ANALYSIS OF CAVERNS IN SQUEEZING GROUND CONDITION BY CONVERGENCE CONFINEMENT METHOD

Abstract

Out of all river valley projects that have been completed over the decades or are proposed, a large number of these belong to Himalayan region. As it is well known that this region contains a very huge hydro potential, engineers have tried since long to utilize this in order to generate required head for power generation. For the same purpose many hydropower projects have been developed. In a hydropower project, underground excavation are are of most importance. They are used in the conduction system in the form of tunnels, pressure shaft, drift galleries and most importantly large power house cavern. Construction of large underground powerhouse has been a challenging work to be carried out in Himalayan region attributed to complex hydro-geological conditions, seismic activities, non homogeneous and anisotropic nature of rocks. Support problem is the most important problem faced by designers. Chibro Underground Powerhouse complex is situated in lesser Himalayan region. It is one of the first ventures of its type in lesser Himalaya. It is located in stratified bed of limestone, closely jointed and having water charged regions. Analysis of cavern for normal as well as squeezing ground condition is performed using finite element method in Phase2 module. Two combination of support systems namely rockbolts with shotcrete and multi strand cables with shotcrete are used for both ground condition. Effects of changing the spacing of supports across the cavern boundary on cavern behaviour are tabulated. As the density of support network is increased, deformations are reduced and stability is reduced. Comparison of results for two type support systems is made for both ground condition. It is inferred from the analysis that when cables are used in place of rockbolts, yielded material reduced. Cables of lower stiffness in comparison to rockbolt results in same displacement with more grip length with less normalized closure. GRC and LDP are plotted for top heading of cavern. The GRC curve is plotted using a plane strain model by internal pressure reduction method. In this method, the internal pressure is reduced from field stress to zero in 10 stages. The ground is deformed continuously till this internal pressure reaches zero. LDP curve is plotted using the method suggested by Vlachopoulos and Diederichs. It is plotted with the help of two finite element analysis results: (a) maximum deformation obtained in plane strain model and (b) radius of plastic zone.