Geotechnical investigation of osogbo-iwo road collapsible soil’s clay fraction and improvement using bentonite

This research investigated the collapsible subgrade soils along Osogbo-Iwo Road in South-Western Nigeria. The road pavement has had a history of incessant differential settlement due to the effect of water on the subgrade material hence the soil’s dominant clay mineral was assessed, and an attempt was made to improve the soil with bentonite. Representative soils were taken from three failed sections of the road and their geotechnical characteristics were determined in the laboratory. The soil was classified according to American Association of State Highway and Transportation Officials (AASHTO) as A-5 (Silty-sand) and ML group (inorganic silts, fine sands with low plasticity) according to the USCS Classification system. The clay fraction (which is 10% of the soil mass) was extracted via sedimentation and centrifugation. From the result of the X-ray diffraction (XRD) and consistency limits tests carried out, Kaolinite was observed to be the dominant mineral of influence. The relatively inert nature of the clay fraction was deduced to be a major contributor to its collapsible nature. Bentonite was added to the soil in concentrations of 1-3 % respectively. The results showed that the untreated samples gave OMC values of between 11.7-14.97 %, MDD (1644-1453.6) kg/m3, UCS (61.98-78.01) kPa, Soaked CBR (2-6) % and a Collapse Index (C.I) of 10-12 % which places the soil under a “Severe trouble” category. 1-2 % bentonite gave the best improvement having moved the soil from the “severe trouble” category to the trouble category.