California Bearing Test

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CALIFORNIA BEARING RATIO 2013

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California bearing ratio test

NAME OF GROUP C :

I. AZRI IZZAT BIN MOHD AZAMI 14DKA12F1061II. AHMAD AFFANDI BIN DULAH 14DKA12F1062

III. MUHAMMAD ZULKARNAIN BIN SAINI 14DKA12F1063IV. AHMAD FIQRI BIN HAMDAN 14DKA12F1065V. MUHAMAD ZAZALI BIN PAIMAN 14DKA12F1066

VI. MOHD SHAFIQ BIN MOHD TAHIR 14DKA12F1067VII. SITI NURJIHAN BINTI SELAMAT 14DKA12F1069

VIII. AIMAN BIN MOHD SAID 14DKA12F1070IX. MUHAMMAD SYUKRI BIN ABDUL SAMAD 14DKA12F1071X. MOHD NORARIF BIN ARIPIN 14DKA12F1072


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CONTENT

NO. CONTENT PAGE

1. INTRODUCTION 35

2. THEORY 6

3. OBJECTIVE 7

4. APPARATUS 8 - 11

5. PROCEDURE 1216

6. DATA AND ANALYSIS 17 - 18

7. QUESTION 19 - 20

8. DISCUSSION 21 - 23

9. CONCLUSION 24

10. REFERENCE AND APPENDIX 25 - 27


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INTRODUCTION

The California Bearing Ratio, was developed by The California State Highways Department. It

is in essence a simple penetration test developed to evaluate the strength of road sub grades.

The Basic CBR Test :

This consists of causing a plunger of standard area to penetrate a soil sample, (this can bein the laboratory or on site). The force (load) required to cause the penetration is plotted

against measured penetration, the readings noted at regular time intervals. This

information is plotted on a standard graph, and the plot of the test data will establish the

CBR result of the soil tested.

The test is fully covered in :-

B.S.1377:Soils for civil engineering purposes: Part 4,Compaction related tests.

The Reason For The CBR Test :

It sounds complicated, but the basis behind it is quite simple. We are determining theresistance of the sub grade, (i.e. the layer of naturally occurring material upon which the

road is built), to deformation under the load from vehicle wheels. Even more simply

put, How strong is the ground upon which we are going to build the road''.

The CBR test is a way of putting a figure on this inherent strength, the test is done in astandard manner so we are able to compare the strengths of different sub grade materials,

and we are able to use these figures as a means of designing the road pavement depth

required for a particular strength of sub grade.


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The stronger the sub grade (the higher the CBR reading ) the less thick it is necessary to

design and construct the road pavement, this gives a considerable cost saving. Conversely if

CBR testing indicates the sub grade is weak (a low CBR reading) we must construct a suitable

thicker road pavement to spread the wheel load over a greater area of the weak sub grade in order

that the weak sub grade material is not deformed, causing the road pavement to fail.

The CBR in spite of its limited accuracy still remains the most generally accepted method

of determining sub grade strength, and as such this information, along with information on traffic

flows and traffic growth is used to design road pavements.

CBR values in relation to site conditions at the time of construction

CBR values "on site" may not bear any relationship to the CBR values employed in theroad design, due to softening from wet weather and trafficking from site vehicles.

This is of course true for any design method you employ if the soil conditions at the timeof construction are different to the soil conditions upon which you based your design. It

could be some time before the properties of the soil revert back to their original

engineering condition, and by this time failure could have occurred.

"Capping layers" have been introduced to help solve the problem of sub-grades wettingup and losing strength during construction by protecting the sub grade from the worst of

the damage caused by site traffic.

The opposite is also true, if CBR values are taken on site after the sub -grade has beenexposed and dry weather has caused the moisture content of the soil to decrease,

increasing soil stiffness, the CBR value will be higher than natural moisture content, this

is an incorrect value for design purposes and if accepted will cause a serious under design

of the road pavement.


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Natural soil moisture content, after drainage, is the correct moisture content fordetermining CBR values for highway design purposes because in the course of time

natural soil moisture conditions will be re-established.

Good drainage is an essential part of road construction to allow the optimumstrength/CBR to be obtained from the soil foundation, whether it be insitu soil or

imported fill.

It of course follows that the drainage must be kept operating efficiently during the life ofthe road to prevent the strength/CBR decreasing through weakening of the foundation by

a rising water table.


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OBJECTIVE

To determine the California bearing ratio by conducting a load penetration test in thelaboratory

The objective of the California Bearing Ratio test is to determine the CBR value for a soilunder consideration as a pavement foundation. This value is a percentage comparison

with the standard crushed rock from California. Thus this test is a comparison test. The

CBR value is used to quantify the response of the pavement foundation and subgrade to

loading1.


