and will also be used to generate hydroelectric power.