a) Group index method: Fine grained soils of each classification group exhibit a wide range of properties in sub grade material. In order to classify the fine soils within one group and for judging their suitability as sub grade material, an indexing system has been introduced in HRB classification which, is termed as group index. Soils are thus assigned arbitrary numerical numbers called as group index (GI), which is function of percentage material passing 0.074 mm sieve, liquid limit and plasticity index of soil and is given by the equation: GI =0.2a +0.005ac=0.01bd According to thisformula, the minimum possible value of group index is zero and maximum value is 20, when the values of soil fraction passing 0.0074 mm sieve, liquid limit and plasticity index are respectively higher than 75, 60, and 30 percent.
b) C.B.R: The design chart is useful to design flexible pavements of road with low to moderate flow of heavy vehicles for design CSA values 1 to 10msa. In this chart the CSA values are plotted on the X axis and the total pavementthickness (in mm) on the Y –axis, both in arithmetic scales. Nine different design curves are given for different sub grade CBR values in the range of 2 to 10% Using the design CBR value of the sub grade soil, the total thickness of flexible pavements is determined using one of the two appropriate design charts for CSA values 1 to 10 msa& for CSA values of 10 to 150 msa given. The critical value of vertical compressive strain on the sub grade on the top of the sub grade or layer 3 and the rut depth are determined.