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.