Temperature and thermal moisture stress calculations are reported for different cavity shapes in which the walls are subjected to short time perturbations of surface thermal boundary conditions. The stress field found by adding these perturbation stresses to the overburden stresses are then used to estimate the probability of surface cracks penetrating into the cavity walls, leading to surface spalling and cavity growth. The paper discusses some preliminary results for tensile failure due to heat sources located on the cavity floor or in a central plume. Directions of future studies are also described.