Geotechnical Properties of Subgrade Soil along Failed Portions of Akungba-Ikare Road Stabilised with Rock Fines

There is an increasing incidence of road failures in Southwestern Nigeria. Many asphalt roads have failed sections soon after construction and rarely reach their design life span before major repair is required. The aim of this study is to conduct geotechnical properties of sub-grade soil along failed portions of Akungba-Ikare road in the Northern part of Ondo State, Southwestern Nigeria with a view of determining the possible causes of the failures experienced along the road. Four samples were collected from four different locations and stabilised with varying percentages of rock fines. Laboratory tests conducted on the soil samples were natural moisture content, liquid limit, plastic limit, grain size analysis, linear shrinkage, specific gravity, compaction and California bearing ratio (CBR). The test results showed that the natural moisture content of the soil samples ranges from 9.3 to 26.4%, the liquid limit from 0.0 to 46.4%, plastic limit from 0.0 to 40.0% and plasticity index from 0.0 to 31.61% which did not conform to specifications. The grain size analysis shows that the amount of fines passing 425µm sieve ranges from 33 to 51% which are higher than the standard specifications. The linear shrinkage ranges between 5.8 and 15.3%. The specific gravity ranging between 2.58 and 2.73 fall within the standard specifications. The maximum dry density ranges between 1497 and 1982 kg/m3 and optimum moisture content between 4.7 and 12.8%. The California bearing ratio (CBR) values for the unsoaked soil samples range from 0 to 5% which fall below the standard. Despite the use of stabilisation material (rock fines) at different percentages, the poor geotechnical properties of the soils as indicated by the low maximum dry density, high linear shrinkage, high liquid limit values, high amount of fines, low CBR values and lack of drainage in the study area is the greater causes of the failure of the road portion, hence standard materials in conformity with the Federal Ministry of Works specifications for road constructions must be adhered to during repairs of the road.