Weakening mechanisms of gypsum interlayers from Yunying salt cavern subjected to a coupled thermo-hydro-chemical environment

Abstract China's rock salt deposits in the Yunying area are typically characterized by interlayers. When forming salt caverns, the gypsum interlayers are soaked in saline brines of different concentrations at various temperatures, which damage and weaken them. Moreover, the suspended gypsum interlayers periodically collapse as the size of the salt cavern increases. This can seriously damage the pipeline and influence the shape of the salt cavern. To predict the critical caving pace of a suspended gypsum interlayer and investigate the weakening mechanism and damage law of gypsum interlayers under corrosive environments, a series of laboratory tests were conducted, including scanning electron microscopy (SEM), micro-computed tomography (MCT) scanning, and indentation tests. In these tests, more than four hundred specimens were prepared and then soaked in 3 types of liquid and at 3 different temperatures. The results have demonstrated that deterioration increases with increasing temperature following soaking in water. More prominent steps on the surface of gypsum were noticed when soaked in high saturation brine, as compared with those in distilled water. The fitting parameters, critical transition force (CTF) and indentation modulus (IM) increased with increasing temperature but remained approximately identical with increasing brine concentration. Additionally, to study the effect of each factor, as well as the effects of interactions between factors, on the response variable (i.e., the CTF and IM), a 2 × 2 factorial design was employed to assess the brine concentrations and temperatures. It verifies that water-temperature has a significant weakening effect on the gypsum, while a much weaker effect from the chlorine ions. Based on these results, a damage law that includes time-dependent behaviour is proposed for describing the thermo-hydro-chemical process, which can be implemented in the Particle Flow Code (PFC). Consequently, this study has important implications for the control of the shape and size of salt caverns, as well as their stability and safety.