Wetting-drying-freezing-thawing cycle effect on the hydro-mechanical behaviour of Yanji swelling mudstone

Swelling geomaterials in northwestern and northeastern China are exposed to both seasonal wetting-drying (DW) and freezing-thawing (FT) processes. The influence of full-process wetting-drying-freezing-thawing (WDFT) cycles on their hydro-mechanical behaviour has not been well investigated. In this study, a series of swelling and compression tests was conducted on Yanji weathered mudstone subjected to different WD, FT and WDFT processes and the effects of seasonal processes and cyclic number on the swelling strain, compression index, rebound index and hydraulic conductivity were experimentally determined. With the increasing WD, FT and WDFT cycles, the starting time of primary swelling decreased first due to the increasing water infiltration with the appearance of large pores, and then increased because of the decreasing swelling potential of compact aggregates after two cycles. Moreover, as the cyclic number increased, the final swelling strain declined. Upon loading, the specimens after cyclic processes exhibited a smaller compression index at low stresses due to their smaller inter-particle distance after swelling, but a larger one owing to the collapse of large pores and cracks at high vertical stresses. After unloading, the rebound index decreased with the increase of cyclic number due to the irreversible collapse of large pores and cracks. The hydraulic conductivity increased with the increasing cyclic number at low vertical stresses (large void ratios). With the further increase of vertical stress, the increase of hydraulic conductivity induced by cyclic processes became indiscernible. Moreover, a comparison among three processes suggested that the WDFT process exerted a more pronounced influence on the hydro-mechanical behaviour of Yanji mudstone than the separate WD or FT process.