Evaluasi Stabilitas Rencana Terowongan Akses Tambang pada Formasi Sedimen Berbasis Deterministik & Probabilistik Monte Carlo

Abstract

Mine access tunnels are essential underground mining infrastructure that support operational activities and ensure production continuity. In sedimentary rock formations, tunnel stability assessment becomes more challenging due to heterogeneous rock mass characteristics and the presence of geological discontinuities. Conventional deterministic approaches, which rely on average geotechnical parameters, are often insufficient to represent the inherent uncertainty of rock mass properties. Therefore, this study aims to evaluate the stability of a planned mine access tunnel in the Tanjung Formation, Tanah Bumbu Regency, South Kalimantan, by integrating deterministic analysis using the Finite Element Method-Shear Strength Reduction (FEM-SSR) and probabilistic analysis using Monte Carlo Simulation. Rock mass characterization was performed using three geotechnical boreholes consisting of ROD, JCond89, lithological descriptions, discontinuity data, and laboratory test results. Geological Strength Indax GSn values were estimated and converted through the Generalized ed 6/17 criterion into equivalent Mohr-Coulomb parameters for numeric Deterministic analysis was conducted using average rock mass propernes, wine probabilistic analysis treated the residual cohesion and residual friction angle of the coal seam as random variables generated using Latin Hypercube Sampling (50 samples). A Response Surface Model (RSM) was subsequently developed and applied in 10,000 Monte Carlo simulations to estimate the Safety Factor distribution and Probability of Failure (PoF). The deterministic analysis produced a critical Strength Reduction Factor (SRF) of 1.32, satisfying the minimum stability criterion. The probabilistic analysis yielded an average SRF of 1.303, ranging from 0.895 to 1.367, with a Probability of Failure of 42.22%. The developed RSM achieved a coefficient of determination (R²) of 0.720, indicating satisfactory predictive capability. Furthermore, continuous discontinuity models exhibited more critical stability conditions than isolated discontinuity models due to the formation of continuous failure paths. The integration of deterministic and probabilistic approaches provides a more comprehensive evaluation of tunnel stability and supports risk-informed design for mine access tunnels in sedimentary rock formations.