Transcript TRANSPORTATION ENGINEERING II

CALIFORNIA BEARING RATIO
(CBR) METHOD
Typical Testing Machine
Introduction
• The California Bearing Ratio devised by
engineers of the California Division of
Highways in nine years period to 1938.
• Most universally accepted pavement
design methods.
• A standard penetration-type loaddeformation test is carried out, and using
the values obtained from the test from an
empirical design chart, the pavement
thickness are calculated.
CBR LABORATORY TEST
Specimen may be
• Prepared
• In-situ
Mould
•
•
•
•
150mm diameter
127.3mm height
Separate base plate
A collar
• The specimen is soaked and the expansion
is measured.
• Load is applied by the loading frame
through a cylinder plunger of 50mm
diameter and penetration is measured.
• Rate of penetration is maintained at
1.25mm/minute.
• Loads are recorded for 2.5 and 5.0mm.
• Load is expressed as a percentage of the
standard load at the respective deformation
level, and is known as the CBR value.
CBR Equipment
Typical Testing Machine
Soaking Samples
CBR Equipment
CBR Test – Sample Preparation
CBR Test – Load-Penetration
Measurement
California Bearing Ratio (CBR)
• Load a piston at a
constant rate
• See what load it
takes to make it
penetrate a
known amount
CBR Equation
x
CBR  100 
 y
x =
material resistance or the unit load on the piston
(pressure) for 2.5 mm or 5 mm of penetration
y =
standard unit load (pressure) for well graded
crushed stone.
For 2.5 mm of penetration = 1000 psi
For 5.0 mm of penetration = 1500 psi
Typical CBR Ranges
General Soil Type
Clean gravels
Gravels with fines
Clean sands
Sands with fines
Silts and clays
USC Soil Type
CBR Range
GW
40 – 80
GP
30 – 60
GM
20 – 60
GC
20 – 40
SW
20 – 40
SP
10 – 40
SM
10 – 40
SC
5 – 20
ML
15 or less
CL
15 or less
OL
5 or less
MH
10 or less
CH (LL>50%)
15 or less
OH
5 or less
California Bearing
Ratio Equipment
California Bearing
Ratio Vs Moisture
content
CBR DESIGN METHOD
• In this method, the CBR values are
used to determine the total thickness
of the flexible pavement and its
various layers.
• Following are the graphs used in the
design of the pavement thickness.
Flexible Pavement Design
Using CBR (Road Note 31)