Rock slope stability and hydromechanical coupling evaluation on the relay ramp-controlled failure mechanism in Mae Moh Mine, Thailand

The complex tectonic history of the Mae Moh mine in Thailand created normal faults, which formed a relay ramp that controlled the failure mechanism of the massive block in the C1-west wall. Geotechnical monitoring data between 2021 and 2023 revealed that the slope deformation was predominantly influenced by mining activities and precipitations, resulting from the hydromechanical (HM) coupling process. A 3D-distinct element model was performed to calculate displacement and the factor of safety (FS) of the mine plans in 2023, 2034 and 2041. The modeling scenarios consisted of dry condition, water table with effective stress analysis, and HM coupling analysis. The numerical analysis revealed the failure mechanisms, including bi-planar compound block sliding, bi-planar rotational block sliding, and toe pushing-up influenced by the dipping of the relay ramp. The maximum displacement and FS obtained from the HM coupling analysis were found to be more critical than those derived from the traditional methods, particularly in 2041, where the excavation created a fully open-pit wall face. Additionally, the sensitivity analysis indicated the instability if the groundwater level reaches the critical value. The monitoring data and numerical model identified that disturbances from stress relaxation due to unloading led to increased permeability in discontinuities, causing slope stability sensitive to changes in the subsurface groundwater during mining activities. These findings emphasize the significant effect of the HM coupling process on slope stability and highly recommend integrating the HM coupling approach into slope design to achieve a more representative and conservative risk assessment.