Integrated geophysical and geotechnical investigation of a bridge site - A case study of a swamp/creek environment in south east Lagos, Nigeria

Abstract

Integrated geophysical and geotechnical investigation was carried out at a bridge site within a creek and swamp environment in parts of Agbowa, South East Lagos. This was with a view to delineating the subsoil sequence and determining the engineering properties. The investigation involved three shell and auger boring and seven Vertical Electrical Soundings (VES) using the Schlumberger electrode array. Analysis of results of the boring lithological logs indicates occurrence of four major layers composed of clay, organic clay, silty sandy clay and sandy deposits to about 40 m depth. Results of the in-situ and laboratory tests reveal that the silty-sandy soils are characterized by low N-values that range from 5 to 15, while the clayey soils are characterized by high void ratio of 1.73 and low Cu values of 22 kN/m 2 with 4 0 indicating the poor strength and highly compressible nature of the subsoil sequence. The electrical resistivity survey results show good correlation with boring logs and further indicated occurrence of a highly resistive (>1000 ohm-m) basal sandy deposit beyond the boring probe to appreciable depth of over 80 m. The generalized subsoil sequence indicates occurrence of old erosional channel cuts through the basal sand and subsequently filled by deposits of thick organic clay/peat. Based on the geotechnical and geoelectrial parameters, the heterogeneous deposits of soft clay, loose silty sand, clayey sandy and organic clay/peat overlying the basal sand within the creek/swamp section are incompetent foundation soil. The highly resistive superficial sandy deposits in the upland section and the basal sand constitute the competent foundation soils in the area. Keywords : Bridge Investigation, Swamp/Creek Environment, Poor Subsoils, Deep Foundations