Fig. 3 Shear displacement-shear stress curves under different conditions. a) Mudstone granules sheared in both dry and wet conditions after 0 dry-wet cycles; b) and e) mudstone granules sheared in dry conditions and wet conditions after 3 dry-wet cycles, respectively; c) and f) mudstone granules sheared in dry conditions and wet conditions after 6 dry-wet cycles, respectively; d) sandstone granules sheared in both dry and wet conditions after 6 dry-wet cycles.
When the granules encountered water, the shear mode of the weathered mudstone samples changed from strain hardening to strain softening (Fig. 2 b vs. Fig. 2 e and Fig. 2 c vs. Fig. 2 f). A higher normal stress led to a greater difference between the peak and residual shear stresses for mudstone_3 and mudstone_6. In contrast, the same strain hardening behavior was observed for wet mudstone_0 and sandstone_6 under 500 kPa of normal stress. This indicated that the attenuation of the rock response due to water induced only a shear stress reduction and not further crushing to reach the residual strength. The difference in the shear stress reduction suggested that the weathering intensity was enhanced with an increasing number of the dry-wet cycles.
According to the Mohr-Coulomb yielding criteria, the cohesion (c ) and internal friction angle (φ ) of the peak strength and residual strength were analyzed (Fig. 4). The reduction in cohesion at the peak strength (c pd) was dramatic in the dry condition once the mudstone granules underwent dry-wet cycling. A further reduction in cohesion between mudstone_3 and mudstone_6 was found in the wet condition (c pw). In comparison to the cohesion, the internal friction angle decreased from mudstone_0 to mudstone_3 by approximately 1° and 6° in the dry and wet conditions, respectively, while the corresponding results were 5° and 12° for mudstone_6. In the residual stage, the residual internal friction angle (φ rw) continued to drop with the dry-wet cycles, while the residual cohesion (c rw) remained constant. In addition, the difference in shear strength between the mudstone and sandstone could be attributed to the lithology, as sandstone minerals are harder than clay minerals but lack interparticle cohesion in the crushed state.