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  • Roadbase Soil Stabilizer
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In situ California Bearing Ratio Testing Method


This method covers the determination of the California Bearing Ratio (CBR) of a soil tested in situ, with a selected overburden pressure,by causing a cylindrical plunger to penetrate the soil at a given rate and comparing the relationship between force and penetration into the soil to that for a standard material. At certain values of penetration the California Bearing Ratio (CBR) is defined in the form of a percentage, as the ratio of the force exerted on the soil to a standard force that would be exerted on a specified crushed rock compacted and confined in a given manner.The CBR test may also be carried out in the laboratory on soil in a mould.

On account of the plunger size the test is appropriate only to material having a maximum particle size not exceeding 20 mm. Hence where material of this size or larger is possibly present beneath the test surface this should be checked after making the test and reported.


  1. The apparatus described in No. 2 to No. 16 is required, most of which is illustrated schematically and assembled in Figure 1. When assembled the overall rigidity shall be sufficient to suit the capacity of the jack.
  2. Cylindrical corrosion-resistant metal plunger, the lower end of which shall be of hardened steel and 49.65 ± 0.1 mm in diameter (nominal cross-sectional area 1935 mm2). The minimum length shall be 100 mm.
  3. Jack for applying the test force through the plunger at a controlled rate. The minimum capacity shall be 45 kN. The minimum extension shall be 50 mm. The mechanism shall suit a constant rate of extension of 1.0 ± 0.2 mm/min. The jack shall be provided with a swivel head to connect with the reaction frame for ease of alignment of the plunger.
  4. Metal extension rods for coarse height adjustment of the plunger. The overall length will depend upon the height of the reaction frame above the ground.
  5. Adjustable metal extension rod, such as a screw thread and bolt assembly, for the initial seating adjustment of the height of the plunger relative to the soil surface.
  6. Reaction load. The provision of the reaction load, incorporating the frame on which to attach the jack, may be in any convenient form such that it acts as a stable deadweight with its normal supports placed sufficiently far from the jack not to influence the results of the soil being tested.

    NOTE 1 The required reaction load will depend on the strength of the soil being tested and may be less than the capacity of the jack. Ballasted vehicles are commonly used.
    NOTE 2 Where the reaction load is on springs, as when using a vehicle, in order to prevent upward movement during the tests affecting the rate of penetration, separate jacks, to take the weight of the vehicle, should be used to remove the load from the springs.

  7. Annular surcharge discs, two discs suitably slotted or consisting of semi-circular segments. Each disc shall have a mass equal to 4.5 kg ± 100 g, an internal diameter between 52 mm and 54 mm and a nominal external diameter of 250 mm.
  8. Two discs, each with a mass equal to 9.0 kg ± 200 g with a similar shape and the same diameters as specified in No.7.
  9. Discs with other masses and shape may be used to represent the actual surcharge. The tolerance shall correspond to that specified above. The bottom disc shall be 250 mm in diameter.
  10. Calibrated force-measuring devices. Three ranges are required depending upon the CBR value as follows:
      1. for CBR values up to approximately 8 % a 2 kN capacity force-measuring device readable to 2 N;
      2. for CBR values from approximately 8 % to approximately 40 % a 10 kN capacity force-measuring device readable to 10 N;
      3. for CBR values above approximately 40 % a minimum of 40 kN capacity force-measuring device readable to 50 N.
  11. The force-measurement devices shall each include a substantial purpose-made transit case in order to prevent damage when the devices are not in use.
  12. Linear measurement system for determining the vertical penetration of the plunger and to enable the rate of penetration to be controlled, consisting of a datum bar, supports and penetration measurement gauge. The datum bar shall be held firmly in position with the supports placed outside the area of influence of the test. The measurement gauge shall be readable to 0.01 mm over a range of at least 25 mm.

    NOTE 1 A dial gauge indicating 1 mm/rev is convenient since the specified rate of penetration of 1 mm/min can be controlled conveniently by keeping the hand of the dial gauge in step with the second hand of a watch.

  13. Clock for controlling the rate of plunger penetration readable to 1 s.
  14. Straight steel cutting edge to prepare a flat area for the test. Suitable dimensions for the cutting edge are 500 mm by 25 mm by 3 mm.
  15. Container for sample of sufficient size for laboratory tests.
  16. Materials. Clean dry sand for placing beneath the lowest surcharge disc on uneven surfaces.

