Before Design of Flexible Pavement by Group Index Method, let us understand What is Flexible Pavement?
A true flexible pavement yields “elastically” to traffic loading. It is constructed with a bituminous-treated surface or a relatively thin surface of hot-mix asphalt (HMA) over one or more unbound base courses resting on a subgrade.
Group Index method of flexible pavement design is an empirical method which is based on the physical properties of the soil sub-grade.
Determination of Group Index Value of Soil SubgradeGroup 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. By sieve analysis test we can determine Group index value of soil subgrade from below equation
GI = 0.2a + 0.005 ac + 0.01bdWhere, a= percentage of soil passing 0.074 mm sieve in excess of 35 per cent, not exceeding 75. b= percentage of soil passing 0.074 mm sieve in excess of 15 per cent, not exceeding 55. c= Liquid limit in per cent in excess of 40. d= Plasticity index in excess of 10.
Group Index Method of Flexible Pavement Design
Data Required for Flexible Pavement Design1. Group index of soil subgrade Group index value range of different soils is given below
- For good soil – 0 to 1
- For fair soil – 2 to 4
- For poor soil – 5 to 9
- For very poor soil – 10 to 20
- <50 - light traffic
- 50-300 - medium traffic
- >300 - Heavy traffic
Flexible Pavement Design Procedure:Before going to design the pavement we must know the structure of flexible pavement
= T - tsbThe combined value of thickness of base and surface course can be found out from above chart form dotted curve with the help of group index value and traffic volume. Or otherwise assume the thickness of surface course (ts) = 5 cm Then we can easily calculate the value of thickness of the base course,
Tb = T-tsb-tsCross section of flexible pavement: