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CALIFORNIA BEARING RATIO

By :- ASOK DACHARLA

M Tech

GMR Institute of Technology

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STANDARD

IS 2720 (Part 16) – 1987

LABORATARY DETERMINATION OF CBR

IS 2720 (Part 31) : 1990

FIELD DETERMINATION OF CBR

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California bearing ratio

This is the most widely used method for the design of flexible pavement.

Developed by The California State Highways Department in 1930.

Main principal is Resistance of the material to uniaxial penetration.

Field and laboratory test.

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California Bearing Ratio (CBR) :-

The ratio of the force per unit area required to penetrate a soil mass with standard penetration plunger at a uniform rate of 1.25 mm/min., to the corresponding penetration load of the standard material is called CBR.

CBR value =

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Significance :- Evaluation of strength of subgrade soil, sub base & base

coarse materials.

The results obtained by this test are used with the empirical curves to determine the thickness of pavement and its component layers.

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Apparatus

Mould

Steel Cutting collar

Spacer Disc

Surcharge weight

Dial gauges

IS Sieves

Penetration Plunger

Loading Machine

Air tight container

Miscellaneous Apparatus

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specifications

Cylindrical mould with inside dia 150 mm and height 175 mm, provided with a detachable extension collar 50 mm height and a detachable perforated base plate 10 mm thick.

Spacer disc 148 mm in dia and 47.7 mm in height along with handle. 

Metal rammers: Weight 2.6 kg with a drop of 310 mm (or) weight 4.89 kg a drop 450 mm.

Weights: One annular metal weight and several slotted weights weighing 2.5 kg each, 147 mm in dia, with a central hole 53 mm in diameter.

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Loading machine: With a capacity of at least 5000 kg and equipped with a movable head or base that travels at an uniform rate of 1.25 mm/min

Metal penetration piston 50 mm dia and minimum of 100 mm in length.

Two dial gauges reading to 0.01 mm.

Sieves: 4.75 mm and 20 mm I.S. Sieves.

Miscellaneous apparatus, such as a mixing bowl, straight edge, scales soaking tank or pan, drying oven, filter paper and containers. 

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PREPERATION OF TEST SPECIMEN

The test may be performed on

Undisturbed test specimen

Remoulded specimen.

Remoulded specimen might be prepared in two methods Statically compacted method Dynamic compaction method

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UNDISTRUBED SPECIMEN

A mould of 150 mm internal diameter is used for collection of field specimen as like core cutter method.

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.

Remoulded specimen

The field density or the value of the maximum dry density estimated by the compaction tests.

Any other density at which the bearing ratio is desired.

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Statically compacted specimen

Calculate the weight of the wet soil at the required water content to give the desired density when occupying the standard mould.

Volume of mould = 2250cc.

Weight of dry soil (W) = 2250 x MDD. W

M = Optimum moisture content obtained from the laboratory compaction test.

Take the weight W (calculated as above) of the mix soil and place it in the mould.

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Place a filter paper and the displacer disc on the top of soil.

Keep the mould assembly in static loading frame and compact by pressing the displacer disc till the level of disc reaches the top of the mould.

Keep the load for some time and then release the load. Remove the displacer disc.

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Dynamically compacted specimen

Take 4.5-5.5kg of soil & mix it thoroughly with required quantity of water (OMC) or field moisture content.

The material used in the remoulded specimen shall pass a 19-mm IS Sieve but is retained on 4.75-mm IS Sieve.

The mould with the extension collar attached shall be clamped to the base plate.

The spacer disc is placed at the bottom of the mould over the base plate & a coarse filter paper is placed over the spacer disc.

If specimen is to be soaked a sample of 50g shall be taken for determination of water content.

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The moist soil is to be compacted over spacer disk in mould by adopting either IS light or heavy compaction.

After compacting the last layer, the collar is removed & the excess of soil above the top of the mould is evenly trimmed off.

The clamps are removed & the mould with the compacted soil is lifted, leaving below the base plate & spacer disc is removed.

A filter paper is placed on the base plate, the mould with compacted soil is inverted & placed over the base plate & clamps are tightened.

Take the weight of the mould along with compacted specimen.

Weight of 2.5-5 kg are placed over the soil in the mould. Then the whole mould is placed in the water tank to allow soaking for 4 days.

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Test for Swelling

This test is optional and may be omitted in case of unsoaked CBR test

Determine the initial height of specimen (h) in mm.

Mount the expansion-measuring device along with the tripod on the edge of the mould and record the initial dial gauge reading (ds).

Keep this set up as such undisturbed for 96 hours noting down the readings everyday against the time of reading.

Maintain a constant water level through out the period of soaking.

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Note the final reading of the dial gauge at the end of soaking period (dh).

Calculation for Swelling

Expansion ratio = ×100

ds = Initial dial gauge reading in mm

df = final dial gauge reading in mm

h = initial height of specimen in mm

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Drain the excess water by placing the mould inclined for about 15 minutes and weigh the mould

Note the weight of mould after soaking period.

The mould is clamped over the base plate & the same surcharge weights are placed on the specimen as the test could be conducted.

The complete assembly is placed under the loading machine.

Put annular weights to produce a surcharge equal to weight of base material and pavement expected in actual construction.

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Each 2.5 kg weight is equivalent to 7 cm construction. A minimum of two weights should be put.

Set the plunger under a load of 4 kg so that full contact is established between the surface of the specimen and the plunger.

The dial gauge of proving ring & the penetration dial gauge is set on 0.

The load is applied through penetration rate of 1.25 mm/min.

The load readings are taken at 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0, 12.5 mm of penetration.

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Detach the mould from the loading equipment. Take about 20 to 50 g of soil from tested specimen and determine the moisture content.

A graph is plotted by penetration (mm) on x-axis & load (kN) on y-axis & the value of load at penetration 2.5 & 5mm are found from the graph.

The following table gives the standard loads adopted for different penetrations for the standard material with a C.B.R. value of 100%.

Penetration of plunger (mm)

Standard load (kg)

2.5 1370 5.0 2055

7.5 2630 10.0 3180

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Cbr graph

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The curve is uniformly convex upwards.

If the initial portion of the curve is concave upwards due to surface irregularities.

Make correction by drawing a tangent to the upper curve at the point of contra flexure as below

Finally, the CBR value is calculated from the formulae.

As per IRC recommendation the min. value of CBR required for a subgrade should be 8%.

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Field cbr testApparatus

Load like truck, tractor.

Mechanical screw jack.

loading plunger.

Proving ring assembly, dial gauge.

Surcharge 5kg in weight.

Two circular slotted weights of 10kg.

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procedure

Circular area of 30cm in diameter is trimmed and leveled and Surcharge load of 15kg is placed on surface.

Dial gauge to measure penetration is attached to the plunger.

A load of 4kg is applied, load and penetration dials are set to zero.

Load is applied to the plunger by means of mechanical screw jack.

Rate of penetration is 1.25mm/minute.

The load readings are taken at 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0, 12.5 mm of penetration.

Moisture content is tested form specimen taken underneath the plunger.

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Setting apparatus

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Note

• CBR specimens tested at field moisture content and density tend to give higher CBR value in laboratory than those obtained in the field (particularly for granular soil)

• Difference is due to the confining effect of rigid mould in laboratory tests

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