Empirical and Numerical Analyses of Tunnel Support Design in Weak Volcanic Rock Masses: A Case Study of the Jatinegara Diversion Tunnel, Central Java Province, Indonesia

This study presents an integrated geotechnical framework combining empirical and numerical analyses to design tunnel support systems for the planned diversion tunnel at Jatinegara dam in weak and heterogeneous volcanic rock masses. Field investigation, borehole analysis, and laboratory testing were used to characterize the rock mass using three empirical classifications: rock mass rating (RMR), Q-system (Q), and geological strength index (GSI). The empirical approach provided preliminary support requirements as the baseline and input parameters for the two-dimensional finite element method (FEM) using commercial software for tunnel stability analysis. The tunnel alignment is delineated into five distinct geotechnical zones according to empirical classifications, with rock mass conditions ranging from "Poor" to "Fair" quality. Furthermore, two-dimensional finite element modeling (FEM) was performed to evaluate tunnel stability under static and pseudo-static seismic loads and compare empirically derived supports with optimized support systems. Result implied that poor zone (zone 1, zone 3, and zone 5) requires systematic rock bolts (3 m length, spacing 1.2 m), 50 cm shotcrete with wire mesh and steel sets with 1.5 m spacing, while fair zone (zone 2 and zone 4) can be supported with systematic rock bolts (2-2.4 m length, spacing 2.1–2.3 m), 50–60 mm shotcrete with wire mesh. Using those support combinations, a strength factor of greater than 1 and a displacement of less than 10 cm produces a reliable and safe design.