Constitutive modelling of jointed rock masses is an important aspect while analysing the structures founded in or on the rocks. The model presented by Huang et. al (1995) has been a endeavour in this direction. The model uses the Normal and Shear stiffnesses of the joints, their orientation and spacing in addition to intact rock properties to model the stress- strain behaviour of the mass. In the present article the model is further modified to take into account the variation in the stiffnesses of the joints at various levels of the applied stress. It is asserted that the stiffnesses depend on the ratio of applied stress and the uniaxial compressive strength of the jointed mass. An experimental programme was planned and executed to validate the suggested model. Cement-sand mortar was used to simulate the rock, and joints were formed by inserting the drawing sheets at proper spacing and orientation while casting the specimen. The spacing between the joints was kept as 25 mm and the orientation was varied from 0° to 90° with direction of loading. The normal and shear stiffnesses were determined by tests performed on the joints. A comparison of the experimental and theoretical results indicates that the modulus of deformation of the mass is under estimated if dilation of the joints is neglected. The stress-strain behaviour can be reasonably predicted if dilation is taken into account except for the situations where the joints are either perfectly parallel or normal to the loading direction. The dilatancy factor is also found to have good correlation with the orientation of the joints.
The assessment of deformability of jointed rock masses is an important and difficult problem while designing structures founded on rock masses. The best solution to estimate the field Values would be in-situ testing, which is very time consuming and expensive. Moreover these tests will represent very small volume of rock mass and their applicability at other locations is always doubtful. Alternatively the approach of constitutive modelling can be used. In this approach the elastic properties of intact rock and joints along with the geometrical properties of the joint surface are used to model the deformability of the whole mass. Huang et al (1995) discussed a model to estimate elastic moduli for jointed rock masses. The model has been developed with three sets of non-orthogonal intersecting joints and considers the effect of dilatation on deformability. Similar model has been discussed by Li (2001) for single set of joints and without considering the effect of dilation. In the present article this model is modified to take into account the variable nature of both the normal and shear stiffness and also to account for dilation of the mass.
The normal stiffness of the joints can be determined in the laboratory by loading them normal to their plane and then plotting the normal stress against the closure of the single joint. It is observed that the stress required to cause unit closure increases exponentially with increasing deformation.