3D numerical modelling studies were carried out to study the behaviour of large twin caverns belonging to a hydroelectric project in Himalayan region. Various stages of extraction of the caverns were characterised by instrumentation studies. The cavern is oriented along the major principal stress direction. Numerical model results were in close agreement with the measured values in the field. Taking this model as the base, parametric studies were carried by varying the angle between cavern axis and direction of major principal stress. Alignment of the caverns were changed at an interval of 15°. Analysis was done for 12 orientations upto an angle of 165°. Maximum stress concentration factors for all orientations were obtained on the upstream side of powerhouse cavern, in the pillar between powerhouse cavern and transformer hall cavern and on the downstream side of transformer hall cavern. Similarly, the displacements in the walls of twin caverns were compared for all orientation of the caverns.
Excavation of large caverns in India and neighbouring countries is on the rise to meet the demands of large underground space requirements of hydro power projects, oil and gas storage, pump house and surge chamber for irrigation purpose etc. Stability of large underground caverns revolves around interaction of surrounding rock mass with the cavern and its support elements. There are two factors, fixed one such as geological setup, in-situ state of stress and variable factors such as shape, size and orientation of cavern, existence of other caverns in the vicinity, method of excavation and finally support elements provided to aid its interaction with the surrounding rock mass, that determines the stability of the cavern.
Optimum orientation for a cavern is the direction which utilizes the rock arching action to the maximum extent. Indian standard, IS 9120–1979 states that a machine hall cavity (main cavern in a hydro project) may be aligned on the basis of an optimum compromise between the direction of the ruling strike and the direction of such features so as to ensure that inferior rock formation is confined to the shortest dimension of the cavity. Empirical guidelines for orientation of the caverns are based on either the joint characteristics or in-situ stress orientation.
Joint characteristics can have major influence on the orientation of caverns. For caverns with long and high walls, it is important to have an angle of at least 25° to the strike of steeply dipping discontinuities. It is necessary to carry out a detailed survey of the bedding or foliation and the jointing of the rock mass so that optimization of the direction of the excavation axis with respect to joint orientation could be done. For openings situated at shallow or intermediate depths, the longitudinal axis of the cavern is ideally oriented along the bisection line of the largest intersection angle of the strike of the two dominant sets of discontinuities (joints, bedding or foliation). Close alignment with any further joint sets may be avoided, so as to reduce the extent of potentially unstable rock (Jack and Parry, 2015).