Physicomechanical properties and degradation mechanism of impurity-bearing gypsum rock subjected to alternating dry and wet conditions

Abstract

Investigating the deterioration effect of dry‒wet cycles on gypsum rocks is highly important for evaluating the stability and safety design of geotechnical engineering projects located in chemical sedimentary strata. In this work, five groups of impurity-bearing gypsum rock samples with different numbers of dry‒wet cycles, namely, N = 0, 10, 20, 30, and 40, were prepared. P-wave tests and uniaxial and triaxial compression tests were subsequently performed on the samples to explore the degradation patterns of their physicomechanical properties caused by alternating dry and wet conditions. The results show that the P-wave velocity, uniaxial compression strength (UCS), elastic modulus and cohesion of the rock decrease logarithmically with N. The sensitivity order of the parameters to dry‒wet cycles is as follows: elastic modulus > UCS > cohesion > P-wave velocity. As N increases, the fracture form of axially compressed samples tends to be more complex depending on the newly induced defects, and the failure of compressed samples with lateral constraints changes from brittle shear fracture to plastic bulging. Furthermore, the deterioration mechanism of the gypsum rock subjected to dry‒wet cycles was revealed via micro tests. Cycling-induced deterioration is closely related to progressive microstructural damage in rock, which is implemented through temperature and water effects involving particle fragmentation, defect proliferation, dissolution of gypsum and disintegration of gray materials containing dolomite, mica, chlorite, quartz, etc. To establish the response relationships between macroscopic deterioration and microstructure damage, a variable containing fractal dimensions was proposed to quantify the damage effect of dry‒wet cycles on the microstructure of rock. Macroscopic deterioration indices characterized by UCS, elastic modulus and P-wave velocity change as logarithmic functions of the microstructure damage variable.