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.