CBR
CBR (CALIFORNIA BEARING RATIO) TEST OF
SOIL(IS-2720-PART-16-1979)
OBJECTIVE
Determination of CBR of soil either
in undisturbed or Remoulded condition
REFERENCE STANDARDS
IS: 2720(Part 16)-1973- Methods of
test for soils: Laboratory determination of CBR.
EQUIPMENTS / APPARATUS
§ Compression machine
§ Proving ring, Dial gauge, Timer
§ Sampling tube
§ Split mould
§ Vernier caliper
Fig:-Showing CBR testing
apparatus
PREPARATION SAMPLE
The test may be performed
(a) On undisturbed soil specimen
(b) On remoulded soil specimen
(a) On
undisturbed specimen
Undisturbed specimen is obtained by
fitting to the mould, the steel cutting edge of 150 mm internal diameter and
pushing the mould as gentky as possible into the ground. When the mould is sufficiently
full of soil, it shall be removed by under digging. The top and bottom surfaces
are then trimmed flat so as to give the required length of specimen.
(b) On
remoulded Specimens
The dry density for remoulding should be either the field density or if the subgrade is to be compacted, at the maximum dry density value obtained from the Proctor Compaction test. If it is proposed to carry out the CBR test on an unsoaked specimen, the moisture content for remoulding should be the same as the equilibrium moisture content which the soil is likely to reach subsequent to the construction of the road. If it is proposed to carry out the CBR test on a soaked specimen, the moisture content for remoulding should be at the optimum and soaked under water for 96 hours.
The dry density for remoulding should be either the field density or if the subgrade is to be compacted, at the maximum dry density value obtained from the Proctor Compaction test. If it is proposed to carry out the CBR test on an unsoaked specimen, the moisture content for remoulding should be the same as the equilibrium moisture content which the soil is likely to reach subsequent to the construction of the road. If it is proposed to carry out the CBR test on a soaked specimen, the moisture content for remoulding should be at the optimum and soaked under water for 96 hours.
Soil
Sample – The material used in the remoulded
specimen should all pass through a 19 mm IS sieve. Allowance for larger
material may be made by replacing it by an equal amount of material which
passes a 19 mm sieve but is retained on a 4.75 mm IS sieve. This procedure
is not satisfactory if the size of the soil particles is predominantly greater
than 19 mm. The specimen may be compacted statically or dynamically.
I. Compaction
by Static Method
The mass of the wet soil at the
required moisture content to give the desired density when occupying the
standard specimen volume in the mould is calculated. A batch of soil is
thoroughly mixed with water to give the required water content. The correct
mass of the moist soil is placed in the mould and compaction obtained by
pressing in displacer disc, a filter paper being placed between the disc &
soil.
II. Compaction
by Dynamic Method
For dynamic compaction , a
representative sample of soil weighing approximately 4.5 kg or more for fine
grained soils and 5.5 kg or more for granular soil shall be taken and mixed
thoroughly with water. If the soil is to be compacted to the maximum dry
density at the optimum water content determined in accordance with light
compaction or heavy compaction, the exact mass of soil required is to be taken
and the necessary quantity of water added so that the water content of soil
sample is equal to the determined optimum water content. The mould with
extension collar attached is clamped to the base plate. The spacer disc is
inserted over the base plate and a disc of coarse filter paper placed on the
top of the spacer disc. The soil water mixture is compacted into the mould in
accordance with the methods specified in light compaction test or heavy
compaction test.
PROCEDURE
1.
The mould containing the specimen
with the base plate in position but the top face exposed is placed on the lower
plate of the testing machine.
2.
Surcharge weights, sufficient to
produce an intensity of loading equal to the weight of the base material and
pavement is placed on the specimen.
3.
To prevent upheaval of soil into the
hole of the surcharge weights, 2.5 kg annular weight is placed on the soil
surface prior to seating the penetration plunger after which the remainder of
the surcharge weight is placed.
4.
The plunger is to be seated under a
load of 4 kg so that full contact is established between the surface of the
specimen and the plunger.
5.
The stress and strain gauges are
then set to zero. Load is applied to the penetration plunger so that the
penetration is approximately 1.25 mm per minute.
6.
Readings of the load are taken at
penetrations of 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 4.0, 5.0, 7.5, 10.0 and 12.5 mm.
7.
The plunger is then raised and the
mould detached from the loading equipment.
Table:-CBR
Wt. of
Rammer :- 2.60 kg Amount of compaction :- light/ heavy
proving
Factor :- 2.36
S.No.
|
Penetration
(mm)
|
Force
details
|
||
Proving
Reading (Div.)
|
Load on
the plunger (Kg")
|
Corrected
Load (Kg)
|
||
1
|
0.00
|
0.00
|
0.00
|
|
2
|
0.5
|
12
|
28.32
|
|
3
|
1
|
22
|
51.92
|
|
4
|
1.5
|
34
|
80.24
|
|
5
|
2
|
48
|
113.28
|
|
6
|
2.5
|
62
|
146.32
|
465.00
|
7
|
3
|
80
|
188.80
|
|
8
|
4
|
100
|
236.00
|
|
9
|
5
|
135
|
318.60
|
1080
|
10
|
6
|
171
|
403.56
|
|
CALCULATION
Load-Penetration
curve:
The load penetration curve is
plotted taking penetration value on x-axis and Load values on Y-axis.
Corresponding to the penetration value at which the CBR is desired, corrected
load value is taken from the load-penetration curve and the CBR calculated as
follows
California
bearing ratio = (PT/PS)x100
Where
PT = Corrected unit
(or total) test load corresponding to the chosen penetration curve, and
PS = Unit(or total)
standard load for the same depth of penetration as for PS taken
from standard code.
Corrected load at 2.5 mm & 5 mm
has been taken from plotted graph
unit load at 2.5 mm penetration =
1370 kg
unit load at 5 mm penetration = 2055 kg
Soaked CBR Value at 2.5 mm (in %) =
150*100/1370
Soaked CBR Value at 5 mm (in %) = 325*100/2055
REsult
The CBR values are usually
calculated for penetration of 2.5 mm and 5 mm. The CBR value is reported
correct to the first decimal place.
CBR value at 2.5 mm = 10.95%
CBR value at 5 mm =7.30%
PRECAUTIONS
§ Clean the sieves with the help of a brush, after sieving
§ While weighing put the sieve with soil sample on the balance
in a concentric position.
§ Check the electric connection of the sieve shaker before
conducting the test.
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