determine the California bearing ratio by conducting a load penetration test in
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
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.
and tool required.
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.
Spacer disc 148 mm in dia and 47.7 mm in height along with handle.
Weight 2.6 kg with a drop of 310 mm (or) weight 4.89 kg a drop 450 mm.
annular metal weight and several slotted weights weighing 2.5 kg each, 147 mm in
dia, with a central hole 53 mm in diameter.
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.
Metal penetration piston 50 mm dia and minimum of 100 mm in length.
Two dial gauges reading to 0.01 mm.
mm and 20 mm I.S. Sieves.
Miscellaneous apparatus, such as a mixing bowl, straight edge, scales soaking
tank or pan, drying oven, filter paper and containers.
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.
= 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)|
The test may
be performed on undisturbed specimens and on remoulded specimens which may be
compacted either statically or dynamically.
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.
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.
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
the collar and trim off soil.
the mould upside down and remove the base plate and the displacer disc.
the mould with compacted soil and determine the bulk density and dry density.
filter paper on the top of the compacted soil (collar side) and clamp the
perforated base plate on to it.
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
W =desired dry density * (1+w) V
W = Weight of the wet soil
= desired water content
volume of the specimen in the mould = 2250 cm3 (as per the mould
available in laboratory)
the weight W (calculated as above) of the mix soil and place it in the mould.
a filter paper and the displacer disc on the top of soil.
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.
the load for some time and then release the load. Remove the displacer disc.
test may be conducted for both soaked as well as unsoaked conditions.
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.
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.
the mould assembly and weights in a tank of water and soak it for 96 hours.
Remove the mould from tank.
the consolidation of the specimen.
the mould assembly with the surcharge weights on the penetration test machine.
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.
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.
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.
water content (%)
of mould + compacted specimen g
of empty mould g
of compacted specimen g
of specimen cm3
water content %
weight of the compacted soil, (W)g
of soaking 96 hrs. (4days).
factor of the proving ring
1 Div. = 1.176 kg
weight used (kg)
2.0 kg per 6 cm construction
content after penetration test %
count of penetration dial
1 Div. = 0.01 mm
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
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.
of specimen at 2.5 mm penetration
of specimen at 5.0 mm penetration
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.