Tunnelling Challenges and Solutions through Adverse Geological Conditions. An Experience from Head Race Tunnel Excavation of 720 MW Mangdechhu Hydroelectric Project; Bhutan

The 720 MW Mangdechhu Hydroelectric project is under construction in Trongsa district of Bhutan. The project envisages construction of 112m high concrete dam across Mangdechhu river to divert flow through 6.5m diameter and 13.5209 km long Head Race Tunnel located along the left bank of the river to an underground Power House Complex for generation of 720MW of electricity.

The 6.5m diameter, 13.5209 km long, horse shoe shaped HRT is being excavated through diverse tunnelling media which comprises of high cover zones where superincumbent cover ranges from 750 to 1000m resulting in squeezing and mild rock bursts, water charged strata, where heavy ingress of water resulted in deterioration of rock mass thus inducing dilation in the crown and ultimate collapse and poor rock mass conditions were loose falls and cavity formations were common. Such challenging zones were successfully tackled by the MHPA by employing advance and innovative approaches comprising of pre-grouting, pipe roofing, fore polling, timely installation of appropriate and adequate support system, inferring anticipated tunnelling conditions ahead of face by probe hole drilling for advance planning of resources and tunneling methodology, using modern and high capacity tunnelling equipments such as 2 and 3 boom jumbos, high capacity dumpers, loaders, high pressure grout pumps and shotcrete machines, with emphasis on continuously reduction of cycle time till an optimum cycle time is achieved and adhered too for different tunnelling conditions.

Using advanced tunnelling methodology and machines and optimizing cycle time, timely excavation of the tunnel is being ensured for scheduled commissioning of the project, despite of considerable variation in anticipated tunnelling conditions as per DPR and as encountered.

This paper exclusively deals with the excavation of HRT through challenging strata and solutions devised to overcome it by following modern tunnelling methods to achieve scheduled excavation while also ensuring stability of the structure.