Laboratory Investigation for Variation of Subgrade Strength Related to Moisture

Abstract

 The design of the pavement layers to be laid  over sub grade soil starts off with the estimation of sub grade  strength and the volume of traffic to be carried. Design of the  various pavement layers are very much dependent on the  strength of the sub grade soil over which they are going to be  laid. Sub grade strength is mostly expressed in terms of CBR  (California Bearing Ratio). Weaker sub grade essentially  requires thicker layers whereas stronger sub grade goes well  with thinner pavement layers. The sub grade is always  subjected to change in saturation level due to precipitation,  capillary action, flood or abrupt rise or subsidence of water  table. Change in moisture level in sub grade causes change  in the sub grade strength. And it becomes quite essential for an engineer to understand the exact nature of dependence of  sub grade strength on moisture variation. An understanding  of the dependence of the CBR strength of local soils on water  content will contribute towards better design and  maintenance practices. Normally CBR test is an easy and  well adopted method conducted on soil samples to measure  the strength of sub grade. However, many other tests are also  considered for assessing the sub grade strength. The strength  of soil, used for sub grade may vary largely on the amount of  saturation in it, i.e. amount of water exposed to the soil.  Hence, in this study an attempt has been made to vary the  degree of soaking and hence the saturation level in various  types of soils and study the engineering properties of soils  including CBR at different saturation levels. It is observed that for fine grained and clayey soil, worst  engineering properties are observed after three days of  soaking.