Engineering geology for site characterization in Tunnel 2 Bintang Bano irrigation area, West Sumbawa, West Nusa Tenggara Province

Site characteristics develop a better understanding of geological conditions to prevent tunnel failure. Tunnel 2 Bintang Bano Irrigation Area in Seloto, West Sumbawa, West Nusa Tenggara Province, was designed with different interpretations of geological conditions that could lead to inaccuracy of the tunnel design. This paper aims to identify the characteristics of the tunnel site area through engineering geology, including morphological and lithological unit, geological structure, and geomechanics, which are rock mass characterization using Rock Mass Rating (RMR) and Tunnelling Quality Index (Q-system), for the excavation planning area. This area is identified as volcanic landforms categorized as lava ridges with steep slopes and alluvial plains with very sloping slopes. The lithologies are altered andesite and gravelly sand unit. Altered andesite with tuff intercalation was found in the subsurface. The tunnel is projected to be excavated in rock mass with dominantly fair rock with ratings (±40≤RMR≤65/±4.33≤Q-System≤8.67) approximately 60% of the excavation area, ±30% area of good rock (63≤RMR≤69/10.67≤Q-system≤11.40) and 10% poor rock (26≤RMR≤40/1≤Q-system≤3.57).