Fig. 6 Particle size distributions of a sheared specimen (left) corresponding to each layer of a cross-section (right).

3.3 Micropore increases for the weathered mudstone granules

The ESEM images in Fig. 7 suggest that the microstructure of the mudstone samples varied with the number of dry-wet cycles. At low magnifications (200 μm and 500 μm), mudstone_0 had no significant microstructure at the particle boundaries, which was the same as that observed in the interior of the particles. The mineral arrangement was relatively consistent according to the homogeneous grayscale, indicating a lower surface roughness for mudstone_0 than for the weathered particles. After the dry-wet cycling treatment, several micropores of approximately 80 μm were found at the particle boundaries in mudstone_3. Many more micropores (approximately 50~300 μm) were observed from the boundary to the inside of the particles in mudstone_6, due to the further development of water-rock interactions. Analogously, at the high magnification (mineral scale), there was almost an absence of cracks and micropores in mudstone_0, while the direction of mineral arrangement was fairly consistent. This kind of homogeneity demonstrated that the mudstone samples without dry-wet treatment were undamaged. On the other hand, the mineral distribution of the weathered granules was disordered, resulting in a higher roughness than that observed for mudstone_0. Thus, the damage feature from the boundary to the inside of particles may be based on the micropore evolution process during dry-wet cycling (Fig. 7).