Evaluation of geological conditions along the proposed northern collector tunnel using electrical resistivity tomography method in gatanga, kenya.

Abstract

The proposed Northern collector Tunnel is located in Gatanga area within the southern slopes of the Abardare ranges in Kenya. Geologically, the area is composed of volcanic rocks comprising of the basalts and the pyroclastics. The mode of formation and deposition of these rocks make them prone to changes due to the dynamics of the earth and therefore influence on their mechanical behavior. Success of any tunnel project during implementation and after commissioning is influenced by the geological conditions. This study set out to investigate the geological conditions at the proposed tunnel location, 800 m length ERT Profile was carried out along the tunnel and supported by other short profiles at the valleys in each site and the results analyzed. The choice of ERT method was dictated by its widely known applicability in investigation of buried objects and structures, and also in exploration for underground water. The results of the study have shown that the upper most rocks have weathered to red brown soils. The basaltic agglomerates are the main rocks in the area and are overlain by pyroclastic rocks of varying thickness. Although no geological structures that are clearly observed at the earth’s surface the resistivity contrast between the rocks and the results from drill core logs reveals highly fractured rocks under the ground and these fractures are traced to correspond to the steep slopes at some sections along the tunnel route. The study also revealed ground water occurrence in all sections of the tunnel. The study recommends for the mitigation measures through pre-installation of horizontal drainage pipes below the adits to invert pore water pressures reducing seepage ingress and maintenance of relatively low hydraulic gradient compared with critical gradient for piping failure.