This method covers the laboratory determination of the California Bearing Ratio (CBR) of a compacted sample of soil – three point method, which is the prescribed method in the Pavement and Materials Design Manual.

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.

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

•        Two metal rammers of weights 2.5 kg and 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.

•        Perforated baseplates, fitted to the CBR moulds in place of the normal baseplate •   Perforated swell plates, 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 to be fitted to the tripod for measuring swell.

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

•        Annular surcharge discs for soaking, each having a mass known to + 50 g. Half circular segments may be used. Combined weight 4.5 kg.

•        Petroleum jelly.

•        A cylindrical metal plunger/piston with diameter 49.65 mm.

•        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

•        Annular surcharge discs for penetration test. Combined weight 5.5 kg.

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 generally considered unreliable in emulating site conditions, and such test results must consequently be treated with care.

Take a portion of material large enough to provide about 25 kg of material passing a 20 mm sieve. Bring the sample to the optimum moisture content (OMC ± 0.3 %) according to the BS Heavy compaction test. The soil shall be thoroughly mixed and shall normally be sealed and stored for at last 24 h before compacting into the moulds.

Test Procedure – Moulding

  1. Weigh the moulds with baseplates attached to the nearest 5 g (m2).
  2. Measure the internal dimensions of the moulds to 0.5 mm.
  3. Attach the extension collar to the moulds and cover the baseplates with a filter paper.
  4. The moist material (at OMC) is transferred to a mixing tray. It is thoroughly but rapidly mixed and then covered by a damp sack which should be kept over the material until compaction is completed so as to keep the moisture content as constant as possible.
  5. Stand the mould assemblies on a solid base, e.g. a concrete floor or plinth.
  6. The first mould is now tamped full of material using the 4,5 kg rammer, five layers and 62 blows per layer. Each layer should occupy about or a little more than one-fifth of the height of the mould. Ensure that the blows are evenly distributed over the surface. The final level of the soil surface should be about 5 – 10 mm above the top of the mould body.
  7. Remove the collar and trim the soil flush with the top of the mould with the scraper, checking with the steel straightedge.
  8. Weigh the mould, soil and baseplate to the nearest 5 g (m3).
  9. A representative sample for moisture content is now taken from the mixing tray.
  10. The second mould is then tamped full of material using the 4.5 kg rammer, five layers and 30 blows per layer. The moulded material is trimmed off, weighed and another representative sample for moisture content is taken from the mixing tray.
  11. The third mould is then tamped full of material, but in this case only three layers of material are compacted and on each layer 62 blows of the 2.5 kg rammer is applied. The moulded material is again trimmed off and weighed.

The compactive effort used for the three moulds is summarized below:

(a)      4.5 kg rammer, five layers and 62 blows per layer.

(b)      4.5 kg rammer, five layers and 30 blows per layer.

(c)      2.5 kg rammer, three layers and 62 blows per layer.

The average of the two moisture content determinations taken after the compaction of the first and second moulds is taken as the moulding moisture content for all three moulds.

Test Procedure – Soaking

  1. Place a filter paper on top of each sample and fit perforated baseplates on top of the moulds and invert the moulds.
  2. Remove the baseplates from the moulds. Fit the collar to the 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. The surface of the moulded material which was against the base plate should now be facing upwards. Place a filter paper on top of the sample followed by the perforated swell plate. Fit annular surcharge discs weighing 4.5 kg around the stem on the perforated swell 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. After 4 days of soaking, 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. Carefully remove the surcharge discs, perforated swell plate and extension collar.
  9. If the sample has swollen, trim it level with the end of the mould.
  10. The sample is then ready for testing.

Test Procedure – Penetration

  1. Place the mould containing the sample, with the top face (originally the bottom face) of the sample exposed, centrally on the lower platen of the testing machine.
  2. Place annular surcharge discs weighing 5.5 kg carefully 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
  5. 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.
  • Perform the penetration test on all three samples.

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.

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 %).

Swell calculation

The Swell (in %) is calculated from the equation:

Where

S is the swell expressed as a percentage of the height of the moulded material before soaking, i.e. 127 mm.

k is the dial gauge reading after 4 days’ soaking

L is the dial gauge reading before soaking.

Calculation of California Bearing Ratio (CBR)

Penetrations of 2.5 mm and 5.0 mm may be used for calculating the CBR value. However, the CBR at 2.5 mm penetration is generally used for assessing the quality of the material.

1.       Record the plunger force value at 2.5 mm penetration from the (corrected) force-penetration curve for each of the three specimens.

2.       Calculate the corresponding CBR values from the equation:

Where

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

3.       Record the plunger force values at 5.0 mm penetration from the (corrected) force-penetration curves.

4.       Calculate the corresponding CBR values from the equation:

Where

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

The CBR at 2.5 mm penetration is generally used for assessing the quality of the material.

References

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

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