Prediction of Ground Behaviour for Rock Tunnelling

Studies have shown that the ground behaviour for rock tunnelling depends on the rock mass strength and the in situ stresses. The rock mass strength, to a considerable extent, is governed by the presence of joints/discontinuities and their properties. Three aspects concerning the joints are primarily responsible for the reduction in the strength and are measurable in the field: (i) Joint frequency or number of joints per metre length in the direction of loading (J_n), (ii) orientation of critical joint set with respect to loading direction (β), and (iii) shear strength along the critical joint set (r). These three factors have been combined into another rock mass parameter, namely, the joint factor (J_f). Thus, joint factor has been selected as one of the parameters for developing the empirical correlation for predicting the ground behaviour. In addition to joint factor, the tunnel size and the tunnel depth are the other two involved parameters. Details of data and the process of developing the empirical correlations are discussed in the paper. The proposed empirical correlations for non-squeezing and squeezing ground behaviours have been validated by comparing the results with the observed data obtained from various sections of one Himalayan tunnel recently excavated in the state of Jammu and Kashmir in India.