PERFORMANCE OF OVERSCORING STRESS MEASUREMENT DEVICES IN VARIOUS ROCKTYPES AND CONDITIONS

The U.S. Bureau of Mines deformation gauge, the Australian CSIRO hollo w-inclusion cell, the South African CSIR triaxial cell and the Univers ity of New South Wales solid-inclusion cell have been tested in the la boratory under simulated field conditions. The four borehole stress me asurement devices were embedded in parallelepiped-shaped blocks of nat ural rock, in blocks of bedded mortar and in different concrete mixes to simulate various degrees of homogeneity and anisotropy. The devices were overcored under a calibrated pressure and changes in strain were measured continuously with a temperature-compensated bridge. The resu lts indicate that Young's modulus and Poisson's ratio need to be caref ully determined from the cores and that the nature of the cell bond sh ould be checked. The manner in which the cell acts affects its perform ance. Whereas all the cells performed to an accuracy of 10% in continu ous and homogeneous rock, none performed well under thinly bedded cond itions because of the development of cracks in the overcore. Inhomogen eity affected the point-contact devices. In mildly bedded rock the mea n value of the combined readings of the U.S. Bureau of Mines gauge and the inclusion cells was close to the true value.