High-resolution seismic refraction tomography for integrated geotechnical characterization of near-surface layers in engineering foundation assessments

Abstract This study presents integrated geophysical and geotechnical assessments using seismic refraction tomography to evaluate subsurface competence and suitability for foundation design within a rapidly urbanizing terrain prone to building subsidence. Seismic refraction surveys were conducted across the study area using a 24-channel ABEM Terraloc Mk-6 digital seismograph. Compressional and shear wave velocities were measured to derive parameters such as porosity, void ratio, density, Poisson’s ratio, shear modulus, Young’s modulus, Lame’s first parameter, material index, N-number, ultimate bearing capacity, and allowable bearing capacity. Two distinct near-surface layers were delineated by the seismic refraction survey, with the thickness of layer 1 ranging from 2.5 to 4.2 m (mean 3.5 m). P-wave velocities of layer 1 (unconsolidated lateritic sand and alluvial sediments) ranged from 406 to 659 m/s, while P-wave velocities of layer 2 (gravelly sand intercalated with sandy clay) ranged from 1378 to 1885 m/s. Porosity values ranged from 50% in layer 1 to 21% in layer 2, showing progressive compaction and pore space reduction. Shear modulus, ultimate bearing capacity, and allowable bearing capacity increased with depth, indicating stronger formation shear strength and stiffness. This demonstrated that layer 2 was more competent for supporting structures, while the shallower layer 1 was weaker and more prone to excessive settlement. The significance of this study lies in its systematic integration of seismic velocity data with multiple foundation design indices, enabling predictive modeling of subgrade competence before construction. This improves the accuracy of near-surface geotechnical characterization and reduces field investigation costs and risks related to differential settlement in civil infrastructure development.