Abstract:
The problem of bad roads in Nigeria has become an embarrassing stigma in many parts of the country. Failure of these roads is due to the non involvement of engineering geologists in their construction. Surface geological mapping, field observations and laboratory geotechnical analysis were employed to evaluate the contributions of engineering geological properties of the sub-grade soils to the occurrence of road failure along Akure-Ikere road, South-western Nigeria. Field observation show that the road is in a very poor state of disrepair as major portions of the road alignment have failed. A total of twenty disturbed samples were collected along the 25km stretch of the road. These samples were subjected to various laboratory tests like soil-index and strength, (Natural Moisture Content, Mechanical Grain Size Analysis, Hydrometer Analysis, Atterberg Limits, Specific Gravity, Linear Shrinkage, Activity, Free Swell, Compaction and California Bearing Ratio) tests. Samples were classified based on USCS and AASHTO. The study area is underlain by two major rock types viz porphyritic granites and charnokitic rocks. The results of the laboratory tests revealed that the natural moisture content ranges from 10.98 to 21.4%, liquid limit from 22.8 to 47.7%, plastic limit from 19.2 to 24.6, plasticity index 3.6 to 26.3%. The grain size analysis shows that the amount of fines ranges from 15.9% to 49%. others are linear shrinkage between 1.4 and 10%, free swell between 25 and 46%, maximum dry density from 1593 kg/m3 to 2016kg/m3,optimum moisture content from 11%-22.4% and CBR between 5% and 48%. The specific gravity ranges from 2.64 to 2.74. Based on AASHTO classification 15% of the samples classified as A-2-4, 5% classified as A-4, 40% classified as A-6, while 40% classified as A-7-6. Dominance of fair to good California bearing ratio values, fair to good maximum dry density, high linear shrinkage and A-7-6, A-6 and A-2-4 soil groups have combined to give fair to good geotechnical properties to the subgrade. Generally the fair to good subgrade geotechnical
properties ratings of the studied road is an indication that subgrade might not be the major cause of failure at this locality.
