Fig. 5 Permeability coefficient of the sheared specimen under different normal stresses.
As shown in the cross-sections of the sheared specimen in Fig. 6, a visual distinction between the dry and wet samples was observed, where mudstone_3 presented more interparticle pores than mudstone_6. Although the crushing character of the shear band was found in the particle size distribution, where the content of the 2~1 mm grain size in the middle of the specimen was higher than those in the upper and lower parts. The dry specimen showed limited fragmentation under shearing and compression, which was reflected in the absence of a thin crushing zone after the ring shear tests (Fukuoka et al., 2007; Kimura et al., 2018). Therefore, the confined water could flow through the macropores and across the shearing zone easily. In contrast, the macropores of the wet specimen were filled with mud and almost disappeared. The particle content finer than 2~5 mm was increased throughout the specimen of mudstone_6 under wet conditions and a normal stress of 800 kPa. Similarly, extensive crushing in the middle part of mudstone_6 was revealed via the further increase of finer particle content, resulting in the strain softening behavior of weathered mudstone granules under wet conditions. The difference in the finer particle content between the dry and wet conditions supported the variation in the macropores observed in the cross-sections.