Mechanical and Thermomechanical Behaviour of Lac Du Bonnet Granite: Some Laboratory Observations

Abstract

Mechanical and thermomechanical rock property tests have been carried out on Lac du Bonnet granitic samples. These tests have provided design data and allowed data interpretations for geosphere modelling, conceptual vault engineering and in situ experiments associated with the Canadian Nuclear Fuel Waste Management Program (NFWMP). The results indicated the following trends with increasing depth: decreased uniaxial compressive strength; decreased tensile strength; decreased Young's modulus; decreased zero-pressure compressive wave velocity; increased Poisson's ratio. The observed trends compared with a slight compositional variation of the Lac du Bonnet granitic pluton with depth. Altered pink granite samples from the upper zones of the formation were stronger in compression and deformed less than the slightly altered greenish grey, or grey granitic samples from the deeper horizons. It is felt that this behaviour can be attributed to stress relaxation in the grey granites after drilling. The strength and deformational properties of the rock samples were relatively unaffected by increasing temperature to 200 degrees C, except at low confining pressures (below 5 MPa). The apparent thermal expansion of the rock samples occurred in multiple stages over the temperature range of 25 degrees C to 200 degrees c. Apparent expansion is lower during the initial stages of heating and may not be detected under in situ conditions if large fractures are present.