Hi,

Group Index method of flexible pavement design is an empirical method which is based on the physical properties of the soil sub-grade. To design the thickness of the pavement you have to go through the following steps:

- Find out the Group Index Value(GI) of the soil.
- Use the design charts to find out the thickness of the pavement and layers.

**Finding GI Value :**Now let's discuss Group Index(GI) first. Group Index is a number assigned to the soil based on its physical properties like particle size, Liquid limit and plastic limit. It varies from a value of 0 to 20, lower the value higher is the quality of the sub-grade and greater the value, poor is the sub-grade.

To find out the value of GI we, can either use the following equation, or we can use the charts

*GI = 0.2a + 0.005 a.c. + 0.01b.d*
where,

a= percentage of soil passing 0.074 mm sieve in excess of 35 percent, not exceeding 75.

b= percentage of soil passing 0.074 mm sieve in excess of 15 percent, not exceeding 55.

c= Liquid limit in percent in excess of 40.

d= Plasticity index in excess of 10.

The second method to find out the GI value is to use two charts whose numerical values added together to get the GI value.

**Using design charts to find the thickness:**

Once the GI value is found next we can use the design charts given by IRC(Indian Roads Congress) to find out the thickness of the pavement and its thickness of various layers. Charts are prepared based on the GI value and one can find the thickness of the pavement to be designed for different traffic volume.

Traffic volume is classified in three categories as Light, Medium and Heavy. When nos. of vehicles per day is less than 50, this is light traffic, greater then 50 and less than 300 is medium and greater than 300 is classified as heavy traffic.

You have follow the corresponding curve for the given traffic and for the one among the three different curves you can find the total value of the thickness of the pavement. Then there are two other curves, one to find out the thickness of the sub-base and one to find the thickness of the base course.

**Limitation of Group Index method:**Limitation of this method is that this is based only on the physical properties of the soil and does not consider the strength parameters.

Thank you!