California Bearing Ratio Test IS 2720

• This standard (Part 16) which deals with the laboratory determination of California bearing ratio was originally published in 1965. This revision brings in line with other international standards specially in regard to the procedure of testing. The California bearing ratio test (usually abbreviated as CBR test) is an adhoc penetration test developed by the California State Highway Department of USA for the evaluation of subgrade strengths for roads and pavements. The results obtained by these tests are used in conjunction with empirical curves based on experience for the design of flexible pavements.

• The test is arbitrary and the results give an empirical strength number which may no: be directly related to fundamental properties governing the strength of soils, such as cohesion, angle of internal friction, etc. However, it should be noted that attempts have been made of late to correlate CBR values to certain properties of soil, such as the bearing capacity and the plasticity index.

• Since the test is an ad hoc test, the procedure should be strictly adhered to if results are to be comparable with those previously obtained and correlated. The results have a direct application to the method of design for which the test has been devised.

This standard (Part 16) covers the laboratory method for the determination of Caylfornia’oearing ratio (CBR) of undisturbed soil specimens obtained from the field and also remoulded specimens of the soil compacted in the laboratory both by the static and dynamic method, and in the soaked and unsoaked state.

• For the purpose of this standard, the definitions given in IS: 2809-1972 and the following shall apply.

• Standard Load - Load which has been obtained from the test on crushed stone which was defined as having a California bearing ratio of 100 percent (see also 6.3).

• California Bearing Ratio (CBR) - The ratio of the 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 of a standard material. The ratio is usually determined for penetrations 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.

• Moulds with Base Plate, Stay Rod and Wing Nut - These shall conform to 4.1, 4.3 and 4.4 of IS: 9669-1980.

• dollar - It shall conform to 4.2 of IS: 9669-1980.

• Spacer Disc - It shall conform to 4.4 of IS: 9669-1980.

• Metal Rammer - As specified in IS: 9198-1979.

• Expansion Measuring Apparatus - The adjustable stem with perforated plates and tripod shall conform to 4.4 of IS: 9669-1980.

• Weights - This shall conform to 4.4 of IS: 9669-1989.

• Loading Machine - With a capacity of at least 5 000 kg and equipped with a movable head or base that travels at a uniform rate of 1.25 mm/min for use in forcing the penetration plunger into the specimen. The machine shall be equi$ped with a load indicating device that can read to an accuracy of one-half percent of loads.

• Penetration Plunger - This shall conform to 4.4 of IS: 9669-1980. To use a plunger of greater length, a suitable extension rod may be used.

• Dial Gauges - Two dial gauges reading to 0.01 mm.

• Sieves - 47.5-mm IS Sieve and 19-mm IS Sieve [see IS: 460 (Part 1)-!378].

• Miscellaneous Apparatus - Other general apparatus, such as a mixing bowl, straightedge, scales, soaking tank of pan, drying oven, filter paper, dishes and calibrated measuring jar.

• The test may be performed:

  • on undistrubed specimens, and

  • on remoulded specimens which may be compacted either statica.lly or dynamically.

• Undistrubed Specimens - Undisturbed specimens shall be obtained by fitting to the mould, the steel cutting edge of 150 mm internal diameter and pushing the mould as gently as possible into the ground. This process may be facilitated by digging away the soil from the outside as the mould is pushed in. When the mould is sufficiently full of soil, it shall be removed by underdigging, the top and bottom surfaces are then trimmed flat so as to give the required length of specimen ready for testing. If the mould cannot be pressed in, the sample may be collected by digging at a circumference greater than that of the mould and thus bringing out a whole undisturbed lump of soil. The required size of the sample to fit into the test mould shall then be carefully trimmed from this lump.

  If the specimen is loose in the mould, the annular cavity shall be filled with paraffin wax thus ensuring that the soil receives proper support from the sides of the mould during the penetration test. The density of the soil shall be determined either by weighing the soil with mould when the mould is full with the soil, or by measuring the dimensions of the soil sample accurately and weighing or by measuring the density in the field in the vicinity of the spot at which the sample is collected inaccordance with the method specified ‘in IS: 2720 (Part 28)-1973 or IS: 2720 (Part 29)-1975. In all cases, the water content shall be determined in accordance with IS: 2720 (Part 2)-1973

• Remoulded Specimens - The dry density for a remoulding shall be either the field density or the value of the maximum dry density estimated by the sompaction tests [see IS: 2720 (Part 7)-l 980 and IS: 2720 (Part 8)-19831 or any other density at which the bearing ratio is desired. The water c&tent used for compaction should be the optimum water content or the field moisture as the case may be.

  • Soil Sample - The material used in the remoulded specimen shall pass a 19-mm IS Sieve. Allowance for larger material shall be made by replacing it by an equal amount of material which passes a 19-mm IS Sieve but is retained on4.75-mm IS Sieve.

