California Bearing Ratio (CBR)
cbr test of soil
The ratio expressed in percentage of force per unit area required to penetrate a soil mass with a circular plunger of 50 mm diameter at the rate of 1.25 mm/min to that required for corresponding penetration in a standard material.
The ratio is usually determined for penetration of 2’5 and 5 mm.
Where the ratio at 5 mm is consistently higher than that at 2’5 mm, the ratio at 5 mm is used.
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| CBR |
CBR TEST APPARATUS
- Moulds with Base Plate, Stay Rod and Wing Nut
- Collar
- Spacer Disc
- Metal Rammer
- Expansion Measuring Apparatus
- Weights
- Loading Machine
- Penetration Plunger
- Dial Gauges
- Sieves
- Miscellaneous Apparatus
- cbr value of soil
PREPARATION OF TEST SPECIMEN
- undisturbed specimens,
- remoulded specimens which may be compacted either statically or dynamically
CBR TEST PROCEDURE
1.Test for Swelling
A filter paper shall be placed over the specimen and the adjust-able stem and perforated plate shall be placed on the compacted soil specimen in the mould. Weights to produce a surcharge equal to the weight of base material and pavement to the nearest 2’5 kg shall be placed on the compact soil specimen. The whole mould and weights shall be immersed in a tank of water allowing free access of water to the top and bottom of the specimen.
The tripod for the expansion measuring device shall be mounted on the edge of the mould and the initial dial
gauge reading recorded. This set-up shall be kept undisturbed for 96 hours noting down the readings every day against the time of reading.
A constant water level shall be maintained in the tank through-out the period.
At the end of the soaking period, the change in dial gauge shall be noted, the tripod removed and the mould taken out of the water tank.
The free water collected in the mould shall be removed and the specimen allowed to drain downwards for 15 minutes.
Care shall betaken not to disturb the surface of the specimen during the removal of tbe water. The weights, the perforated plate and the top filter paper shall be removed and the mould with the soaked soil sample shall be weighed and the mass recorded.
2.CBR Penetration Test
The mould containing the specimen, with the base plate in position but the top face exposed, shall be placed on the lower plate of the testing machine.
Surcharge weights, sufficient to produce an intensity of loading equal to the weight of the base material and pavement shall be placed on the specimen.
If the specimen has been soaked previously, the surcharge shall be equal to that used during the soaking period. To prevent upheaval of soil into the hole of the surcharge weights, 2’5 kg annular weight shall be placed on the soil surface prior to seating the penetration plunger after which the remainder of the surcharge weights shall be placed.
The plunger shall be seated under a load of 4 kg so that full contact is established between the surface of the specimen and the plunger.
The load and deformation gauges shall then be set to zero ( In other words, the initial load applied to the plunger shall be considered as zero when determining the load penetration relation ).
Load shall be applied to the plunger into the soil at the rate of 1’25 mm per minute. Reading of the load shall be taken at penetrations of 0.5, 1.0, 1.5, 2.0, 2.5, 4.0, 5.0, 7.5, 10.0 and 12.5 mm (The maximum load and penetration shall be recorded if it occurs for a penetration of less than 12.5 mm).
The plunger shall be raised and the mould detached from the loading equipment. About 20 to 50 g of soil shall be collected from the top 30 mm layer of the specimen and the water content determined according to IS : 2720 ( Part 2 )-1973*. If the average water content of the whole specimen is desired, water content sample shall be taken from the entire depth of the specimen.
The undisturbed specimen for the test should be carefully examined after the test is completed for the presence of any oversize soil particles which are likely to affect the results, if they happen to be located directly below the penetration plunger.
The penetration test may be repeated as a check test for the rear end of the sample.
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| SET-UP FOR CBR TEST |
CBR TEST CALCULATION
1.Expansion Ratio
The expansion ratio based on tests conducted as Soaked method, shall be calculated as follows:
Expansion Ratio = ((dt - ds) /h) *100
where,
- dt=final dial gauge reading in mm,
- ts=initial dial gauge reading in mm, and
- h=initial height of the specimen in mm
The expansion ratio is used to qualitatively identify the potential expansiveness of the soil.
2.California Bearing Ratio (CBR)
The CBR values are usually calculated for penetrations of 2’5 and 5 mm. Corresponding to the penetration value at which the CBR values is desired, corrected load value shall be taken from the load penetration curve and the CBR calculated as follows:
CBR = (Pt /Ps) *100
where,
- PT=corrected unit ( or total ) test load corresponding to the chosen penetration from the load penetration curve,
- Ps=unit ( or total ) standard load for the same depth of penetration
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| CORRECTION LOAD PENETRATION CURVES |
Generally, the CBR value at 2’5 mm penetration will be greater than that at 5 mm penetration and in such a case, the former shall be taken as the CBR value for design purposes. If the CBR value corresponding to a penetration of 5 mm exceeds that for 2’5 mm, the test shall be repeated.
If identical results follow, the CBR corresponding to 5 mm penetration shall be taken for design.
CBR TEST RESULTS
The results of the CBR test are,
- CBR value
- expansion ratio
CBR TEST OBSERVATIONS
Reference:
IS 2720-16 (1987): Methods of test for soils, Part 16: Laboratory determination of CBR [CED 43: Soil and Foundation Engineering]
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