Shallow Seismic Investigation of Laterite Layer Thickness for Road Sub-base Assessment in Anambra State, South East Nigeria

Road construction in Anambra State depends heavily on laterite as a sub-base material, yet the variable thickness and engineering properties of laterite layers across the state result in inconsistent pavement performance and premature road failure. This study applies shallow seismic refraction to characterise laterite layer thickness and velocity along proposed road alignments in Awka South Local Government Area. Seismic refraction surveys were conducted along four profiles totalling 960 metres, using a 12-channel seismograph with a sledgehammer source at 1-metre geophone spacing for shallow resolution. First arrivals from forward and reverse shots were used to construct time-distance plots interpreted by the delay time method. Compressional wave velocity models indicate a two-layer to three-layer subsurface, with the upper laterite horizon having velocities of 450 to 950 metres per second overlying weathered basement at 700 to 1,800 metres per second. Laterite thickness ranges from 1.2 to 5.8 metres along the surveyed profiles, with the thinnest zones corresponding to areas of past erosion. The velocity structure is correlated with Dynamic Cone Penetrometer resistance values obtained from concurrent geotechnical tests to develop a local velocity-CBR relationship applicable to the region. The derived thickness model directly informs the road design specification for the depth of sub-base compaction and pavement layer thicknesses. Keywords: seismic refraction, laterite thickness, road sub-base, Awka, Anambra State