Cyclic triaxial mechanical properties and damage failure of dolomite in acidic solutions with different concentrations

Given that dolomite is prone to strength degradation and susceptible to water-sand ingress under physicochemical actions, this study aims to investigate these phenomena, along with the sanding mechanism in the Xiaopu Tunnel of the Yunnan Dianzhong Water Diversion Project, using a combined experimental and modeling approach for systematic analysis. Triaxial cyclic loading-unloading tests were first conducted on dolomite samples soaked in sulfuric acid solutions of varying concentrations, with synchronous monitoring of their mechanical responses (e.g., peak strength, deformation modulus, porosity changes). These tests, combined with observations of macroscopic morphology and mass changes during soaking, revealed a four-stage degradation pattern of dolomite in sulfuric acid: water absorption, dynamic equilibrium, dissolution, and stabilization. Key quantitative relationships established that as sulfuric acid concentration increased (from 0% to 15%), the peak strength of dolomite decreased significantly (by 7.49% to 24.99%), while porosity markedly increased (by 45% to 130%). Further post-failure analysis (fracture surface observation) and scanning electron microscopy (SEM) micro-characterization uncovered the intrinsic mechanisms of acid-induced damage: the acid solution not only promoted macroscopic crack propagation and increased fracture surface roughness but also triggered severe structural deterioration at the microscale, including enlarged crystal spacing, dissolution of gel-like substances, formation of intra-crystalline pores, weakened interparticle cementation, and development of macropores. The extent of this deterioration was positively correlated with acid concentration. Based on the experimentally revealed chemo-mechanical coupling damage mechanism between acid and rock, this study established, for the first time, a multi-scale predictive model capable of quantitatively correlating acid concentration, microstructural deterioration, and degradation of macroscopic mechanical properties. The development of this model not only deepens the quantitative understanding of the dolomite sanding mechanism but also provides a crucial theoretical tool for assessing surrounding rock stability and predicting risks in similar water diversion tunnel engineering. Addressing the specific risks of water and H⁺ erosion in the Xiaopu Tunnel, the research findings directly informed the engineering reinforcement strategy: concrete lining is recommended as the primary load-bearing structure, supplemented by surrounding rock surface protection measures, to effectively mitigate the acid-induced damage process and enhance the long-term stability of the surrounding rock.