Experimental study on the water-weakening shear behaviors of sandstone joints collected from the middle region of Yunnan province, P.R. China

The paper presents an experimental study on the effects of water on the shear behaviors of sandstone joints collected from middle region of Yunnan province. A series of direct shear tests on specimens with different immersion time in water were performed under the constant normal loading condition. Experimental results showed that the peak shear strength of the sandstone joints can be lowered by about 50% due to immersed in water for 32 days. Under the low normal loading condition, the peak shear strength declined until the immersion time up to 8 days and beyond the point onward, the values were not sensitive to the immersion time. Under the high normal loading condition, we can observe a decline trend in the peak shear strength and the influence of water became less prominent with the increase of immersion time. The effect of water on the peak shear displacement was less obvious as the normal stress is low, while it increased with the increase in immersion time under the high normal loading condition. The variations of residual shear displacement exhibited fluctuation as the immersion time increased and no obvious regularity can be observed. Shear-induced normal displacement during the process of shearing generally switched from dilatancy to contraction (negative dilatancy) as immersion time increased. Water-induced reduction in secant shear stiffness under the normal stresses of 2.0 and 4.0 MPa can be observed for specimens immersed in water for 2 days, and beyond the point onward the stiffness decreased slightly with the increase of immersion time. One of possible weakening mechanisms would be related to the fracture energy and the friction coefficient of the cement particles within the intact block, and the other would be related to the capillary forces and the chemical and/or physio-chemical effects among contact particles distributed on the surfaces.