Fig. 4 Shear strength parameters (cohesion c and internal friction angle φ ) of the four kinds of samples. Note:c pd/φ pd representsc and φ at the peak strength of dry samples.c pw/φ pw representsc and φ at the peak strength of wet samples.c rw/φ rw representsc and φ at the residual strength of wet samples.

3.2 Permeability of sheared specimens

The permeability coefficient of the sheared specimen in Fig. 5 revealed negative correlations between the permeability and normal stress, and the permeability and number of dry-wet cycles. A linear reduction in the permeability coefficient with increasing normal stress was found for all the mudstone specimens, although the weathered specimens in dry conditions presented a considerable fluctuation, which may be attributed to the heterogeneity of particles. Under the softening impact of water, the permeability coefficient of weathered mudstone granules decreased sharply with increasing normal stress when compared to the results of mudstone_0 and was close to 1.0×10-5 cm/s. The permeability difference between the dry and wet conditions of the weathered samples varied from 10 to 45 times with increasing normal stress at a limited shear displacement of 20 mm. Under the wet condition, a small reduction in the permeability of mudstone_0 was observed, but its permeability was still lower than that of most dry specimens. By fitting the data in a linear relation, the reduction in the slope from dry to wet conditions demonstrated that the change in permeability induced by the increasing normal stress was decreasing. The influence of weathering intensity on permeability was lower for mudstone_3 and mudstone_6 because of the decreasing discrepancy under wet conditions, while this trend was not apparent in dry conditions. Due to the low cohesion of the samples, the permeability of the sandstone was higher than that of the mudstone, so it was more difficult to obtain the mudstone results under the same test conditions. Thus, the difference in the permeability coefficient controlled by lithology was not discussed in this study.