Integrating Surface Geological Mapping and Multi-Year Subsurface Data to Identify Lateral Lithological Variations Along the Jatinegara Diversion Tunnel, Central Java, Indonesia

Abstract

Tunnel design in tropical regions requires careful attention to subsurface variability due to intense weathering and complex lithology. The ongoing design of the Jatinegara Diversion Tunnel, with a diameter of 6 meters and a length of 439 meters, relies solely on the 2024 borehole data without considering the recent 2017 data, revealing a contrast subsurface geological characteristic. This study investigates lateral lithological variations along the tunnel alignment by integrating surface geological mapping with multi-year borehole data from 2017 and 2024. The methodology includes geological mapping of a 2.0 × 2.0 km area, corebox analysis, surface-subsurface stratigraphic correlation, and rock mass quality evaluation using the Rock Mass Rating (RMR) system. Results show the tunnel traverses five zones with distinct rock mass characteristics. Zone 1, Zone 3, and Zone 5 consist of andesitic breccia with RMR values ranging from 20.21 – 39.57, indicating poor quality of rock mass. Zone 2 comprises interbedded carbonate sandstone and siltstone overlain by andesitic breccia, with RMR values ranging from 42.82 – 52.40 (fair). Zone 4 consists of interbedded carbonate sandstone and siltstone overlain by massive sandstone with siltstone intercalation with an RMR of 52.43 (fair). Given these rock mass conditions, the recommended excavation method is a top heading and bench approach with 1.0–1.5 meters advance in the top heading with support system consist of rock bolt, shotcrete, and steel set. These findings highlight the importance of integrating multi-year geotechnical data to support an adaptive and safe tunnel design in complex geological environments.