Biaxial tests of some overcored samples from AECL's underground research laboratory, Lac du Bonnet, Manitoba

Abstract

In support of Atomic Energy of Canada Limited's in situ stress measurement program at the Underground Research Laboratory, biaxial tests and biaxial tests with axial load were conducted at the Canada Centre for Mineral and Energy Technology (CANMET) to examine the effect of axial and side loads on the strains measured on the inside and on the outside of overcored rock samples obtained in the URL and the elastic constants derived from them. The rock is a massive, grey, medium to coarse grained porphyritic granite. With axial loading, cell pressures as high as 36 MPa could be applied on the overcored specimens without causing disking. Without axial loading, a cell pressure as low as 13 MPa could cause disking. There were fairly good agreements between the secant Young's modulus values obtained on the outside and on the inside of the hollow rock specimens in both the biaxial tests and the biaxial tests with axial load. The secant Young's modulus values in the axial direction were found to be approximately 10 GPa higher than the modulus values in the circumferential direction, indicating that the rock specimens were highly anisotropic and the anisotropy was probably caused by the microcracks present in the specimens. The secant Young's modulus values obtained in the biaxial tests with axial load at pressure levels above 28 MPa were very high. Selected uniaxial compression tests should be conducted on the overcored specimens to check the effect of end constraint on the modulus values as well as to shed some light on the discrepancies in some of the axial strain readings.