APPLICATION OF GEOPHYSICAL METHODS IN FOUNDATION INVESTIGATION FOR CONSTRUCTION PURPOSES AT OLKARIA (V) FIELDS, KENYA.

Abstract

Geophysical methods were used in structural foundation investigation at Olkaria (V) field in Kenyan rift valley prior to the construction works. Geophysical techniques were of preference because they are faster, cost effective and non-destructive compared to conventional way such as boring and drilling which are expensive, invasive and provide information in discrete area. Geophysical investigations provide a wider picture of the subsurface that aid in evaluation of ground fitness for construction purposes. The objective of this investigation was to determine the depth to bedrock that forms a firm foundation for development purposes. Electrical resistivity and seismic methods were used in ground assessment for its suitability in structural development. Data was collected using electrical resistivity imaging system, geographical positioning system (GPS), receiver and seismograph. The RES2DINV and Geometrics SeisImager Software respectively were used to process results obtained from these investigations. The methods used gave results that are significant in the preliminary stages of site assessment for foundation works. The techniques probe the depth to the bedrock and reveal weak soils zones. The analysis and interpretation of geophysical data provide a better understanding into the subsurface geology for foundation works. Compact rock materials were observed from a depth of 14m below ground level extending down to greater depths. Further, the area was characterized by weak Silty material, which has low bearing and consolidation ability. Such materials are prone to frequent washout under conditions of percolating waters hence require utmost care during the placement of engineering structures. Utilization of piles is necessary and should be anchored to a depth greater than 14m below ground level such that they rest directly on the competent bed. In-situ compaction should go before utilization of reinforced concrete amid the development of shallow foundation to achieve the maximum compaction limits due to the anticipated load of the proposed infrastructure. Equally, excavation of the top soil is necessary and thereafter refilling with competent material such as gravel and laterite in order to have a strong basement