Experimental investigation of the influence of dry-wet, freeze-thaw and water immersion treatments on the mechanical strength of the clay-bearing green sandstone

The presence of water in sandstones has caused many engineering geology disasters. In order to investigate the weakening mechanism of the clay-bearing green sandstone caused by water at different ambient temperature, the main water treatments have been conducted in this research, including dry-wet cycles, water immersion and freeze-thaw cycles. It shows that the repeated dry-wet cycles have little effect on the mechanical strength of this sandstone due to the lack of expansive clay minerals like montmorillonite. However, the water immersion and cyclic freeze-thaw treatments have caused a remarkable reduction of the strength. The UCS (uniaxial compressive strength) of this green sandstone has decreased by 27.2% from the dry to saturated state and it continues to decline for another 31.1% after 60 times of freeze-thaw cycles. In addition, the internal friction angle has a little increment but the cohesion decreases quickly with increasing freeze-thaw cycles. The adverse influence of the freeze-thaw damage on the TCS (triaxial compressive strength) would be inhibited by the confining pressure, because the pores and microcracks were compacted under a high confining pressure. The novel findings of this study are that the reversible physical softening should be responsible for the water weakening of this clay-bearing green sandstone above 0 °C, and the freeze-thaw damage is irreversible due to the production of plastic frost heaving deformation below 0 °C. Therefore, it is suggested that the effect of water weakening on the mechanical properties of the green sandstone should be considered in the wet state regardless of the number of dry-wet cycles, especially in cold regions.