E is a key figure for the description of rock deformation behaviour. It quantifies the material stiffness under uniaxial conditions with S11 (in the case of a fluid saturated porous rock σ11) as the only non-zero stress. Nevertheless, it can be determined with any kind of compressional experiment, may it be uniaxial and unconfined (UCS test) or triaxial (so called single-stage and multi-stage experiments) using cylindrical test plugs with a length to diameter ratio of 2:1. Young’s modulus is determined as the gradient of the load path in a stress-strain (or σ-ε) diagram of an unloading-loading hysteresis carried out as soon as the stress-strain curve of the experiment has reached the linear part. The corresponding equation is Hooke’s law applied to the axial stress and axial strain:
E = ΔS11/Δε11 (modified from Zoback 2007).
Young’s modulus usually is given in GPa. Brittle, strong rocks (granite, basalt, gneiss) have a high Young’s modulus in the range of 50 GPa to 100 GPa, weak, (semi-)ductile rocks (poorly consolidated sandstone, mudstone, some shale and marble) have a low Young’s modulus of 0.1 GPa to 50 GPa.