Fig. 7 Variation in the microstructure of the mudstone particles with an increasing number of dry-wet cycles at different magnifications. The sketch at the bottom describes the damage degree related to the variation in microstructure.

3.4 Variation in the granule shapes

Here, the shape coefficient results for the entire 2~5 mm particle size range are shown in Fig. 8. Compared to mudstone_0, the weathered mudstone granules exhibited a distinct variation phenomenon regardless of the reduction or increment in the four coefficients. Regarding the grain size composition, the UC and SC dropped continuously to approach 1.0, which indicated a better uniformity and sorting when the mudstone granules underwent dry-wet cycling. The four groups of specimens had no distinct difference in the CC, which fluctuated around the value of 1.05. The FD describes the autocorrelation of the change rate of the perimeter (c ) in response to the change in area (S ), which can be simplified as
where a is constant. The increase in the FD for weathered specimens, both mudstone and sandstone, indicated a strong correlation between perimeter and area. A slight change in area could induce a considerable variation in the perimeter for a higher FD. Another coefficient expressing the relationship between the particle perimeter and area, the AFF, increased with the number of dry-wet cycles and approached 1.0, which corresponds to the maximum value for the circle. Therefore, the complexity of the mudstone particle boundaries decreased with area variation. It can be deduced that the shapes of the mudstone granules tended to change from polygons to circles with further weathering via dry-wet cycling, increasing the uniformity of the particle size distribution. In addition, among the four specimens, the sandstone exhibited generally low values of UC, SC, CC, and FD but the highest AFF. This difference can be attributed to the lower boundary of fracture surface complexity and better resistance to weathering of sandstone under dry-wet cycling.