Parametric studies are performed using the distinct element computer program UDEC-BB for a large cavern in the Himalayas. The studies provide insights into some important deformation mechanisms in a numerical system of blocks. The sensitivity studies involving changes in joint spacings (block size) revealed that the deformations around an opening are dependent on the size or the number of blocks adjacent to the excavation. Large size blocks deform mainly through translational shear upon which rotational shear may occur. In a model in whieh the block size is small compared to the tunnel dimensions, the tendency to dilation across an increased number of non-planar joints may contribute to interlocking of blocks. The BB joint behavioural model which allows the modelling of the dilation accompanying shear and hence the build-up of higher normal stresses, predicts smaller deformations than the Mohr-Coulomb model in which the dilation angle is constant. The sensitivity studies involving changes in key BB joint shear strength parameters have shown that, within the range of values considered, the model output is relatively insensitive to assumed joint strength parameters joint roughness coefficient and (at. However, an increase in the joint wall compressive strength contributes to an increase in the joint shear strength resulting in a marked reduction of displacements around the opening.