Effect of Clay on the Strength of Compacted Subgrade Soil for the Design of Flexible Pavements

Abstract

Design of the various pavement layers is very much dependent on the strength of the subgrade soil over which they are going to be laid. As subgrade gives support to the overlying pavement layers, subgrade should possess sufficient stability under adverse climate and loading conditions. Subgrade strength is mostly expressed in terms of California Bearing Ratio (CBR) value. Most often clay is present in subgrade soil which affects the strength characteristics of the subgrade soil. The presence of clay in the subgrade influences in the CBR value which affects the pavement thickness. Therefore, this investigation is carried out to check the possibility of using clay mix soil in subgrade for the construction of flexible pavements. In this project work, fine sand of Karnaphuli river having fineness modulus (FM) of 1.47 was mixed with different percentage of clay content (0%, 10%, 15%, 20%, 25% and 30%). Since the subgrade is always subjected to change in its moisture content due to rainfall, capillary action, overflow or rise of water table, both the unsoaked and soaked CBR values were determined to check the effect of CBR value on the pavement thickness. The engineering properties especially maximum dry density (MDD) and optimum moisture content (OMC) were also determined with the above percentage of clay soil. It was found from the result that fine sand containing 20% of clay can be used as subgrade material in road construction considering the dry density and CBR values.