Characteristics of mudstone interlayers and mechanical failure model development during salt cavern gas storage construction

To accurately predict the collapse step distance of mudstone interlayers during cavern leaching and ensure the stability of salt cavern gas storage facilities, this study conducts an in-depth analysis of the structural characteristics and mechanical properties of mudstone interlayers, establishing a predictive model for interlayer fracture. Macroscopic analysis and point load tests on mudstone interlayer samples from Submember 3b of Member 3 of the Buxin Formation in the Sanshui District salt mine revealed the following: (1) the structural characteristics of the mudstone interlayer vary significantly from top to bottom, with the lower section developing multi-angle fractures (partially filled with salt rock), while the upper part remains dense; (2) consequently, the interlayer is categorized into an upper high-strength mudstone and a lower low-strength mudstone, with point load tests confirming that the uniaxial compressive strength of the high-strength mudstone is approximately 5.5 times that of the low-strength mudstone; (3) under brine soaking, the high-strength mudstone exhibits significant softening effects within the initial 14 days, after which its strength tends to stabilize; the long-term strength is primarily influenced by intrinsic interlayer characteristics, such as the proportion of high-strength mudstone and interlayer thickness. Based on these properties, the progressive fracture mechanism of the mudstone interlayer is revealed, which can be divided into three stages: low-strength mudstone fracture, high-strength mudstone fracture, and operational-stage fracture. Ultimately, a fracture distance prediction formula is established that comprehensively considers soaking time and interlayer characteristics (high-strength mudstone proportion and thickness). The study clarifies that: (1) buckling failure occurs in the interlayer when the high-strength mudstone proportion exceeds a critical value (approximately 0.2); (2) when the high-strength mudstone proportion is relatively low, non-buckling failure is more likely to occur. These research findings provide a theoretical basis for predicting and controlling interlayer collapse during the leaching of salt cavern gas storage facilities.