Nepalese headrace tunnel makes good progress

22 November 2022

Tunnelling is progressing on the 13.1km headrace tunnel for the Sunkoshi Marin Diversion Multipurpose Project in southern Nepal.

Since China Overseas Engineering Group Co Ltd and China Railway No 2 Engineering Group Co Ltd launched the 6.4m diameter Robbins double shield TBM last month it has bored more than 500m. It recorded a best week of 224.6m and a best day of 36.6m.

The tunnel is connected to a new 28.6MW surface powerhouse on the Marin River to improve electricity supply in the area. It will provide farmland irrigation in the districts of Dhanusha, Mahottari, Sarlahi, Rauthat and Bara in the Terai Plain, diverting water from the Sunkoshi River to the Marin River.

The headrace tunnel is being bored through granitic rock with maximum overburden of 1,320m. Geological conditions may include squeezing ground, fault zones and water inflows. In particular, a major fault zone is located approximately 4km into the tunnel at Dhanamana Khola.

Liu Fengfan, project manager for contractor B-2/COVEC, said the machine was customised for the conditions.

“The TBM shield is a tapered design to reduce the risk of becoming stuck in squeezing ground; there is an enclosed cutterhead design to reduce the collapse of surrounding rock as well. The TBM has overcut capabilities and a high thrust. It has also been designed to be equipped with an extra high torque cutterhead drive, as well as strong auxiliary thrust to be used in squeezing ground or weak fault zones to keep the machine from becoming stuck,” he said.

The major fault zone will be reached in around June next year.

“We have prepared the advance geology forecast along with technical measures and equipment to cope with the extraordinary geological conditions. These include advance drainage holes, pre-excavation grouting (both ordinary and chemical), and other methods based on our experience.  When we reach the major fault zone at Dhanamana Khola we will go slow, with more probe drilling while in the fault zone,” said Liu.

The TBM has a good track record, having bored a 12.2km tunnel for the Bheri Babai Diversion Multipurpose Project (BBDMP), where it finished nearly one year early and achieved a 1,202m advance in one month.

“The Robbins TBM at BBDMP successfully broke through the 12.2km tunnel and overcame extraordinary geological conditions especially in fault zones. Its stable and excellent performance and function have been verified very well,” said Liu.