Evaluation of engineering geological condition and empirical design of excavation method and support system of the diversion tunnel of Krekeh Dam, West Nusa Tenggara

Abstract The initial design of the Krekeh Dam diversion tunnel was developed using the Rock Mass Rating (RMR) method based on limited core drilling data, which potentially reduced the accuracy of excavation sequencing and support system recommendations. This study aims to reevaluate rock mass quality and engineering geological conditions using both the RMR and Q-System methods, and to propose more reliable excavation and support designs. Data were obtained through geological mapping, core drilling analysis, and laboratory testing. The tunnel alignment comprises three lithological units: Tuffaceous Sandstone with Intercalated Siltstone and Tuff, Andesitic Breccia, and Tuff Breccia. Based on classification results, the alignment is divided into three zones: Zone 1 (RMR: fair; Q: poor), Zone 2 (RMR: poor; Q: very poor), and Zone 3 (RMR: fair; Q: poor). Excavation in Zones 1 and 3 is recommended using a top heading and bench method with an advance rate of 1.5–3 m, while Zone 2 requires a reduced rate of 1–1.5 m. According to RMR, support in Zones 1 and 3 includes 4 m bolts (1.5–2 m spacing), crown wire mesh, and shotcrete (5–10 cm crown, 3 cm walls). Zone 2 requires 4–5 m bolts (1.0–1.5 m spacing), wire mesh at crown and walls, 10–15 cm shotcrete at the crown, 10 cm at walls, and light-to-medium steel ribs at 1.5 m spacing. The Q-System suggests for Zone 1: 2.54 m bolts at 1.7–2.1 m spacing with 6–9 cm fiber-reinforced shotcrete; for Zone 2: 2.54 m bolts at 1.3–1.5 m spacing with 9–12 cm shotcrete; and for Zone 3: 2.54 m bolts at 1.7–2.1 m spacing with 5–6 cm shotcrete.