A laboratory study on effect of test conditions on subgrade strength

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. Subgrade strength is mostly expressed in terms of CBR (California Bearing Ratio). Weaker subgrade essentially requires thicker layers whereas stronger subgrade goes well with thinner pavement layers. The pavement and the subgrade mutually must sustain the traffic volume. The Indian Road Congress (IRC) encodes the exact design strategies of the pavement layers based upon the subgrade strength which is primarily dependant on CBR value for a laboratory or field sample soaked for four days. The subgrade is always subjected to change in its moisture content due to rainfall, capillary action, verflow or rise of water table. For an engineer, it’s important to understand the change of subgrade strength due to variation of moisture content. This project is an attempt to understand the strength of subgrade in terms of CBR values subjected to different days of soaking and the corresponding variation in moisture content. It is observed that the CBR decreases and the moisture content increases for high degree of soaking.