Geotechnical Investigation of Soils and Underlying Basement Rock for the Construction of Southern Parkway Bridge at Federal Capital Territory (FCT), Abuja, Northcentral Nigeria

Abstract

It has been shown in several studies that factors such as composition, texture, degree of weathering, and presence of fissures or discontinuities have a first order control on engineering properties and behavior of soils and rocks. This paper investigates geotechnical properties of soils and the underlying basement rock at the site of the proposed Southern Parkway Bridge in Federal Capital Territory (FCT), Abuja, Nigeria with a view to assessing their strength and suitability to carry the load of the bridge and support the road under construction. The study involved two exploration borings to about 30 m on either side of the road using percussion drilling rig. A total number of six soil samples and four rock samples were collected for various geotechnical analyses. The borehole logs showed that the subsoil is composed of a suite of residual materials such as silty laterite with traces of clay, underlain by migmatite as the basement rock; with a subtle difference in the composition of materials from the two drilled borehole pits. The results of the particle size analysis revealed that the soils are generally poorly graded, while the Atterberg’s Limits indicated that the analyzed soils have high natural moisture content (14.3 – 21.3%), high Liquid Limit (31 – 49%), high Plastic Limit (22 – 34%) and high Plasticity Index (9 – 16%); all suggesting that the studied soils (regolith) can be regarded as poor for sub-base or base course materials, and thus could not be considered suitable for bridge construction. The results of unconfined compressive strength (170.21 – 184.63 MPa) and point load strength (3.42 – 6.60 MPa) show that the underlying basement rock (migmatite) has medium to high strength, and this implies that it has the desired load-bearing capacity to support the overlying weight of the proposed bridge. It is therefore concluded that the regolith at the site for the proposed bridge are unsuitable to support the bridge and for road construction, while the underlying basement rock has the desired capacity or strength to carry the bridge. We therefore recommended that the soils should be excavated and replaced with a higher grade, better quality material of improved geotechnical properties.