Evaluation of Subgrade Soils using California Bearing Ratio (Cbr) in Parts of Rivers

Subgrade soils are characterized by their resistance to deformation under load, which can be a measure of strength (stress needed to break a material) or stiffness (the relationship of stress and strain in the elastic range or how well a material is able to return back to its original shape and size after applying stress). The greater the resistance to deformation a subgrade is the more load it can support before reaching a critical deformation value. The California Bearing Ratio (CBR) test is a simple strength test that compares the bearing capacity of a soil with that of a well graded crushed stone (a high quality crushed stone material should have a CBR of 100). The collected soil samples were taken to the laboratory for the 24 hours soaked California Bearing Ratio (CBR) test, Standard Proctor Compaction, moisture content and classification. These various tests were carried out according to AASHTO 1993, ASTM D-1883 and BS 1377 standard test procedures. The California Bearing Ratio (CBR) of the samples range from 12 - 34%. The plasticity index (PI) of the samples fall within 10 – 20%. The study has established the need for the upgrade of the performance of the studied material for use as subgrade. In view of this fact therefore, possible options of upgrade were considered in line with economic considerations. The two options that are fit for the upgrade are the over – excavation and stabilization options. The over - excavation method of enhancement of the performance of the subgrade materials that have been studied will require the excavation of the sub- grade soil to the depth of 0.7m and the subsequent replacement with higher quality materials. The stabilization method prescribed will require addition of asphalt as the binder. The choice of method adopted shall strictly be guided by the cost implication of the implementation of the method.Keywords: Subgrade soils, California Bearing Ratio (CBR) test, Plasticity Index (PI), Rivers State, Nigeria.