Vertical electrical resistivity investigation of foundation conditions near a buried stream channel in part of Port Harcourt, River State, Nigeria

Abstract

Four Vertical Electrical Soundings have been carried out for building sites using Schlumberger array within a buried River channel near Okilton close, Port Harcourt. The objective was to delineate the different geoelectric and geologic parameters of the subsurface as a means of determining its effect on foundation. Five geoelectric layers were obtained from the interpretation of the field curves by computer modelling.This includes the topmost layer which consists of silt, sand and organic materials (humus). The topmost layer is followed in succession by clayey layer, clay/silt, fine sand and coarse sand. The topsoil and clay layers resistivities range between 28.8 to 168 ohm’s meters and 115.3 to 120.5 ohm’s meters respectively. The maximum resistivity obtained for the fine sand layer ranges between 421 to 885 ohm’s meters. The low resistivities of the subsurface layers may be attributed to the high moisture content, presence of clay and the soft subsurface materials of the buried river channel. The clay is susceptible to expansion on moisture influx and contraction when it gives out the water. The clay and soft materials that filled the buried channel release the moisture and consolidate gradually when a building is on it. The subsurface materials therefore have great effect on the long term stability of the building foundations in the area due to gradual subsidence of the buildings. The low apparent resistivities of the subsurface layers are also an indication that the corrosion of the area is high to moderate. The results show that the durability and lifespan of civil engineering structures and underground utilities in the area will be reduced. It is advisable to replace the topsoil and the clay layer with more competent materials before the foundation of any building in the area and there should be a drainage system in the area.

Keywords: Okilton, Buried channel, Building, Foundation and subsidence