Abstract:
Causes of failure on the Ikere-Igbara-Odo road were investigated in this study. Geological mapping was carried out and three (3) rock samples were taken for Petrographic studies. 16 disturbed soil samples were also collected for geotechnical analyses which include the determination of specific gravity, natural moisture content, consistency limits, linear shrinkage, grain size analysis, compaction, California Bearing Ratio (CBR) and clay mineral composition.
The Petrographic analysis revealed the rock types to be biotite granite, biotite-hornblende granite and quartzite. The natural moisture content of the soils in the study area range between 7.64- 41.18%, the liquid limit between 18.4-53.5%, the plastic limit between 12.1-43.6% and the plasticity index between 3.4-29.2%. Shrinkage limit is between 5.3-14.4% while the grain size analysis revealed that the percentage passing through sieve No. 200 is between 26.6-77.2%. The CBR ranges between 14-63% while the compaction characteristics of the soil indicated a maximum dry density (MDD) of between 1622kg/m3 – 2098kg/m3 at optimum moisture content (OMC) of 13-22%. The clay mineral composition consists mostly of Montmorillonite in the soils from the failed portion; and Illite, Kaolinite and Halloysite in the soils from areas with little or no symptoms of failure.
The engineering geological data on the areas of intense failure revealed that the presence of montmorillonite with its swelling and shrinkage characteristics is a major cause of failure. Hence, the residual soil in these failed parts of the study area is not suitable as subgrade material and should therefore be excavated to a reasonable depth and a more competent material introduced as subgrade.
