Near-Surface Geophysical Characterisation of an Arch dam Foundation in Kano State

This study applies near-surface geophysical methods to characterise the foundation conditions at a proposed small arch dam site in Kano State, North West Nigeria. The engineering performance of arch dams is critically dependent on the competence and homogeneity of the abutment foundation rock, and geophysical investigation provides cost-effective pre-design information to guide foundation grouting and drainage design. This study deploys seismic refraction and electrical resistivity tomography at a proposed dam site in Garun Mallam local government area. Seismic refraction uses two geophone strings of 24 channels at 3-metre spacing for forward, backward, and cross shots. ERT is acquired along four profiles across the river valley using the dipole-dipole array with 5-metre electrode spacing and 10 levels of investigation. P-wave velocity models and resistivity sections are integrated to characterise the degree of rock mass weathering, fracture zone distribution, and depth to fresh competent rock. Findings reveal a weathered rock zone with P-wave velocities below 1,000 metres per second extending to depths of 3 to 8 metres at the abutments, underlain by partially weathered rock at 1,000 to 2,500 metres per second and fresh granite at velocities exceeding 3,000 metres per second. ERT sections show a zone of low resistivity below 50 ohm-metres on the left abutment, interpreted as a water-bearing fracture zone that requires grouting. The fresh rock surface at the proposed dam base is at 6 to 12 metres depth. The study recommends supplementary borehole investigation of the identified fracture zone and grouting treatment before dam construction.

Keywords: dam foundation, seismic refraction, electrical resistivity, weathered rock, Kano State.