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| California bearing ratio test procedure pdf |
CBR Test of Soil
California bearing ratio test apparatus
- Cylindrical mould of internal diameter 150 mm and height 175 mm
- Spacer height - 47.7 mm, Volume of the mould- 2250 cc.
California bearing ratio test procedure pdf download link given below.
Specimen
The specimen shall have 'field density' or 'maximum dry density' at optimum moisture content, The water content used for compaction is 'optimum moisture content' or the field moisture."
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| cbr test apparatus specifications |
Soil sample
Material shall pass 20 mm IS Sieve. Allowance for larger materials shall be made by replacing it by an equal amount of material which passes a 19 mm IS Sieve but is retained cn4.75 mm IS Sieve. Normally, the test specimens should be prepared by static compaction. However, if not possible, dynamic method may be used as an alternative.
CBR mould volume calculation - Statically compacted specimen
The weight of wet soil at the required moisture content to give the intended density when occupying the standard test mould is calculated as given below
Weight of dry soil = 2209 d gm, where d is the required dry density in gm/cc
Weight of wet soil = ((100+m)2209)/100 d gm
Where m = required moisture content in percent. The soil lumps are broken down and stones larger than 20 mm are removed., The soil is mixed with water to give the required moisture content. The correct weight of wet soil is placed in the mould. A filter paper is placed in the mould, after initial tamping with a steel rod. This is followed by the 5cm displacer disc. The specimen is compressed in the machine until the top of the displacer is flush with the top of the collar.
The load is held for about 30 seconds and then released. In some soil types, it may be necessary to reapply load to force the displacer disc slightly below the top of the mould so that on rebound the right volume is obtained
Dynamic compaction
The soil is mixed with water to give the required moisture content. The mixture is then compacted into the mould in three layers using a standard soil rammer. After compaction, the soil is trimmed flush with the top of the mould with the help of a metal straight edge. The mould is weighed fully and emptied to enable determination of wet bulk density. From this, the dry density is calculated, knowing the moisture content.
At the same moisture content, further specimens are then prepared to different dry densities by varying the number of blows applied to each layer. From this is known the amount of compaction that fill the mould uniformly with calculated weight of wet soil.
Soaking of the specimen
The specimen in the mould is soaked in water for 96 hours. After 96 hours of soaking the mould is taken out of the water and the free water collected in the mould should be removed without disturbing the surface of the specimen. Then the specimen is allowed to drain for 15 minutes.
Test procedure The mould containing the specimen is placed on the CBR testing machine (Fig 4.1). A surcharge weight is placed on the top of the specimen in order to offset the weight of the base material of the mould. The assembly is placed under the plunger of the loading frame.
Testing and Recording
The stress and strain gauged is set at zero. The initial load applied on to the plunger shall be considered as zero for the purpose of determining the relationship between the load and the penetration. Then, the load is applied to the plunger so that the re of penetration is at 1.25 mm per minute. Reading of the load is taken at 0.0,0.5, 10 15,2.0, 2.5.4.0, 5.0, 7.5, 10.0, and 12.5 mm.
Test analysis : CBR value
A graph is plotted with penetration on x axis and unit load on y axis. Alternatively, the load may be converted into pressure and one more graph may be plotted for pressure versus s penetration. The curves are mainly convex upwards, although the initial portion of the second curve is concave upwards due to surface irregularities. Then, a correction is applied by drawing a tangent to the upper curve at the point of contra flexure as shown below. The origin is shifted to the point, where the tangent cuts 'x' axis and the converted stress is read for the penetration measured from the new origin.
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| load penetration curve |
The C.B.R value is calculated using the relation:
- CBR = (Ps/Pt)100
- where Ps = Load (pressure) sustained by the specimen at 2.5 or 5.0 mm Penetration
- Pt = Load (pressure) sustained by standard aggregates at the corresponding penetration level. The standard load values are 1370 and 2055 kg (70 and 105 kg/cm2) respectively at 2.5 and 5.0mm penetration.
Generally, the C.B.R value at 2.5 mm penetration will be greater than that of 5.0 mm penetration. In such a case, the value for 2.5 mm shall be taken for design purposes. If the C.B.R value corresponding to a penetration of 5.0 mm exceeds that for 2.5 mm, the test shall be repeated. If the results of the repeated tests are also the same, the C.B.R corresponding to 5.0 mm penetration shall be taken for design.
CBR test is not used for: / Criticism against CBR test -
The criticism against CBR test is on the following considerations.
- It is an adhoc and arbitrary test and the result does not give a direct indication of the strength of the soil.
- The method becomes inappropriate for materials with CBR value of more than 60.
- Presence of coarse grained particles in the specimen distort results. Materials passing 20mm sieve alone can be used in the test and the specimen has to be prepared for the test.
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