EXPERIMENTAL STUDY OF THE STRENGTH OF ROCKS
An experimental investigation of the homogeneous Solenhofen limestone and a number of other rocks was made to determine their their strengths while under moderate hydrostatic pressure at room temperature. The other rocks were fossiliferous limestone, shaly limestone, marble, granite, diabase quartzite, slate, soapstone, verde antique, and sandstone. The minerals tested were pyrite, quartz, microcline, and fluorite. Three types of experimental procedures were used: compression of solid cylinders, crushing of hollow cylinders, and punching of disks. The specimens were all carefully shaped right circular cylinders, and during the tests were jacketed to prevent penetration by the pressure fluid. The hydrostatic pressures ranged from 1 atmosphere to 4000 kg/cm 2 . Measurements were made to obtain strains and stresses developed in each specimen for deformation to the point of rupture or into the plastic range; the duration of the tests was 1–8 hours. All rock specimens exhibited a range of elastic linearity of stress with strain. Under moderate hydrostatic pressures, the limestones and marbles could be made to flow plastically to large deformations, and some heating experiments on limestone demonstrated an increase of plasticity with heating. None of the silicate rocks and minerals exhibited any plastic behavior. The experimental data were examined with reference to various criteria of failure. The maximum shear-stress criterion was found to be reliable in predicting the yield point for the limestones, and was found to be an approximate guide to failure of limestone by rupture. The silicate rocks failed by rupture, and their rupture strength was increased by hydrostatic pressure. A rough, empirical criterion of failure was found for rupture of silicate rocks, namely, linearity of maximum shear stress with mean stress.