The California Bearing Ratio (CBR) test is a measure of the resistance of a material to penetration of a standard plunger under controlled density and moisture conditions. It was developed by the California Division of Highways as a method of classifying and evaluating soil subgrade and base course materials for flexible pavements. The strength of the subgrade is the main factor in determining the required thickness of flexible pavements for roads and airfields. The strength of a subgrade, subbase and base course materials are expressed in terms of their California Bearing Ratio (CBR) value. The CBR value is a requirement in design for pavement materials of natural gravel.

This method covers the laboratory determination of the California Bearing Ration (CBR) of a compacted sample of soil dynamically compacted by metal rammers – one-point method. The CBR value is the resistance to a penetration of 2.5 mm of a standard cylindrical plunger of 50 mm diameter, expressed as a percentage of the known resistance of the plunger to 2.5 mm in penetration in crushed aggregate, (taken as 13.2 kN).

Required equipment

•        Test sieves sizes 20 mm and 5 mm.

•        A cylindrical metal mould, i.e. the CBR mould, having a nominal internal diameter of 152 mm and a height of 127 mm. The mould shall be fitted with a detachable baseplate and a removable extension. The internal face shall be smooth, clean and dry before use.

•        A metal rammer of either 2.5 kg or 4.5 kg

•        A steel rod

•        A steel straightedge

•        A spatula

•        A balance, capable of weighing up to 25 kg readable to 5 g.

•        Apparatus for moisture content determination.

•        Filter papers 150 mm in diameter.

Sample preparation

The CBR test shall be carried out on material passing the 20 mm test sieve. If the soil contains particles larger than this, the fraction retained on the 20 mm test sieve shall be removed and weighed before preparing the test sample. If this fraction is greater than 25 %, the test is not applicable. The moisture content of the soil shall be chosen to represent the design condition for which the test results are required.

Take a portion of material large enough to provide about 6 kg of material passing a 20 mm sieve. Bring the sample to the required moisture content. The soil shall be thoroughly mixed and shall normally be sealed and stored for at last 24 h before compacting into the mould. The specified effort of compaction shall correspond to the 2.5 kg rammer method – BS Light or the 4.5 kg rammer method – BS Heavy (or to an intermediate value).

Test procedure

  1. Divide the prepared quantity of soil into three (or five) portions equal to within 50 g and seal each portion in an airtight container until required for use, to prevent loss of moisture.
  2. Stand the mould assembly on a solid base, e.g. a concrete floor or plinth.
  3. Place the first portion of soil into the mould and compact it, so that after 62 blows of the appropriate rammer the layer occupies about or a little more than one-third (or one-fifth) of the height of the mould. Ensure that the blows are evenly distributed over the surface.
  4. Repeat using the other two (or four) portions of soil in turn, so that the final level of the soil surface is not more than 6 mm above the top of the mould body.
  5. Remove the collar and trim the soil flush with the top of the mould with the scraper, checking with the steel straightedge.
  6. Weigh the mould, soil and baseplate to the nearest 5 g (m3).

Soaking

Required equipment

•        A perforated baseplate, fitted to the CBR mould in place of the normal baseplate

•        A perforated swell plate, with an adjustable stem to provide a seating for a dial gauge.

•        Tripod, mounting to support the dial gauge.

•        A dial gauge, having a travel of 25 mm and reading to 0.01 mm.

•        A soaking tank, large enough to allow the CBR mould with a baseplate to be submerged, preferably supported on an open mesh platform.

•        Annular surcharge discs, each having a mass known to + 50 g. Half-circular segments may be used.

•        Petroleum jelly.

