Study on the Influence of Unsaturated-saturated State Change on the Shear Strength of Ion-adsorption Rare Earth Ores

There exists an intrinsic correlation between the injection process in ion-adsorption rare earth ore and the transition of ore bodies from unsaturated to saturated states. Identifying relatively stable and unstable zones within the mine under such saturation-dependent changes is crucial for ensuring safe mining operations and facilitating landslide early-warning monitoring. To quantitatively evaluate the effect of unsaturated-saturated state transitions—Triggered by solution injection—On slope stability in ion-adsorption rare earth deposits, an orthogonal experimental design was adopted, incorporating five controlling factors—Initial dry density, initial moisture content, leaching agent concentration, leaching agent type, and grain size distribution—Each evaluated at three discrete levels. Systematic unsaturated direct shear tests and saturated consolidated-undrained triaxial tests were performed to determine the sensitivity of key shear strength parameters—Namely, effective cohesion and effective friction angle under unsaturated conditions, and undrained cohesion and total friction angle under saturated conditions—To variations in each factor. The results further quantify how incremental changes in factor levels govern the magnitude and trend of strength parameter evolution. The experimental results demonstrate that, in unsaturated ion-adsorption rare earth ore, the initial moisture content—Quantified as matric suction—Exerts a dominant influence on shear strength primarily through enhancement of effective cohesion, whereas leaching agent concentration governs the effective friction angle. In contrast, under saturated conditions, leaching agent concentration modulates shear strength predominantly via its effect on undrained cohesion. Throughout the solution injection process, spatial heterogeneity in shear strength persists within the ore body, with coexisting high-strength and low-strength zones. Relative to conventional ammonium sulfate-based leaching agents, the optimized composite leaching agent—Comprising equimolar ammonium sulfate and magnesium sulfate [m(MgSO₄)∶m(NH₄)₂SO₄=1∶1]—Achieves dual benefits: Significant reduction in ammonia nitrogen emissions and measurable improvement in slope stability at active mining sites. The results in the study can provide a significant guideline for safe and green mining of ion-adsorption rare earth minerals.