Commissioning the 6.4m diameter Robbins double shield TBM by China Overseas Engineering Group Co Ltd and China Railway No 2 Engineering Group Co Ltd in the previous month, it has excavated over than 500m of the 13.1km headrace tunnel for the Sunkoshi Marin Diversion Multipurpose Project in southern Nepal and tunneling process in this project is advancing.
With a weekly performance of 224.6m and a daily of 36.6m performance, it has recorded a best efficiency.
In order to improving electricity supply in the area, the tunnel is linked to a new 28.6MW surface powerhouse on the Marin River. It is due to 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 geological condition of headrace tunnel’s path comprises of granitic rock with maximum overburden of 1,320m, while it 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.
According to Liu Fengfan, project manager for contractor B-2/COVEC: “The machine was customized 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.”
Liu also added: “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.”
The date of reaching to the major fault zone will be around June next year.
Considering the experience of digging 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, this TBM has a good track record.
“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.