Pinching-off and disking of rocks

Bridgman's ‘pinching-off’ experiment, in which hydrostatic pressure is applied to a portion of the length of a circular cylinder with free ends, has been repeated for jacketed and unjacketed rock materials. For unjacketed specimens, failure is found to occur at pressures somewhat greater than the tensile strength. For jacketed specimens, failure takes place at much higher pressures, of the order of the unconfined compressive strength or somewhat less. Multiple failures into several pieces commonly occur. According to elastic theory, quite high tensile principal stresses can occur near the discontinuity in applied surface pressure, but failure is not always related to these. Experiments were also made in which a piece of core was left projecting in an axial drill hole in a cylinder which was stressed perpendicularly to its axis. It was found that the projecting core broke off before general failure occurred; this break took place when the average load stress over the cross section was of the order of, but less than, the uniaxial compressive strength. These observations are relevant to the breaking up of core into disks when diamond drilling into highly stressed ground is being done. Experimental drilling in the laboratory shows that such disking does occur and that the thickness of the disks diminishes as the stress increases.