GEOTECHNICAL SUBGRADE SOILS PERFORMANCE EVALUATION IN CASE OF HUMBO-GESUBA GRAVEL ROAD

Abstract

The geotechnical performance evaluation of subgrade soils of Humbo-Gesuba gravel Road, Southern Ethiopia, is investigated to evaluate their performance as subgrade. Here, Humbo-Gesuba road is a road for which a thin layer of gravel was found spread and compacted directly on the subgrade without any subgrade investigation. The road is with some parts having no defined road alignment and road formation width. A total of 26 subgrade samples along the road is collected and subjected to index properties test, compaction test, California Bearing Ratio (CBR), and CBR swell test under ASTM and AASHTO standards for soil testing. Soil index properties revealed that the subgrade soils composed mainly of fine contents and percentage passing 0.075mm sieve ranged from 56.07% to 94.55%. The maximum dry density ranged between 1.31 g/cc and 1.62 g/cc while their optimum moisture contents ranged from 20.5% to 36.5%. The liquid limit ranged between 34% and 91%; the plastic limit ranged between 13% and 46% while the plasticity index ranged from 8% to 47%. The three-point CBR (96 hours - soaked) at 95% MDD ranged between 2.2% and 8.7%. The study concluded from the results of the compaction test, Atterberg limits test, and CBR test that the subgrade soil samples of chainages 8+000-16+000, about 35% of the total, perform poorly to use as either subgrade or road construction materials. Most Humbo-Gesuba road subgrade soils rate poor to fair as subgrade materials. The study has established the need for the upgrade of the performance of these unsuitable materials for use as the subgrade. 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 subgrade soil to the depth of 1m, 1.3m, and 1.7m and the subsequent replacement with higher quality materials for subgrade soils of plasticity index of 20%-30%, 30%-40%, and 40%-50% respectively. The stabilization method prescribed will require the addition of asphalt as the binder. The choice of the method adopted shall strictly be guided by the cost implication of the implementation of the method.