Correlation between the dynamic cone penetration index and the subgrade resilient modulus obtained from the Falling Weight Deflectometer

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Modern pavement design characterizes the subgrade soil in terms of the resilient modulus. This parameter is determined in a non-destructive in-situ method using the falling weight deflectometer (FWD) and in the laboratory from the repeated load triaxial test. These methods however are both expensive and time consuming and are not readily available in most highway departments in developing countries. On the other hand, the simple and inexpensive dynamic cone penetrometer has been extensively used for pavement in-situ subgrade characterization. The objective of this thesis is to find a correlation between the results of the Dynamic Cone Penetrometer index, DCPI and the output of the Falling Weight Deflectometer (FWD) test for purposes of estimating the subgrade resilient modulus. The study road is a major urban arterial road in Accra with varying terrain, comprising of a hill flanked by two valleys. The study road was sectioned into three according to the terrain, i.e. section 1 and 3 are in the valley whiles section 2 is on the hill. Fifty-two FWD deflections and Dynamic Cone Penetrometer field tests were conducted at these sections. Laboratory tests, performed on soils recovered from five trial pits sunk within this varying terrain, revealed two types of soils, namely, A-2-7 soil on the hill and A-6 soil in the valley. The results of the output of the FWD sensors were analyzed and correlated with the DCP penetration index, DCPI, for the subgrade soils and the unbound material lying on the subgrade. Statistical regression model for predicting the subgrade resilient modulus was developed based on the the results of the Dynamic Cone Penetrometer index, DCPI and the output of the Falling Weight Deflectometer (FWD) test for the subgrade soils. This was extended to the unbound material overlying the subgrade soil. The statistical regression model for predicting the resilient modulus based on the field tests of FWD and DCP penetration index results were developed on section by section basis for the subgrade soils and also for the unbound material. Combined sections with similar soil characteristics were also considered for both the subgrade and the unbound material. The model correlates the subgrade resilient modulus to the DCP penetration index, DCPI. Poor agreement were obtained between the resilient modulus of the of subgrade soils (A-6 and A-2-7) on the study road and the DCP penetration index, DCPI.
A Thesis Submitted to the College of Engineering Kwame Nkrumah University of Science and Technology in partial fulfilment of the requirements for the degree of Master of Science in Road and Transportation Engineering.