Reducing the cost of road construction through targeted geotechnical and geophysical investigations - a case study of road section re-design in the Hwereso valley of Ghana

摘要

The Hwereso valley that is crossed by a section of the main highway linking the north and south of Ghana, is a 2.0km wide flood plain formed by tributaries of the Hwere River in the Ashanti Region of Ghana. Prior to its reconstruction in 2004, as part of the general upgrading of the Accra-Kumasi highway, this section of the road was subjected to perennial flooding and pavement failures as a result of an incompetent clay sub grade. The original road upgrade design recommended the construction of a bridge with piled foundation across the Hwere and an embankment across the valley. Through detailed geotechnical investigation targeted at this section of the road during the construction stage, the soil profile along the centerline of the road was defined in detail and a complete characterization of the subsoil soil materials undertaken. The depth to bedrock was determined from seismic refraction surveys. Based on the results of these detailed investigations, the drainage structures and foundation design were revised to a large, single bay box culvert with 3No helper pipe culverts. The idea of a 2km long embankment across the valley was abandoned and subsoil drainage system consisting of French drains was adopted to eliminate accumulation of water around the culverts inlets. The redesign enabled the project to avoid the expensive piling exercise and large volume earthworks of the embankment construction recommended in the initial upgrade design thereby making savings in the project cost and construction time.