GEOTECHNICAL SITE EXPLORATION USING SEISMIC REFRACTION AND RESISTIVITY TECHNIQUES AT CONDOMINIUMS SITE LOCATED AT THE CENTRAL PART OF YENEGUA, BAYELSA STATE, NIGERIA.

Abstract

Over the past thirty years in West Africa and Nigeria in particular, the use of geophysical techniques in site investigations for civil engineering works is gradually gaining grounds. Use of geophysical methods in geotechnical investigations has the ability of bringing out the subsurface image of a construction site which is very important to the civil engineer (Goldstein, 2009; Benson and Yuhr, 2002). As a matter of fact, lack of knowledge about the subsurface strength distribution at a site before putting up a structure is in pretense a risk to the inhabitants and people living in its environment. Conventionally, questions pertaining to the foundation of buildings come when issues of buildings submerging under their load arise at a later time. Early discovery of subsurface conditions that may constitute potential danger to a building is crucial at the commencement and can be achieved with geophysical survey. Undetected cavities, fissures and other near surface features such as high clay content are the sources and risk to buildings put up without any geotechnical investigations.

Adversely, many buildings are constructed on soils with insufficient bearing ability to support the gravity of the structure. These soils may be partly made of expansive clays that contract or expand, as the moisture content is altered and can cause the foundation of these buildings to collapse resulting in unexpected breakdown from cracks if the moistening and drying of the clay is abnormal. Geological features and conditions beneath the surface such as voids, conduits, fractures, nearness of the water table to the surface, depth to bedrock are some of the prominent threats to the foundations of buildings (Andrews et al., 2013).  Refraction seismic is one of the most competent geophysical tools that has gained much attraction in site investigation for civil engineering work. It is a simple and effective means of acquiring valuable p-wave velocity information about a large volume of the subsurface in 2D. One leading fundamental use in the then days of the seismic refraction technique was to estimate the depth to bedrock, Besides seismic refraction, geoelectrical resistivity imaging technique is also increasingly becoming popular in environmental and engineering investigations (Aizebeokhai, 2010; Coskun, 2012; Yilmaz, 2011). Two dimensional poly-electrode electrical imaging system which simultaneously takes into accounts sounding and profiling has successfully been used to map areas with fairly complex geology (Aizebeokhai et al., 2010; Amidu and Olayinka, 2006). Depending on how the survey is designed and carried out, the electrical resistivity tomography can be used to acquire data that will give subsurface images in either two or three dimensions. Lately because automated data acquisition systems and efficient user friendly inversion softwares are accessible. The electrical resistivity imaging technique has the potential to give more reliable images of the subsurface (Aning et al., 2013).