Measurement and Analysis of Pore Water Pressure in Large Caverns and Shafts in a Hydro-electric Project

With an objective to understand the behaviour of pore water pressure around large underground openings, piezometers were installed in the rock around the desanding chambers, surge shaft, its drainage galleries, and the transformer cavern of a large hydro-electric project. The monitored data indicated that the pore water pressure was insignificant in the rock surrounding the desanding chambers prior to lining. On completion of the concrete lining of the chambers, the maximum pore water pressure was about 50 kPa as the flow of water was restricted. When one of the desanding chambers was charged with water, the piezometers around it indicated an excessive increase in the pore water pressure due to seepage of water from the chamber. Thus the piezometer readings were useful in taking appropriate remedial measures. In the surge shaft, higher values of pore water pressure were recorded on the hill side of the shaft as compared to the valley side. In one of the drainage galleries, the pore water pressure gradually increased to 450 kPa, which was released by drilling relief holes in the area. In the other drainage gallery, no significant change in the pore water was recorded. In the transformer cavern, the recorded pore water pressures were within 20 kPa.