Monitoring the stability of hydro-caverns with rock mechanics instrumentation - a case study

Abstract

The Ghatghar pumped storage hydro-electric project is under construction in the Thane district of Maharastra state in India for the generation of 250MW of electricity. The scheme consists of two underground caverns, a machine hall, a transformer hall and intersections. The interconnections between the caverns and connections to the surface, as well as to the reservoir, are made through a network of tunnels. The rock mass is predominantly of basaltic formation. The joints are mostly tight and undulating with generally rough to very rough contact surfaces. The rock mass characterization was conducted using Bieniawski’s RMR (1979) and Barton’s Q. The associated values vary from 66 to 81 and from 5.9 to 25, respectively—which indicate very good rock mass conditions. The support requirements for these structures were estimated from empirical approaches and numerical modeling. The support system consists of 6m long fully grouted rock bolts at 2m grid spacing with 50mm thick SFRS for the two caverns and 7m long rock bolts around the intersections of all openings. An extensive instrumentation scheme was implemented in these structures as a means to evaluate the efficacy of the support system and to have detailed knowledge of the rock mass behavior during and after construction. The instrumentation consisted of: tape extensometers for measuring convergence; multi-point and single point borehole extensometers for radial displacements; load cells for rock load and strain meters for rock strains. These instruments were embedded during the process of cavern excavation. The rock mass deformations around the cavern openings, as indicated by the MPBXs (Fig. 1), the convergence measured by the tape extensometer, and the rock bolt tension measured by the load cells were too low to be significant and collectively indicate that the roof and the walls of both the caverns and of the bus ducts and passageways are stable, indicating that the support system is adequate. It was, however, suggested that the vulnerable wedges should be identified on account of the jointing; these would need to be stitched to the rock mass and the wedge stability be monitored by site specific arrangements. This paper presents in detail the instrumentation scheme implemented at this project plus the analysis of the monitored data, highlights the stability status of the caverns, and recommends the future strategy for monitoring.