Thermally induced shear reactivation of critically-stressed smooth and rough granite fractures

Abstract

Many candidate geological repositories have been reported to have relatively high ratios of in-situ horizontal stress to vertical stress. There can be a substantial increase in the horizontal stress due to the thermal stress resulting from radioactive decay, and accordingly fractures – particular the shallowly dipping ones – could be reactivated. The associated permeability increase with fracture shear dilation and large fracture slip could threaten the long-term safety of the deep geological repository. In this study, we elaborately investigated fracture reactivation under the coupled thermo-mechanical loading based on laboratory experiments. Critically-stressed single fractures in ca. 100 mm cubic granite specimens were heated in the minimum principal stress direction, through setting a target temperature of 150 °C at the heating source. Expansion in the maximum principal stress direction was restricted. Sawcut, laser-marking, and tensile-splitting fractures with low to high joint surface roughness (JRC=0~13) were compared. Fracture slip and shear dilation were directly measured by using clip-on displacement transducers and the measurements were further validated by the digital image correlation analysis. Temperature is non-uniformly distributed in the rock, and decreases with the increasing distance to the heating source. Thermally induced fracture slip grows slowly (compared with the stick-slip) but progressively with the peak slip rate of 2.69 – 6.65× 10−2 μm/s in different test cases. Shear dilation during thermoshearing can be ignored in the sawcut fracture, and a tendency that shear dilation is higher for the fracture with a larger JRC was confirmed.