Abstract: The objective was to analyze the water-weakening feature and the weakening mechanism of the slate taken from the engineering of the west route of water transfer from south to north in China. Some experiments were done including measuring the change of water absorptions, contact angles, the microstructures of the minerals, the porosities and the strength of the rocks in different absorption by SEM, powder X-ray diffraction analysis and the triaxial compression experiment. Some conclusions are drawn from these experiments as follows: (1) when flooding into water, the absorption was affected by the location and density of the layer surfaces, and increased with the dipping time in the first few days till it kept stable at last; (2) in the SEM images the grains of the slates bulged and the structure relaxed, which made the porosity increased without confinement and the volume bulge lagged behind the water absorption; (3) the triaxial compression results show that the slates weakened and the peak strength of the slates declined with the absorption increased in minus logarithm law; (4) from the variety trends of microstructure and contact, the capillary force and the tension force of the slates decreased with the absorption, which made the adhesive force declined. In the macroscopic view, it is shown that the slates weakened with water. The slate is anisotropic which made the volume dilation in the perpendicular direction to the layers bigger than the parallel direction. As a result, the slate are easier to failure following the layer surfaces.

Key words: slate, water-weakening, microstructure, contact angle, rock mechanics