Soaking procedure

  1. If a solid baseplate have been used, this shall be removed from the mould and replaced with a perforated baseplate.
  2. Fit the collar to the other end of the mould, packing the screw threads with petroleum jelly to obtain a watertight joint.
  3. Place the mould assembly in the empty soaking tank. Place a filter paper on top of the sample followed by the perforated swell plate. Fit the required number of annular surcharge discs around the stem on the perforated plate.
  4. Mount the dial gauge support on top of the extension collar, secure the dial gauge in place and adjust the stem on the perforated plate to give a convenient zero reading.
  5. Fill the soaking tank with water to just below the top of the mould extension collar. Start the timer when the water has just covered the baseplate.
  6. Record readings of the dial gauge each day.
  7. Take off the dial gauge and its support, remove the mould assembly from the soaking tank and allow the sample to drain for 15 min.
  8. Remove the surcharge discs, perforated plate and extension collar. Remove the perforated baseplate and refit the solid baseplate if available.
  9. If the sample has swollen, trim it level with the end of the mould.
  10. The sample is then ready for test in the soaked condition.

Penetration test procedure

Required equipment

•        A cylindrical metal plunger.

•        A CBR compression machine. The machine shall be capable of applying at least 45 kN at a rate of penetration of the plunger of 1 mm/min to within ± 0.2 mm/min.

•        A loading ring.

•        A dial gauge with 25 mm travel, reading to 0,01 mm for measuring the penetration of the plunger into the specimen.

•        A stopwatch

•        The CBR mould

•        Surcharge discs

Procedure

  1. Place the mould with baseplate containing the sample, with the top face of the sample exposed, centrally on the lower platen of the testing machine.
  2. Place the appropriate annular surcharge discs on top of the sample
  3. Fit into place the cylindrical plunger on the surface of the sample.
  4. Apply a seating force to the plunger, depending on the expected CBR value as follows:

For CBR value up to 5% apply 10 N

For CBR value from 5% to 30% apply 50 N

For CBR value above 30% apply 250 N

  • Record the reading of the loading ring as the initial zero reading (or reset the loading ring to read zero).
  • Secure the penetration dial gauge in position. Record its initial zero reading, or reset it to zero.
  • Start the test so that the plunger penetrates the sample at a uniform rate 1mm/min.
  • Record readings of the force gauge at intervals of penetration of 0.25 mm, to a total penetration not exceeding 7.5 mm).
  • If a test is to be carried out on both ends of the sample, raise the plunger and level the surface of the sample by filling in the depression left by the plunger. Check for flatness with the straightedge.
  • Remove the baseplate from the lower end of the mould, fit it securely on the top end and invert the mould. Trim the exposed surface if necessary.
  • Carry out the test on the base by repeating steps 1 – 8.
  • After completing the penetration test or tests, determine the moisture content of the test sample.

Calculation and plotting

1)       Calculate the force applied to the plunger from each reading of the loading ring observed during the penetration test.

2)       Plot each value of force as ordinate against the corresponding penetration as abscissa and draw a smooth curve through the points.

Calculation of California Bearing Ratio (CBR)

Penetrations of 2.5 mm and 5.0 mm are used for calculating the CBR value.

1.       Record the plunger force value at 2.5 mm penetration from the (corrected) force-penetration curve.

2.       Calculate the corresponding CBR value from the equation:

Where

P is the plunger force (in kN) at 2.5 mm penetration.

3.       Record the plunger force value at 5.0 mm penetration from the (corrected) force-penetration curve.

4.       Calculate the corresponding CBR value from the equation:

Where

 P is the plunger force (in kN) at 5.0 mm penetration.

4.       The higher of these two values is taken as the CBR value.

5.       If a test is carried out on both ends of the sample, the above calculation procedure, steps 1 – 5, is repeated.

Example of a CBR curve

Density Calculations

1)       Calculate the internal volume of the mould, Vm (in cm3).

 2)      Bulk density. The initial bulk density, ρ (in kg/m3), of the sample, is calculated from the equation:

Where

m3      is the mass of soil, mould and baseplate (in g)

m2      is the mass of the mould and baseplate (in g)

Vm     is the volume of the mould body (in cm3).

3)       Dry density. The initial dry density, ρd (in kg/m3), of the sample is calculated from the equation:

Where

w is the moisture content of the soil (in %).

4)       If the dry density of the soaked soil is required, calculate it from the equation:

Where

A is the area of cross-section of the mould (in mm2)

x is the increase in sample height after swelling (in mm).

References

British Standards Institution. (n.d.). Bs 1377: Part 4: 1990: Methods of test for soils for civil engineering purposes.

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