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  1. Remove from the test area any material which is not representative of the soil to be tested, and prepare a circular area of about 500mm in diameter such that it is flat and horizontal, taking special care with the central area on which the plunger will bear.
  2. The minimum spacing between adjacent tests shall be 250mm.
  3. Position the reaction load and its supports such as the jacks when using a vehicle, so that the cyclindrical plunger after assembly is directly over the central area to be tested.
  4. Fit the jack (in the fully retracted position), extension rods, force-measurement device and cyclindrical plunger on to the reaction load fram such that the whole assembly hangs vertically with the lower face of the plunger about 25mm above the soil surface to be tested.
  5. Carefully lower the cyclindrical plunger so that its lower surface just comes into contact with the soil. Ensure the assembly in vertical.
  6. Place a sufficient number of surcharge discs, one on top of another, around the central test area and plunger to correspond with the specified overburden pressure for the test. Select the number nearest to the specified value.
  7. Assemble and position the linear measurement system as shown typically in Figure 1.
  8. Apply a seating force to the plunger depending on the expcted CBR value as follows:
    1. CBR value up to 5% : 10N
    2. CBR value from 5% to 30% : 50N
    3. CBR value above 30% : 250N
  9. Record the reading of the force-measuring device as the initial zero reading (because the seating force is not taken into account during the test) or reset the force measurement device to read zero.
  10. Reset to zero the penetration measurement gauge or record its initial zero reading.
  11. Start the test so that the plunger penetrates the soil at a uniform rate of 1 ± 0.2mm per minute, and at the same instant start the clock.
  12. Record the force measurement in kN at intervals of penetration of 0.25mm, to a total penetration not exceeding 7.5mm.

    NOTE 1 If the operator plots the force penetration curve as the test is being carried out, the test can be terminated when the indicated CBR value fails below its maximum value. Thus if the CBR at 2.5mm were seen to be 6%, the test could be stopped and result reported as :
    2.5 mm penetration = 6%
    5.0 mm penetration < 6%
    NOTE 2 Where the available reaction is found to be inadequate to acheive a penetration of 2.5mm the maximum recorded force on the plunger and the corresponding penetration from the start of the test (see Step No. 11) should be reported.

  13. At the completion of the test and after removal of the surcharge discs and any sand that may have been used, take a sample of about 350g from beneath the central tests area for dispatch to the laboratory for determination of its moisture content as specified in BS 1377-2 :1990. Record a description of the soil and whether any soil particles greater than 20 mm in size were present beneath the plunger.
  14. Where the bulk density has to be determined the test shall be made in an appropriate manner according to the grading at a location just outside the area influnced by the CBR test.

Calculations, plotting and expression of results

  1. Force-penetration curve. Calculate the force applied to the plunger from each reading of the force-measuring device observed during the penetration test.

    NOTE 1 Alternatively, readings of the force-measuring device may be plotted directly against penetration readings. Forces are then calculated only at the appropriate penetration values as in Step No. 6 (after correction if necessary).;

  2. Plot each value of force as ordinate against the corresponding penetration as abscissa and draw a smooth curve through the points.
  3. The normal type of curve is convex upwards as shown by the curve labelled Test 1 inFigure 2, and needs no correction.
  4. If the initial part of the curve is concave upwards as shown for Test 2 (curve OST in Figure 2), the following correction is necessary. Draw a tangent at the point of greatest slope, i.e. the point of inflexion, S, and produce it to intersect the penetration axis at Q. The corrected curve is represented by QST, with its origin at Q from which a new penetration scale can be marked.
  5. If the graph continues to curve upwards as for Test 3 in Figure 2, and it is considered that the penetration of the plunger is increasing the soil density and therefore its strength, the above correction is not applicable.
  6. The standard force-penetration curve corresponding to a CBR value of 100 % is shown by the heavy curve in Figure 3. The forces corresponding to this curve are: 11.5 kN at 2 mm penetration, 13.2 kN at 2.5 mm, 17.6 kN at 4 mm, 20 kN at 5 mm, 22.2 kN at 6 mm and 26.3 kN at 8 mm. 
  7. The CBR value obtained from a test is the force read from the test curve (after correction and calculation if necessary) at a given penetration expressed as a percentage of the force corresponding to the same penetration on the standard curve. Curves representing a range of CBR values are included in Figure 3.
  8. Penetrations of 2.5 mm and 5 mm are used for calculating the CBR value. From the test curve (with corrected penetration scale if appropriate) read off the forces corresponding to 2.5 mm and 5 mm penetration. Express these as a percentage of the standard forces at these penetrations, i.e. 13.2 kN and 20 kN respectively.
  9. Take the higher percentage as the CBR value.
  10. If the force-penetration curve is plotted on a diagram similar to Figure 3, the CBR value at each penetration can be read directly without further computation if the correction described in Step No.1 for Test 2 is not required. The same diagram can be used for small forces and low CBR values if both the force scale (ordinate) and the labelled CBR values (abscissa) are divided by 10 as shown in brackets in Figure 3.

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