Expedient for IN-SITU foundation exploration in Lagos, South West, Nigeria

Abstract

Fagboyin MB, Afolabi YT

In areas underlain by stratified rocks, it is difficult to obtain undisturbed samples at depth for satisfactory geotechnical results when analysed in the laboratory. Considering the complexity of its subsurface geology, a study on IN-SITU geotechnical exploration tool for foundation investigation in Lagos was conducted. Layer apparent resistivity (ρa) was measured with ABEM SAS 300 Terrameter using Schlumberger array at AB/2 ranging from 1.0 to 50.0 m. The resistivity curves were interpreted using the partial curve matching techniques, and computer iteration with RESIST software. Cone penetration test (CPT) was conducted with penetrometer unit of a cone apex angle 60° over an area of 10 cm2 . Winch and push rods were used to force the cone into the ground, while penetration resistance (qc) was read from pressure gauge at 5 locations. Plots of qc against depth of penetration were made in reverse order to obtain point resistance-depth curves. Allowable bearing capacity (ABC) of beds at depths of interest was estimated using Meyerhoff’s formula in kN/m2 . The geological conditions to depth of about 13 m, from Shomolu to Ajah revealed a maximum of 5 layered soil types. Across the study area, the fourth layer of 0.5 to 1.1 m thickness range with minimum resistivity of 17 _m, constitutes the competent layer. Although, the resistivity value is characteristic of organic clay/coarse sand, the ABCs for shallow foundations, regardless of the width at depths of between 4.25 and 9.75 m are 288.9 and 675 kN/m2 , respectively. The geoelectric sections revealed broad composite nature of the underlying strata, while the CPT curves furnished information about the lithology variability of the underlying soils.