Investigation of hydrogeology condition in Manikin Dam Diversion Tunnel, Timor Island, Indonesia: A Case Study

Abstract

The seepages phenomenon happened in the construction site of the Manikin Dam Diversion Tunnel, Timor Island, Indonesia, has hampered the progress of construction. It seems impossible since the tunnel construction site was at the Bobonaro Complex. This condition leads to an investigate the possibility of another seepage occurrence along the tunnel construction site since more than 500 m long tunnel remains to be excavated. This study was conducted by direct observation method of the geological and hydrogeological conditions. Interpretation to direct field of geological mapping result, 10 borehole cutting samples, 6 lines of geo-electrical survey data, and face-mapping of tunnel excavation data resulting the surface and subsurface geological conditions. Hydrogeological mapping was performed to portray groundwater conditions at the tunnel site. The research reveals that the study area consists of scaly clay (fragmented with exotic block) and carbonate sandstone units. The scaly clay is impermeable, while the carbonate sandstone is permeable, but the fractured exotic blocks also act as an aquifer. The groundwater flows to the tunnel comes from this fractured zone in the exotic blocks. Based on this condition, other seepage phenomena could happen, as shown in the interpretation of the geo-electrical survey.