Geotechnical Investigation and Effect of Moisture Content on Subgrade CBR Values; Arbaminch-Chencha Existing Road; Ethiopia

Abstract

In the flexible pavements sub-grade is the undermost layer for pavement structure constructed on it and used to resist traffic load. Hence, subgrade geotechnical behaviors are considered as measuring parameters of the pavement design and performance. But, here Arbaminch-Chencha road is constructed as gravel road twelve years so far without any geotechnical investigation. The aim of this research is geotechnical investigation and effect of moisture content on CBR of the subgrade soil of Arbaminch-Chencha road. This is to classify and characterize this subgrade according to its geotechnical properties, to identify effect of moisture on subgrade CBR for selected three soil classes. To achieve these objectives, seventeen representative test samples collected along the existing road. Laboratory investigations performed for in-situ moisture content, specific gravity, grain size analysis, Atterberg's limit test for all pit samples. And then soils are classified. After classification of soils; laboratory moisture-density relationship and CBR values are determined. The effect of moisture on its CBR was observed for specified soil classes. For all tests the apparatus and the procedures used for analysis were done according to ASTM and AASHTO standards. According to AASHTO soil classification, Arbaminch-Chencha road subgrade soils classified as coarse and fine grained soil; but dominated by fine grained soil class. As per observed, laboratory maximum dry density of this soil ranges from 1.36 gm./cc to 2.11 gm./cc and OMC range from 10 % to 30%. And four day soaked CBR Values are ranges from 3 % to 49 % under specification of 95% MDD. Effect of moisture revealed as, CBR value reduced 4% to 28% at dry side whereas 25% to 59% at wet side of OMC and MDD of specified soil classes. Saturation after 48 to 96 hours, CBR reduced from 17% to 30% whereas 96 to 144hours reduced 7% to 20%. But, continuous and linear reduction observed in finer soil classes.