Stratigraphic Influence on Geotechnical Properties of Subgrade Soils along the Irrua-Auchi Road, South Eastern Nigeria

Abstract

Geotechnical investigation of the Irrua – Auchi road was carried out to determine the engineering properties of the subgrade soils and evaluate the pavement conditions with a view to establishing the causes of failures along portions of the road. This involved a study of the route geology, measurement of pavement deflection using the Benkelman beam device and in-situ dry density test of the subgrade soils using the nuclear gauge. Other tests included the routine geotechnical laboratory tests to determine index properties, compaction and CBR characteristics of the subgrade soils. Results indicated that the road traversed terrain underlain by the Ajali sandstone, the Imo shale and the Ameki Formations. The subgrade soils, composed mainly of fine grained materials classified as  A-4 to A-2-4., were  characterized by  plasticity index  of  4 -19%,  MDD  of 1.83 – 1.92 Mg/m³, 48- hour soaked CBR of 0.86 to 1.12 to show the fairly similar geotechnical properties of the soils. The segment of the road underlain by the Ajali derived soils was characterized by average relative compaction of 0.96 and average deflection of 0.26 mm and is classified as stable. The segments underlain by the Ameki and the Imo shale derived soils were characterized by average relative compaction of 0.89 and 0.88  and average pavement deflections of 0.39 mm and 0.61 mm respectively and are accordingly classified as less stable and least stable. Conclusively, in this study, the pavement deflections revealed pattern that reflect the influence of soil structures derived from stratification of the underlying geologic units.

Keywords: key words, Subgrade soils, Engineering properties, Pavement deflection, Route geology