To
determine the California bearing ratio by conducting a load penetration test in
the laboratory.
The
california bearing ratio test is penetration test meant for the evaluation of
subgrade strength of roads and pavements. The results obtained by these tests
are used with the empirical curves to determine the thickness of pavement and
its component layers. This is the most widely used method for the design of
flexible pavement.
This
instruction sheet covers the laboratory method for the determination of C.B.R.
of undisturbed and remoulded /compacted soil specimens, both in soaked as well
as unsoaked state.
Equipments
and tool required.
1.
Cylindrical mould with inside dia 150 mm and height 175 mm, provided with a
detachable extension collar 50 mm height and a detachable perforated base plate
10 mm thick.
2.
Spacer disc 148 mm in dia and 47.7 mm in height along with handle.
3.
Metal rammers.
Weight 2.6 kg with a drop of 310 mm (or) weight 4.89 kg a drop 450 mm.
4.
Weights. One
annular metal weight and several slotted weights weighing 2.5 kg each, 147 mm in
dia, with a central hole 53 mm in diameter.
5.
Loading machine.
With a capacity of atleast 5000 kg and equipped with a movable head or base that
travels at an uniform rate of 1.25 mm/min. Complete with load indicating device.
6.
Metal penetration piston 50 mm dia and minimum of 100 mm in length.
7.
Two dial gauges reading to 0.01 mm.
8.
Sieves. 4.75
mm and 20 mm I.S. Sieves.
9.
Miscellaneous apparatus, such as a mixing bowl, straight edge, scales soaking
tank or pan, drying oven, filter paper and containers.
DEFINITION
OF C.B.R.
It
is the ratio of force per unit area required to penetrate a soil mass with
standard circular piston at the rate of 1.25 mm/min. to that required for the
corresponding penetration of a standard material.
C.B.R.
= Test load/Standard load ´ 100
The following table gives the standard loads adopted for different penetrations for the standard material with a C.B.R. value of 100%
| Penetration of plunger (mm) | Standard load (kg) |
| 2.5
5.0 7.5 10.0 12.5 |
1370
2055 2630 3180 3600 |
The test may
be performed on undisturbed specimens and on remoulded specimens which may be
compacted either statically or dynamically.
Undisturbed
specimen
Attach
the cutting edge to the mould and push it gently into the ground. Remove the
soil from the outside of the mould which is pushed in . When the mould is full
of soil, remove it from weighing the soil with the mould or by any field method
near the spot.
Determine
the density
Prepare
the remoulded specimen at Proctor’s maximum dry density or any other density
at which C.B.R> is required. Maintain the specimen at optimum moisture
content or the field moisture as required. The material used should pass 20 mm
I.S. sieve but it should be retained on 4.75 mm I.S. sieve. Prepare the specimen
either by dynamic compaction or by static compaction.
Take
about 4.5 to 5.5 kg of soil and mix thoroughly with the required water.
Fix the extension collar and the base plate to the mould. Insert the spacer disc over the base (See Fig.38). Place the filter paper on the top of the spacer disc.
Compact the mix soil in the mould using either light
compaction or heavy compaction. For light compaction, compact the soil in 3
equal layers, each layer being given 55 blows by the 2.6 kg rammer. For heavy
compaction compact the soil in 5 layers, 56 blows to each layer by the 4.89 kg
rammer.
Remove
the collar and trim off soil.
Turn
the mould upside down and remove the base plate and the displacer disc.
Weigh
the mould with compacted soil and determine the bulk density and dry density.
Put
filter paper on the top of the compacted soil (collar side) and clamp the
perforated base plate on to it.
Calculate
the weight of the wet soil at the required water content to give the desired
density when occupying the standard specimen volume in the mould from the
expression.
W =desired dry density * (1+w) V
Where
W = Weight of the wet soil
w
= desired water content
V =
volume of the specimen in the mould = 2250 cm3 (as per the mould
available in laboratory)
Take
the weight W (calculated as above) of the mix soil and place it in the mould.
Place
a filter paper and the displacer disc on the top of soil.
Keep
the mould assembly in static loading frame and compact by pressing the displacer
disc till the level of disc reaches the top of the mould.
Keep
the load for some time and then release the load. Remove the displacer disc.
The
test may be conducted for both soaked as well as unsoaked conditions.
If
the sample is to be soaked, in both cases of compaction, put a filter paper on
the top of the soil and place the adjustable stem and perforated plate on the
top of filter paper.
Put
annular weights to produce a surcharge equal to weight of base material and
pavement expected in actual construction. Each 2.5 kg weight is equivalent to 7
cm construction. A minimum of two weights should be put.
Immerse
the mould assembly and weights in a tank of water and soak it for 96 hours.
Remove the mould from tank.
Note
the consolidation of the specimen.
Place
the mould assembly with the surcharge weights on the penetration test machine.
(Fig.39).
Seat
the penetration piston at the center of the specimen with the smallest possible
load, but in no case in excess of 4 kg so that full contact of the piston on the
sample is established.
Set
the stress and strain dial gauge to read zero. Apply the load on the piston so
that the penetration rate is about 1.25 mm/min.
Record
the load readings at penetrations of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0,
7.5, 10 and 12.5 mm. Note the maximum load and corresponding penetration if it
occurs for a penetration less than 12.5 mm.
Detach the mould from the loading equipment. Take about
20 to 50 g of soil from the top 3 cm layer and determine the moisture content.
Optimum
water content (%)
Weight
of mould + compacted specimen g
Weight
of empty mould g
Weight
of compacted specimen g
Volume
of specimen cm3
Bulk
density g/cc
Dry
density g/cc
Dry
density g/cc
Moulding
water content %
Wet
weight of the compacted soil, (W)g
Period
of soaking 96 hrs. (4days).
Calibration
factor of the proving ring
1 Div. = 1.176 kg
Surcharge
weight used (kg)
2.0 kg per 6 cm construction
Water
content after penetration test %
Least
count of penetration dial
1 Div. = 0.01 mm
If
the initial portion of the curve is concave upwards, apply correction by drawing
a tangent to the curve at the point of greatest slope and shift the origin (Fig.
40). Find and record the correct load reading corresponding to each penetration.
C.B.R. = PT/PS ´ 100
where
PT = Corrected test load corresponding to the chosen penetration from
the load penetration curve.
PS = Standard load for the same penetration taken from the table I.
| Penetration Dial |
Load Dial |
Corrected Load |
||||||||
|
|
C.B.R.
of specimen at 2.5 mm penetration
C.B.R.
of specimen at 5.0 mm penetration
The
C.B.R. values are usually calculated for penetration of 2.5 mm and 5 mm.
Generally the C.B.R. value at 2.5 mm will be greater that at 5 mm and in such a
case/the former shall be taken as C.B.R. for design purpose. If C.B.R. for 5 mm
exceeds that for 2.5 mm, the test should be repeated. If identical results
follow, the C.B.R. corresponding to 5 mm penetration should be taken for design.