This document discusses the group index method for flexible pavement design. It begins by defining group index as a number from 0-20 assigned to soil based on physical properties like particle size, liquid limit, and plastic limit. Lower values indicate better soil quality. Group index is determined mathematically using a provided equation or graphically. Required data for design includes group index, traffic volume, and flexible pavement structure. Total thickness is selected from a chart based on group index and traffic volume. Thickness of sub-base is also from a chart based only on group index. Remaining thickness is allocated to base and surface courses. An example problem demonstrates calculating group index and designing pavement layers.

2. Submitted to : Sir Atif
Submitted By: Usama Aslam
(15-CLE-47)
NFC-IEFR-FSD
Pavement and Foundation
Engineering

3. What is Group index
method?
 Group Index method of flexible pavement design is an
empirical method which is based on the physical
properties of the soil sub-grade.

4. Determination of Group Index
Value of Soil Subgrade
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.
It can be found by 2 types:
(i) Mathematically (ii) Graphically

5.  By sieve analysis test we can determine
Group index value of soil subgrade from
below equation mathematically
 GI = 0.2a + 0.005 ac + 0.01bd
 Where,
 a= that portion of %age of soil particles passing U.S
sieve #200, greater than 35 and not exceeding 75 (0-
40)
 b= = that portion of %age of soil particles passing U.S
sieve #200, greater than 15 and not exceeding 55 (0-
40)
 c= that portion of LL greater than 40 and not exceeding
60 (0-20)
 d= that portion of the plasticity index greater than 10
and not exceeding 30 (0-20)

6. Data Required for Flexible
Pavement Design
 1. 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
 2. Traffic volume
 It is the measure of Annual average daily traffic, peak-hour
traffic. It is denominated by commercial vehicles/day or
CVPD.
 It is classified in three categories. Based on number of
vehicles per day.
 If no. of vehicles per days is
 <50 – light traffic
 50-300 – medium traffic
 >300 – Heavy traffic

7. Flexible Pavement Design
Procedure:
 Before going to design the pavement we must know the
structure of flexible pavement

8. Calculation total thickness (T):
 From the below chart for given group index of soil
subgrade and traffic volume value select appropriate
thick curve value of “combined thickness of surface,
base and sub-base line” which will give the total
thickness of pavement.
(Note: thick line indicates the total thickness value and
the dotted line indicates thickness of surface and base)

9. Calculation thickness of sub-base
course (tsb):
 From the below chart for given group index of soil
subgrade select appropriate curve value of “thickness of
sub base only” which will give the thickness of the sub-
base course.
 The curve highlighted in below diagram

10. Calculation of thickness of base
and surface course (tb & ts):
 Thickness of surface and base course = total thickness –
sub-base thickness
 = T – tsb
 The 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-ts

11. Cross section of flexible
pavement:
 The group index method is essentially an empirical
method based on the physical properties of the subgrade
soil and it does not consider the strength characteristics
of soil and is therefore open to question regarding its
reliability.

12. Numerical Problem:
The laboratory data for a soil is given below. Using these
values first find the Group Index Value and than by using
the value of Group Index Design the Flexible Pavement.
Using the Traffic Volume of 200

13.  Let first find out the GI of the Soil.
 Formula for Group Index :
 GI=0.2a+0.005ac+0.01bd

 a = 70-35 = 35 (range 0 to 40 )
 a = 35
 b = 70 – 15 = 55 > 40 use b = 40
 c = 49 – 40 = 9 in the range of (0 to 20)
 d = 49-24 = 15 in the range of ( 0 to 20)

 GI = (0.2)(35) + (0.005)(35)(9) + (0.01)(40)(15)
 GI = 14.5use 15
 GI = 15 (Very poor Soil)

14. As the Traffic Volume is 150 and the category of the traffic is
Medium traffic.
We have the value of GI and also traffic category so by using graph
we can find out the thickness of each layer and also the total
thickness of flexible pavement.
By using the graph.
Thickness of Sub-Base = Tsb = 280 mm
Additional thickness of sub-base = 140 mm
Total Thickness of Sub-Base = Tsb = 280 + 140 =
420 mm
Total thickness of Pavement = T = 500 mm

15.  Assume the thickness of Surface Course = Tsur = 50
mm
 So by using these value we can find out the value of
Base Course.
 TB = T - Tsb - Tsur
 TB = 500 – 420 – 50 = 30 mm