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  • Roadbase Soil Stabilizer
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California Bearing Ratio Test


This method covers the laboratory determination of the California Bearing Ratio (CBR) of a stabilized soil in compliance with British Standards. The principle is to determine the relationship between force and penetration when a cylindrical plunger of a standard cross-sectional area is made to penetrate the soil at a given rate. At certain values of penetration the ratio of the applied force to a standard force, expressed as a percentage, is defined as the California Bearing Ratio (CBR).


  • Because of the size of the sample and of the plunger the test is appropriate only for materials having a maximum particle size not exceeding 20 mm.
  • The procedure for soaking the specimen, if required, is included. An in-situ test following the same principle can also be carried out.

Preparation of test sample

  • 1.      Dynamic compaction by hand or mechanical rammer is used to achieve a specified compactive effort in this testing method.
  • 2.      Assemble the mould with its baseplate, weigh to the nearest 5 g and record the mass (m2 ).
  • 3.      Fit the collar and place a filter paper in the bottom.
  • 4.      Calculate and prepare the mass of stabilized material necessary to give the required density as for the static compaction procedure.
  • 5.      Divide this quantity into five equal parts by mass, each sealed in a container to prevent loss of moisture by evaporation.
  • 6.      Then compact the stabilized material into the mould with the collar attached, in five equal layers using the 4.5 kg rammer.
  • 7.      Give each layer sufficient blows distributed evenly over the surface to ensure that the layer after compaction occupies approximately one-fifth of the height of the mould.
  • 8.      When compacted in this manner the final level of the stabilized material shall be just above the mould.
  • 9.      Remove the collar and trim the material flush with the top of the mould using a palette knife and check with a steel straightedge.
  • 10.  Determine the mass (m3 ) of the mould with the baseplate attached and containing the specimen to the nearest 5 g.
  • 11.  After compaction store the specimen for a period, the length of which will depend upon the process and the type of stabilizer employed.
  • 12.  Normally, the specimen is kept at constant moisture content during this period: this may be achieved by coating the ends of the specimen with wax, or placing the end caps on the mould and sealing them with petroleum jelly.
  • 13.  Keep the specimen at 20 ± 2 °C and record the average temperature and duration of storage.
  • 14.  If it is required to soak the specimen, expose the two ends and attach a perforated base plate separated from the specimen by a filter paper.
  • 15.  Screw a collar to the top and seal the joint with petroleum jelly.
  • 16.  Place the specimen in a bath of water and keep the water level just below the top of the collar.
  • 17.  Observe the time taken for the water to reach the top of the speciman and, if this has not occured within 3 days, flood the top of the specimen and leave it to complete its normal soaking period.
  • 18.  The normal soaking period is 4 days and the bath shall be kept at a temperature of 20 ± 2 °C.
  • 19.  If during soaking, measurement of any swell that has ocured is required, place a filter paper and perforated plate on top of the specimen and suitably mount a dial gauge, reading to 0.01mm, to follow any movement of the surface.
  • 20.  When soaking is complete remove the specimen from the bath and allow to drain for 15min.
  • 21. Replace the baseplate, remove the collar and perforated plates, weigh to the nearest 5g and record the mass of the assembly (m4 ).

Penetration test procedure


  • 1.      Before testing remove the end-caps or the protective wax and filter paper sealing the specimen. If the specimen has been soaked, remove the perforated top plate and filter paper.
  • 2.      Place the mould, containing the specimen, with baseplate in position but with the top face exposed, on the lower platen of the testing machine.
  • 3.      Seat the plunger under a force of 50 N for a CBR value of up to 30 % or 250 N for a CBR value of 30 % to 300 %. Allow the plunger to penetrate the specimen at a uniform rate of 1 mm/min.
  • 4.      Take readings of the force at intervals of penetration of 0.25 mm to a total penetration not exceeding 7.5 mm.
  • 5.      Raise the plunger and remove any material projecting over the top of the mould.
  • 6.      Fill any depressions left by the plunger completely with dry fine sand and trim up the face of the specimen by a further pass of a palette knife and check with the straightedge.
  • 7.      Then secure the second baseplate to the upper end of the mould.
  • 8.      Invert the specimen and remove the baseplate from the untested end of the mould.
  • 9.      Repeat the procedure described in Step 1 to Step 4 on this end of the specimen.
  • 10.  After the penetration tests have been completed remove the specimen from the mould.
  • 11.  Break it up and take a representative sample of the stabilized material for the determination of its moisture content.
  • 12. Do not include the dry sand, used in the end first tested, in the moisture content sample.


Calculation and plotting

Force-penetration curve
  • 1.      Plot a graph showing force on the specimen against penetration and draw a smooth curve through the plotted points.
  • 2.      If the initial portion of the curve is concave upwards (which may be due to surface irregularities) a correction should be made by drawing a tangent to the curve at the point of the greatest slope. Take the intersection of this tangent and the penetration axis as the new origin.
  • 3.      The corrected force/penetration curve to be used in the calculations is this tangent from where it cuts the penetration abscissa to its point of contact with the curve and thence the curve itself.
  • 4.   To use this corrected curve either: transpose the co-ordinates to the right so that the tangent passes through their origin; or transpose the corrected curve to the left so that the new and original origin coincides as shown in Figure 1.

Calculation of California Bearing Ratio
  • 1.      CBR curves are given in the force/penetration diagram shown in Figure 1.
  • 2.      The standard forces for 100 % CBR are as given in Table 1.

Test report
  •  The test report shall affirm that the California Bearing Ratio was determined in accordance with 4.5 of BS 1924-2:1990 and whether or not a certificate of sampling is available. If available a copy of the certificate of sampling shall be provided.
  • The report shall contain the following additional information:
    •  sample identification;
    • the CBR values obtained on the top and bottom of the specimens; 
    •  the method of compaction;
    •  the moisture content and dry density at which the specimens were prepared and/or the moisture content and dry density at which they were tested;
    • the age at which the specimens were tested and, where applicable, the period of soaking;
    •  conditions of curing and storage;
    • the amount of swell during soaking (if measured).

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