Geophysical investigation of the causes of highway failures in Niger Delta Sedimentary Basin (a case study of the eastern part of east- west road), Nigeria

Abstract

The causes of the East-West highway failure in the Niger Delta Sedimentary basin have been investigated. The Vertical Electrical Sounding using the Schlumberger techniques was utilized. The aim of the investigation was to delineate the geoelectric and geologic parameters of the subsurface as a means of determining the causes of the highway failure. The survey was carried out in three failed and three stable portions of the highway. Five geo-electric layers were obtained from the interpretation. The subsurface layers are topsoil, clay to sandy-silt, fine sand, medium sand, medium-coarse sand and coarse sand. The resistivities of the top to the forth geoelectric layer for the failed segments ranges from 28.8-164Ω-m, 108.1-120.5Ω-m, 237.5-442.6Ω-m and 418.3-884.9Ωm. Similarly, the resistivities of the stable segments for the first four layers ranges between 209.6-348.47 Ω-m, 372.8-1247.8 Ω-m, 567-2977 Ω-m and 562-1419.04 Ω-m. Comparison of the resistivity of corresponding layers show that the stable portions of the road are more resistive than the failed segments. The low resistivities of the sub-grade in failed segments have been attributed to the presence of high moisture clay in the subsurface. The failures in the road arise from the differential settlement of the sub- grade clay. Other factors also attributed to the road failure from walkover survey are floods, buried channels, poor design and specification, poor workmanship and lack of drainage system. It is hereby recommended that the clay zones be replaced with more competent sands. The road should be built to specification with high quality materials and also drainage be included in the ongoing expansion and construction work.

Keywords: Highway, Geophysical Survey, Failed portion, Stable portion, Geological Factors