THE EFFECT OF SOAKING PERIOD OF SUBBASE MATERIAL ON ITS CALIFORNIA BEARING RATIO VALUE

Abstract

A flexible pavement structure consists of several layers of which the most bottom layers are the subbase and pavement foundation or subgrade. A pavement structure is a layer structure which supports the vehicle load on its surface and transfers and spreads the load to the subbase and subgrade. The effects of weak subgrade and poor subbase course materials on pavement design, construction, and performance prediction are very significant. This study examined The Effect of Soaking Period of Subbase Material on Its California Bearing Ratio Value. The main objective of this study was evaluating The Effect of Soaking Period of Subbase Material on Its California Bearing Ratio Value. Sub-base material was sampled from two borrow sites located in Hawassa Zuria and Wendo Genet Woredas, Sidama Region, Ethiopia. (i.e. designated by borrow site one (BS-1) finchawha site), and borrow site two (BS-2), Wosha site respectively) The laboratory tests were conducted to determine the physical properties of the sub-base material. Laboratory investigations performed for Moisture content, Grain size analysis, Atterberg’s limit (LL, PL, PI, and LS) test. And then soils are classified. After classification of soils, Moisture – density relationship and CBR values are determined. The CBR tests were performed on samples for different soaking conditions and periods (0, 1, 2, 3, 4, 5, 6 and 7 days). The results showed that the sub-base material is classified as well-graded gravel (GM) and poorly graded gravel with clay and sand (GC-GP) according to the Unified Soil Classification System (USCS), while A-2-4 (0) with respect to AASHTO soil classification system. The Average maximum dry density and optimum moisture content were 1.91 g/cm3 and 13.65%, respectively. The obtained unsoaked CBR value for BS-1 and BS-2 were 61% and 46%, respectively. The soaked CBR values for 1 day of soaking period for BS-1 is 58%, but the soaked CBR value for the soaking period from 2 to 7 days remains unchanged and it was 55%. In addition, the soaked CBR values for 1 to 7 days of soaking period for BS-2 were 46%, 42%, 38%, 35%, 32%, 31%, 30% and 30%, respectively. The findings indicate that the CBR value decreased as the soaking period increased specially for plastic soils than non-plastic soil, implying a reduction in the CBR value of the sub-base material. A soaking period of 96 hours may be most reliable for cohesive soils; its effectiveness for coarser cohesionless soils appears limited, as indicated by the stabilization of CBR values beyond this duration. Therefore, it is recommended to consider the effect of soaking period during the design phase of the pavement structure and while conducting investigations on different soil groups which are solely used for subbase material to ensure the long-term performance.