Mapping Weak Soil Zones Using Geoelectrical Resistivity for Bridge Construction in Rokan Hulu, Indonesia

The geotechnical composition of the soils in the Rokan Hulu Regency, Riau, predominantly weak soils, has significant implications for bridge construction within the region. Excessive loading on these weak soil zones can instigate subsidence, thus compromising the stability of the bridge foundation by inducing tilting. This study is aimed at mapping these weak soil zones in the context of a bridge construction project within the Rokan Hulu district of Riau. Investigation of these zones was conducted via a geoelectrical resistivity approach using a dipole-dipole configuration across five lines. The lengths of these lines ranged from 65 to 200 meters, with an electrode inter-distance of 10 meters. The outcome of this analysis was depicted in both two-dimensional and three-dimensional models. The layer exhibiting the lowest resistivity value, interpreted as sandy clay rock, was identified as having high porosity, thereby serving as an aquifer and being indicative of a weak soil zone. As revealed by the two-dimensional and three-dimensional models, the depth of the weak soil zone varied between 0-40 meters, denoted by a low resistivity value of 0-15 Ω.m, and was interpreted as sandy loam. Two weak soil zones were identified within the foundation area of the Hulu Rokan Bridge, located in the northwest (5-40 m) and southeast (65-80 m) sections from the river bank. Moreover, the distribution of these weak soil zones was observed to extend from the Rokan River towards the southeast and northwest. Based on these findings, it can be inferred that the foundation in the northwest is more vulnerable due to the relatively shallow presence of the weak soil zone. Consequently, soil improvement measures, including compaction and stabilization, are recommended.