Evaluation of Pavement Instability Section Using Integrated Geophysical and Geotechnical Methods in a Sedimentary Terrain, Southern Nigeria

Integrated geotechnical and geophysical investigations was carried out in order to harness the cause of the incessant subsidence of road pavement along section of Portharcourt - Eket road Southern Nigeria. The geotechnical investigation involves boring of boreholes and collection of samples for laboratory analysis. The geophysical investigation involves the Vertical Electrical Sounding (VES) technique using the Schlumberger configuration, and the horizontal profiling/vertical electrical sounding techniques using dipole-dipole configuration. The borehole log generally delineates three subsurface layer, which are the top brown yellowish soft clayey sand and dark brown loose medium fine sand, follow by a 13.5 - 15 m thick organic shale layer. These layer is considered in-competent due to its poor engineering properties and is suspected to be potential cause of the perpetual failure of the road section. The basal layer, which consist of the greyish dense coarse sand formation is the only competent layer on or within which the foundation of the road pavement can be placed, due to it good engineering properties. The geoelectric section generated from the VES interpreted results shows that the study area was underlain by four geoelectric layers namely, the lateritic sand/sand, the silty/sandy shale, organic shale, and the coastal sand. The organic shale that underlain VES 1 and 6 locations, is suggestive of zone of poor engineering material and is suspected to be the cause of the perpetual failure of the pavement. However, the last observed geoelectric layer is the coastal sand. The resistivity values of these coastal sand vary from 546 ohm-m to infinity. Meanwhile the depth to this layer varies from 7.4 to 69.4 m. This layer is the only competent layer on or within which the foundation of the road pavement can be placed. The result of the 2D imaging also delineate the soft diapiric shale at the distance of 60 - 110 m and 50 – 130 m at bridge 1 and 2 respectively. It is therefore concluded that the foundation of the road within the subsiding locations should be anchor on a pile.