Evaluation Of Subsurface Competence Using Shear-Wave Velocity and Geotechnical Indices from MASW Data in FCE(T) Akoka Campus, Lagos

Abstract

The increasing incidence of structural defects such as wall cracks, tilting, settlement, and sinking of buildings within Federal College of Education (Technical), Akoka Campus, Lagos, necessitated an assessment of subsurface competence for safe construction. Seven MASW profiles were acquired along six traverses using a 24-channel ABEM seismograph, A 6kg sledgehammer and a metallic strike plate as the seismic source. Data acquisition involved 9-11 shot points per profile with multiple stacking to enhance signal quality. The acquired data were processed using Seisimager/SW software to obtain shear wave velocity (Vs) models and geotechnical parameters were derived to evaluate soil competence. The results revealed three subsurface layers across the profiles comprising silty clay, clay, sandy clay/clayey sand, sand/gravelly sand and firm soil. Vs values ranged from 97 m/s to 1042 m/s. Computed geotechnical indices showed low concentration index values (3.01-3.34), low material index values (-0.98 - -0.70) and stress ratio values ranging from 0.74 to 0.99 indicating predominantly incompetent to slightly competent near surface materials while more competent layers occurred at deeper horizons. The study recommends proper ground improvement measures prior to heavy construction activities to ensure long-term stability and safety.