Weathered Layer Determination and Its Effects on Engineering Structures: Case Study of Parts of the Niger Delta

Abstract

Failures associated with civil engineering structures in the study area with specific reference to roads and buildings have been attributed to lack of proper understanding of the weathered layer characteristics. This study was therefore carried out using downhole seismic refraction data from twenty borehole locations within parts of PortHarcourt, Niger Delta, Nigeria to determine the weathered layer characteristics of the area. The survey was carried out with the Geometrics Stratavisor NZ11 instrument using a set of hydrophones arranged in a definite manner in each of the boreholes. The data was processed using Udysys software and analyzed graphically using the seismic refraction interpretation technique. Several contour and analytical maps were generated for the weathered and consolidated layers. Similarly, four interpretative cross sections namely NW-SE, NE-SW, N-S and E-W were used for the purpose of correlation. Results of the study revealed that the velocity of the weathered layer ranged between 144 and 996m/s with a regional average of 407 m/s. The results also revealed that the thickness of the weathered layer ranged between 3.0 and 9.6m with a regional average of 5.0m. Velocity of the consolidated layer varied between 1449 and 1812m/s with a regional average of 1738m/s which is adjudged sufficiently competent to withstand engineering structures. It is therefore recommended that construction of heavy buildings and roads especially bridges in the area should have their foundations laid to a minimum depth of 9.6m to enable the infrastructure sit on the consolidated layer.