Preliminary Geotechnical Assessment of Residual Tropical Soils around Osogbo Metropolis as Materials for Road Subgrade

Failure of roads is common in most part of the tropical region of the world. Majority of which can be attributed to poor selection of soil materials for road construction and climate. This study is aimed at investigating the properties of lateritic soil as material for road construction. Following an initial visit to the study area, soil samples were taken and subjected to geotechnical analyses such as particle size analysis, natural moisture content, Atterberg limits, compaction test, California Bearing Ratio (CBR), and direct shear test. All the tests were carried out according to the BS standard 1377 (1990). Results revealed a well graded fairly coarse soil with natural moisture content varying from 6.2-29.4 %. Atterberg limits shows a medium plasticity soil with medium compressibility and the soils classified within the A ? 2, A ? 6 and A ? 7 of the AASHTO classification scheme. In terms of soil strength, the maximum dry density (MDD) and optimum moisture content varies from 1210-1520 kg/m3 and 13-24 %, respectively while California Bearing Ratio (CBR) value varied between 2.27- 45.32 % at 2.5 mm and 3.01 - 44.59 % at 5.0 mm. The shear strength of the samples ranged from 31.28-186.41 kN/m 2 . Based on the results, the study concluded that the clayey nature of the soils was responsible for poor geotechnical properties which were generally below the specifications of the Federal Ministry of Works and Housing (FMWH), hence mostly not suitable in its present condition but could be improved to satisfy the conditions for use as sub- grade and sub-base soil in road construction.