Unconfined compressive strength
UCS (also frequently named C0)Critical stress value at which a rock body fails brittlely under compresion. It may be determined directly via a uniaxial test under unconfined compressive conditions (σ1>>σ2=σ3=0 MPa) or by a series of triaxial tests (σ1>σ2=σ3≠0 MPa). In the latter case it is necessary to determine peak strength at several confining pressure stages (either via various single stage tests or via one multi stage experiment) and to draw a regression line through all peak strength data points to unravel the peak strength at 0 MPa as the UCS. This indirect way to determine UCS yields higher stress values than the direct measurements due to the fact that during confining pressure buildup in triaxial deformation tests microfractures are closing which will remain open during an unconfined compressive test. Consequently, only indirect UCS determination gives data for intact rock and only those data can be used for comparison with other data for the same rock type. Rock UCS values roughly range from 1 MPa (weakly consolidated sandstone) to 350 MPa (granite and basalt).