High temperature water seepage in head race tunnel of 1500 mw nathpa jhakri hydroelectric project – A case study

Himalayas are the youngest fold mountains in the world presenting fragile geological conditions. In recent years extensive tunnelling is taking place in these mountains, may it be for hydroelectric projects or for railway and road tunnels. Typically in the Himalayan Region about 10–20% of tunnelling can be expected to pass through very good and good rock mass and 35–45% each in fair and poor/very poor rock masses. About 5–10% rock mass encountered can be expected to fall in the extremely poor rock class, i.e., shear zones comprising crushed/fractured rock material along with clay gauge having very low strength which is easily crushable with hands and squeezing in nature. Often, this fragile material in shear zones is accompanied by heavy ingress of water. At some locations water at high temperature can be encountered where geothermal conditions exist. Hot water ranging in temperature from 35°C to 40°C was encountered, though in small quantity, at number of places during excavation of HRT of Nathpa Jhakri Project but it did not cause any serious problem. However, while excavating HRT through one of the adits, hot water with temperature 52°C to 53°C and in large quantity (60lit/sec) suddenly appeared at one location in January, 1995 increasing total discharge of hot water to 100 lit/sec. Excavation work of HRT through this adit continued for more than 2 years till August 1997 with the temperature of water and rock varying from 52°C to 66°C. The paper describes how the problems faced during tunnel excavation through hot water seepage zone were overcome and what steps were taken to make lining safe against corrosive action on concrete lining and reinforcement.