Influence of volcanic rock mass characteristic in the construction of the Bintang Bano irrigation tunnel in Sumbawa Island – Indonesia.

The Bintang Bano irrigation tunnel, located in the Seloto sub-district in Sumbawa Island, Indonesia, has currently being constructed in the weak volcanic rocks of hilly area at an elevation of 60 m above the mean sea level. Highly weathered volcanic rock masses located at the top of the tunnel inlet had only a uniaxial compressive strength of 1.0 MPa, and geological strength index (GSI) of 29. With these rock conditions, the tunnel experienced difficulties in excavation. Particularly, the roof of the tunnel inlet collapsed not only due to rock conditions, but also lack of confining pressure, where its height was only 10 meters from the surface. Accordingly, primary supports, including wire-meshed shotcrete of 0.15 m thickness and steel ribs of 0.5 m spacing, were immediately installed after excavation. Although, these immediate installments caused a delay in construction time, this was the only effective method available during construction; otherwise, the tunnel inlet would totally ruin, and it might be hard to repair. [ABSTRACT FROM AUTHOR]