An increasing need to determine the in-situ stress state through indirect methods such as core disking has spawned much research over the past few years. While laboratory and field work have brought knowledge to the topic, many issues associated with these approaches have led to a reliance on numerical models for analysis. It is imperative that these numerical methods replicate the failure process and disk thicknesses, which are then related to a component of the in-situ stress state.</DISS_para>
<DISS_para>The constitutive model of tensile softening was used to both replicate previous laboratory experiments and match the general relation between disk thickness and major principal stress. The failure process was analyzed in detail for various stress magnitudes along with size effect. Also, laboratory experiments involving the excavation of cores in a stressed rock were performed to investigate the effect of core damage on measured elastic parameters. It was determined that when the vertical stress σv = 0 and horizontal stresses σH = σh = 75 - 85% Co, where Co = uniaxial compressive strength, Young's modulus can decrease about 10% from the coring process.