Construction of Road/ Highway
There are two types of Road Construction.
1.) Flexible Roads/Pavement
2.) Rigid Roads/Pavement
In Flexible Road Construction
Total length of thickness=570mm
a) GSB Layers=300 mm
b) Wet mix macadam (WMM) Layer=120 to 150 mm
c) Dense Bituminous Macadam (DBM) = 80 mm
d) Bituminous Course (BC)/Wearing Course = 40 mm
Total =300+150+80+40
=570 mm
California bearing Ratio Method
The California bearing ratio of a soil which ranges from 0 to 100 % , gives a value about the quality of a material contrast to that of a magnificent base material for which the C.B.R. is assumed to be 100 percent.
There are many versions of C.B.R. method of flexible pavement design.
This is a penetration test developed by the California Division of Highways to evaluate the stability of soil sub-grade and other flexible pavement.
It contains a standard piston have a diameter of 50 mm which is used to penetrate the soil at the standard value of 1.25 mm/min.
Test Procedure
(a) Laboratory CBR apparatus contain a mold of 150 mm diameter with a base plate and a collar, a loading framework and dial gauges to evaluate the penetration.
(b)The specimen of soil is soaked in water for 4 days, and swelling and water absorption values is noted.
(c) Load is applied on the samples by a standard plunger with diameter of 50 mm at the rate of 1.25 mm/minutes.
(d) A graph is plotted between the load or pressure and penetration.
(e) Load values are noted corresponding to the 2.5 mm and 5.0 mm penetration.
(f) Calculate the CBR value corresponding to 2.5 mm and 5.0 mm.
CBR% = (Load carried by specimen at 2.5 or 5.0 mm penetration/ Load carried b standard aggregates at a corresponding penetration level ) x 100
Standard Load Values
Penetration (mm) Load (Kg) Pressure (Kg/Cm^2)
2.5 1370 70
5.0 2055 105
(g) Average CBR value of three test specimens is reported as the CBR value of the sample. Table.1 Typical CBR values of soils and their rating
Soil Range of CBR values Rating
Clay. 2-5 very poor Subgradade
Silt 5-8 Poor Subgrade
Sand 8-20 Fair to good Subgrade
GRAVEL 20-30 Excellent Sub grade
