ABSTRACT
A study on the response of circular opening in rock mass has been made, with regard to its deformation behaviour and its effect on the support system design. The rock-support interaction forms the base upon which the procedure to predict the deformation behaviour of the rock mass has been developed. In the light of evident shortcomings in various strength criteria applicable to rocks, the one proposed by Ramamurthy et al., (1985) and Ramamurthy and Arora (1993) with consideration of joint factor has been used in the analysis. The procedure developed can be expected to yield reliable results as the strength criterion is a result of vigorous experimentation on intact and jointed rocks. The rock quality was assessed in terms of joint factor, a weakness coefficient of rock mass, which includes the combined influence of joint frequency, strength along the sliding joint and the inclination of the sliding joint. The results of the analysis can be depicted in the form of ground reaction curves and variation in radius of broken zone, which form the basic requirement for effective prediction of rock mass behaviour and subsequent preliminary support design. Parametric study involving the effect of change in parameters on the rock mass behaviour has been carried out with a view to assess the extent of support design revision (if necessary) in field conditions where the rock mass condition may vary from location to location along the alignment of the tunnel concerned.
A detailed study was carried out to bring out the influence of modulus, poisson’s ratio, insitu stress, rock quality changes in the plastic and brittle zones and their influence on brittle-ductile transition and on the extent of broken zone. The results have been analysed to cover a range of rocks having compressive strength varying from 25 MPa to 150 MPa. The insitu stress significantly influences the deformations and the extent of broken zone. The extent of linearity of ground reaction curve in the early stage of deformation is governed by pi/po = 1/(B1 + 1) which corresponds with the brittle-ductile boundary of the rock mass.