  • Statically Compacted Specimens - The mass of the wet soil at the required moisture content to give the desired density when occupying the standard specimen volume in the mould shall be calculated, A batch of soil shall be thoroughly mixed with water to give the required water content. The correct mass of the moist soils shall be placed in the mould and compaction obtained by pressing in the displacer disc, a filter paper being placed between the disc and the soil.

  • Dynamically Compacted Specimen - For dynamic compaction, a representative sample of the soil weighing approximately 4.5 kg or more for fine-grained soils and 5.5 kg or more for granulars Qils shall be taken and mixed thoroughly with water. If the soil is to be compacted to the maximum dry densiy at the optimum water content determined in accordance with IS: 2720 (Part 7)-1980 or IS: 2720 (Part 8)-1983, the exact mass of soil required shall be taken and the necessary quantity of water added so that the water content of the soil sample is equal to the determined optimum water content.

    • The mould with the extension collar attached shall be clamped to the base plate. The spacer disc shall be inserted over the base plate and a disc of coarse filter paper placed on the top of the spacer disc. The soil-water mixture shall be compacted into the mould in accordance with the methods applicable to the 150 mm diameter mould specified in IS: 2720 (Part 7)-1983 or IS: 2720 (Part 8)-1983. If other densities and water contents are desired, they may be used and indicated in the report.

    • The extension collar shall then be removed and the compacted soil carefully trimmed even with the top of the mould by means of a straightedge. Any hole that may then develop on the surface of the compacted soil by the removal of coarse material, shall be patched with smaller size material; the perforated base plate and spacer disc shall be removed, and the mass of the mould and the compacted soil specimen recorded. A disc of coarse filter paper shall be placed on the perforated base plate, the mould and the compacted soil shall be inverted and the perforated base plate clamped to the mould with the compacted soil in contact with the filter paper.

  • In both cases of compaction, if the sample is to be soaked, representative samples of the material at the beginning of compaction and another sample of the remaining material after compaction shall be taken for the determination of water content. Each water content sample shall weigh not less than 100 g for fine-grained soils and not less than 500 g for granular soils.

    If the sample is not t6 be soaked, a representative sample of material from one of the cut-pieces of the material after penetration Shall be taken to determine the water content. In all cases, the water content shall be determined in accordance with IS: 2720 (Part 2)-1973.

• Test for Swelling

  • A filter paper shall be placed over the specimen and the adjustable 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 as such undisturbed for 96 hours (see Note) noting down the readings everyday against the time of reading. A constant water level shall be maintained in the tank throughout the period.

    At the end of the soaking period, the final reading of the 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 downward for 15 minutes. Care shall be taken not to disturb the surface of the specimen during the removal of the 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.

• Penetration Test (see Fig. 1) 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 upheavel 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 stress and strain gauges shall then be set to zero. The initial load applied to the plunger shall be considered as the zero load when determining the load penetration relation. Load shall be applied to the penetration plunger so that the penetration is approximately 1.25 mm per minute. Reading of the load shall be taken at penetrations of 0.0, 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 it if 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 pcnet.ration plunger.

  • The penetration test may be repeated as a check test for the reserve end of the sample.

Expansion Ratio - The expansion ratio based on tests conducted as specified in 5.1 shall be calculated as follws:

$\mathrm{Expansion\; ratio}=\frac{d_f-d_s}{h}\times 100$
where,

• d_f = final dial gauge reading in mm,

• d, = initial dial gauge reading in mm,

• h = initial height of the specimen in mm.

Load Penetration Curve - The load penetration curve shall be plotted (see Fig. 2) This curve will be mainly convex upwards although the initial portion of the curve may be concave upwards due to surface irregularities. A correction shall then be applied by drawing a tangent to the upper curve at the point of contraflexure. The corrected curve shall be taken to be this tangent plus the convex portion af the original curve with the origin of strains shifted to the point where the tangent cuts the horizontal strain axis as illustrated in Fig. 2.

Bearing Ratio - Corresponding to the penetration value at which the California bearing ratio is desired, corrected load value shall be taken from the load penetration curve and the California bearing ratio calculated as follows:

$\mathrm{California\; bearing\; ratio}=\frac{p_t}{p_s}\times 100$
where,

• P_T = corrected unit (or total) test load corresponding to the chosen penetration from the load penetration curve

• P_S = unit (or total) standard load fo’r the same depth of penetration as for P, taken from Table 1.

The CBR vlues are usually calculated for penetration of 2.5 mm and 5 mm. Generally, the CBR value at 2.5 mm penetration will be greater than that at 4 mm penetration and in such a case the former shall be taken as the CBR value for design purposes.

Fig.1 Setup for cbr test

Fig.2 Correction load penetration curves

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 bearing ratio corresponding to 5 mm penetration shall be taken for design.