Structure of a road | Methods for testing soil use as sub grade

Tuesday, September 20, 2011 10:50
Posted in category Transportation Engineering
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Structure of a road:

The structure of a road is composed of following parts

  • Sub grade
  • Sub base
  • Base
  • Surface course

Definitions of parts of structure of a road:

Sub grade:

The part of soil which carries the entire load of moving vehicles is known as sub grade.

Sub base:

The layer above sub grade is known as sub base. It is hard layer.


The structure built to resist stresses is known as base. It may either be

  • Hard
  • Soft

Surface Course:

It is the layer in contact with the traffic loads and often it contains the highest quality material. The characteristics provided by the surface course are friction, smoothness, noise control, rut and shoving resistance and drainage.

Methods for testing soil use as sub grade:

Group index method:

Before design of any pavement soil tests are done in order to check its consistency. The type of soil which has least group index is best for use as sub grade. The soil having zero group index is best for use if it is available.

Group index is calculated by following formula

                                          G.I=0.2a + 0.005 ac + 0.01bd

California bearing ratio method:

  • It varies from 1 to 100 %.
  • It gives information about quality of material used as a sub grade.
  • For an ideal material it is assumed as 100%.
  • For rest it is compared with ideal one.
  • It is abbreviated as C.B.R
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