Estimation of shear strength recovery and permeability of single rock fractures in shear-hold-shear type direct shear tests

The evolution of the long-term mechanical, hydraulic, and transport characteristics of rock fractures should be predicted in advance by considering the issue of the underground deposits of the energy byproducts of high-level radioactive waste. This paper presents slide-hold-slide direct shear-flow coupling experiments conducted for mortar and granite specimens with single fractures so as to investigate the effects of load holding on the mechanical properties of rock joints. From the experimental results, it is confirmed that the shear strength of the mortar replica specimens increases and the permeability decreases during 3 days of load holding. However, no significant changes are observed for the mechanical or the hydro-mechanical properties of the granite specimen even after 20 days of load holding. It is also confirmed that the shear strength of the mortar specimens increases in both short- and long-time holding cases. However, the evolution of shear strength recovery is found to be different for the two cases. This is because the dominant factor in the shear strength recovery during short-time holding may be attributed to a purely mechanical process, like creep deformation at the contacting asperities, while the shear strength recovery during long-time holding is affected by both mechanical and chemical processes. Moreover, to reproduce the shear strength recovery during short-time holding, we develop a direct shear model by employing a temporal variation in the dilation that occurs during load holding. The model predictions are in relatively good agreement with the experimental observations in the range of the short-holding period. (C) 2011 Elsevier Ltd. All rights reserved.