The CBR test of soil is fully described in IS-2720-PART-16-1979 for civil engineering purposes. It was first developed by the California State Highway Department and is used in the field as an arbitrary strength test which is considered to stress soils and replicate wheel loads.
The CBR test forms part of the site investigation and is used to determine the thickness of materials needed for the proposed road construction.
To determine the California Bearing Ratio (CBR) by conducting a load penetration test in the laboratory.
- 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 the handle.
- Metal rammers. Weight 2.6 kg with a drop of 310 mm (or) weight 4.89 kg a drop 450 mm.
- 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.
- Loading machine. With a capacity of at least 5000 kg and equipped with a movable head or base that travels at a 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.
- Sieves. 4.75 mm and 20 mm I.S. Sieves.
- Miscellaneous apparatus, such as a mixing bowl, straight edge, scales soaking tank or pan, drying
The CBR 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 the unsoaked state. oven, filter paper and containers.
Definition of CBR
It is the ratio of force per unit area required to penetrate a soil mass with the standard circular piston at the rate of 1.25 mm/min. to that required for the corresponding penetration of a standard material.
CBR Value = (Test load/Standard load) x 100
The following table gives the standard loads adopted for different penetrations for the standard material with a CBR value of 100%
Table- 1 (Standard Load in KG)
|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.
Preparation of Specimen
- The cutting edge to the mould and push is attached to the ground.
- The soil is removed from the outside of the mould which is pushed in.
- When the mould is full of soil, it is removed from weighing the soil with the mould or by any field method near the spot.
Determine the density
- The remoulded specimen is prepared at Proctor’s maximum dry density or any other density at which C.B.R is required.
- The specimen is maintained 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. The specimen is prepared either by dynamic compaction or by static compaction.
- 4.5 to 5.5 kg of soil is taken and is thoroughly mixed with the required water.
- The extension collar and the base plate are fixed to the mould.
- The spacer disc is inserted over the base.
- The filter paper is placed on the top of the spacer disc.
- The mix soil is compacted in the mould using either light compaction or heavy compaction.
- For light compaction, the soil is compacted in 3 equal layers, each layer being given 55 blows by the 2.6 kg rammer.
- For heavy compaction the soil is compacted in 5 layers, 56 blows to each layer by the 4.89 kg rammer.
- The collar is removed and soil is trimmed.
- The mould is turned upside down and the base plate and the displacer disc are removed.
- The mould with compacted soil is weighed and the bulk density and dry density is determined.
- Filter paper is put on the top of the compacted soil (collar side) and the perforated base plate is clamped on to it.
- The weight of the wet soil is calculated 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 x (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 the laboratory)
- The weight W (calculated as above) of the mixed soil is taken and it is placed in the mould.
- A filter paper and the displacer disc are placed on the top of the soil.
- The mould assembly is kept in a static loading frame and the displacer disc is compacted by
pressing till the level of the disc is reached the top of the mould.
- The load is kept for some time and then the load is released. The displacer disc is
- 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, a filter paper is put on the top
of the soil and the adjustable stem and perforated plate is placed on the top of the filter paper.
- Annular weights are put to produce a surcharge equal to the 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 is immersed and weighed in a tank of water and soaks it for 96 hours.
- The mould is removed from the tank.
- The consolidation of the specimen is noted.
- The mould assembly is placed with the surcharge weights on the penetration test machine.
- The penetration piston is set 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
- The stress and strain dial gauge is setting to read zero.
- The load is applied on the piston so that the penetration rate is about 1.25 mm/min.
- The load readings are recorded 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.
- The maximum load and corresponding penetration is noted if it occurs for a penetration
less than 12.5 mm.
- The mould from the loading equipment is detached.
- 20 to 50 g of soil from the top 3 cm layer is taken and determine the moisture content.
Observation and Recording
For Dynamic Compaction
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) =
For static compaction
Dry density (g/cc) =
Moulding water content (%) =
Wet weight of the compacted soil, (W g) =
Period of soaking = 96 hrs (4days).
For penetration Test
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 the greatest slope and shift the origin. Find and record the correct load reading corresponding to each penetration.
C.B.R. = PT/PS x 100
where PT = Corrected test load corresponding to the chosen penetration from the load penetration
curve, and PS = Standard load for the same penetration taken from table 1.
Interpretation and recording
C.B.R. of specimen at 2.5 mm penetration =
C.B.R. of specimen at 5.0 mm penetration =
C.B.R. of specimen at 2.5 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.
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