Integration of field mapping, lineament interpretation and electrical resistivity tomography survey to characterize granitic rock mass lining by shotcrete [Integrasi pemetaan lapangan, tafsiran lineamen dan survei tomografi keberintangan geoelektrik dalam pencirian jasad batuan granit terlindung shotcrete]

Abstract

Rock mass characterization is important to identify the condition of rock especially in major infrastructure like a tunnel. Rock mass characterization can be done either before, during or after construction completed. In this case study, this characterization was done after almost 20 years of tunnel operation. Characterization could not be done directly to the field as half of the rock mass along tunnel was covered by shotcrete layer. To overcome this problem, three integration methods which are field mapping, satellite image interpretation and geoelectrical tomography resistivity survey been approached. Satellite image interpretation is to emphasise negative lineament structure tracing while field mapping is to emphasis the location of groundwater intrusion and major tectonic structures like fault, joint and shear zone. Geoelectrical tomography resistivity survey was conducted using ABEM SAS 400 system and dipole-dipole array while raw data were processed using Res2DINV software. Data analysis showed three difference resistivity values which are high resistivity (> 1500 Ω.m), moderate (100-500 Ω.m) and low (<25 Ω.m). Findings from field mapping and lineament interpretation were used to interpret pseudosection. All three resistivity values were interpreted as high water content zone (low resistivity), lineament effected and highly fractured granitic rock mass (moderate resistivity) and low fractured granitic rock (high resistivity).