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THEORY

California bearing ratio (CBR) test is developed by California Division of Highway for

evaluating the strength of subgrade soil and base course materials for flexible pavements. After

Second War, the U.S crop of Engineer adopted CBR test for use in the design of base course for

the airfield pavements. The test is empirical and the results of this test cant be related accurately

with the fundamental properties of the material but useful in the design of flexible pavements.

Unless the test procedure is strictly followed, dependable results cant be obtained.

Based on the extensive CBR test data collected, empirical design charts were developed by the

California State Highway Department, correlating the CBR value and flexible pavement

thickness required. Indian Road Congress (IRC) also uses this type of chart.

As per IS: 2720 (part XIV)-1979, the CBR is define as the ratio of force per unit area required

to penetrate a soil mass with a circular plunger of 50mm diameter at a rate of 1.25 mm/min to

force per unit ara required for corresponding penetration of a standard material.

Mathematically,

CBR (%) =

Table 1.1. Standard Load Values on Crushed Stones for different penetration values :

Penetration

(mm)

2.5 5.0 7.5 10.0 12.5

Standard Load

(kg)

1370 2055 2630 3180 3600

Unit Standard

load (kg/)

70 105 134 162 183


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Hence, CBR is the relative measure of the strength of the soil or aggregate with respect to

crushing. Standard load is that load which has been obtained from tests on crushed stone whose

CBR is taken to be 100% (table 1.1). the ratio is usually determined for penetration of 2.5 mm

and 5.0 mm.

A graph (load vs penetration) of load as ordinate and penetration as abscissa is drawn and CBR

value for 2.5mm and 5.0 mm penetration are calculated for each specimen from these graph.

Sometimes the initial portion of the curve is concave due to surface irregularities. In such a case

correction is applied by drawing a tangent at the point of greatest slope. The point where this

tangent meets the abscissa is the corrected zero reading of penetration.

Generally, CBR at 2.5mm is greater and this value is adopted. If CBR at 5.0 mm is greater, the

test is repeated. If again such condition arises, the higher value is adopted. According to IRC, if

the variation of results between three specimen is beyond the maximum permissible varations

(table 1.1) the test is repeated for next three specimens and average of these six specimen is

adopted.

Table 1.2 maximum Permissible Variations in CBR

Range of CBR values

(%)

Up to 10 10-30 30-60 Above 60

Maximum permissible

variation (%)

3.0 5.0 10.0 Not significant


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CBR test can be done in the field or in the lab but the method is different. CBR test is Lab may

be performed on :

A. Undisturbed soil specimen : Due to high degree of disturbances in the soil sample, thetest on undisturbed soil specimen is not generally done.

B. Disturbed or Remoulded soil Specimen: The test on demand or Remoulded soil specimenis generally done. Test specimens are prepared by compacting soil either by static

Compaction or Dynamic Compaction.

1. Statically Compacted Specimen:When static compaction is adopted the specimen is called Statically Compacted

Specimen. For static compaction, the batch of soil is mixed with water to give the

required moisture content; the correct weight of moist soil to obtain the desired

density is placed in the mould and compaction is attained by pressing the spacer disc

using a compaction machine or jack. Generally, preparation of specimen by static

compaction is not adopted.

2. Dynamically Compacted Specimen:When dynamic compaction is adopted the specimen is called Dynamically

Compacted Specimen. Preparation of disturbed soil specimens by dynamic

compaction is more commonly adopted. Dynamic compaction can be achieved either

by IS Light Compaction or by SI Heavy Compaction

a) IS light Compaction (proctor test)For this, soil is compacted in three layers each of compacted thickness about 44

mm by applying 56 evenly distributed blows of the 2.6 kg Rammer from 31 cmheight.


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b) IS Heavy Compaction (Modified Proctor Test or Modified AASHO Compaction)For this, soil is compacted in five layers each of compacted thickness about 26.5

mm by applying 56 evenly distributed blows of the 4.89 kg Rammer from 45cm

height.

3. The CBR test may be done either in Soaked Specimen or in Unsoaked Specimen. Butthe standard test requires soaked test hence soaked specimens are commonly prepared

for CBR value. Here we are doing is also CBR test in Lab on disturbed (remoulded)

soil Specimen compacted by dynamic. Compaction with IS heavy compaction

(Modified Proctor Test or Modified AASHO Compaction). Then the specimens are

soaked for test on Soaked Specimens.


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PROCEDURE

Normally 3 specimens each of about 7 kg must be compacted so that their compacteddensities range from 95% to 100% generally with 10, 30 and 65 blows.

Weigh of empty mould Add water to the first specimen (compact it in five layer by giving 10 blows per layer) After compaction, remove the collar and level the surface. Take sample for determination of moisture content. Weight of mould + compacted specimen. Place the mold in the soaking tank for four days (ignore this step in case of

unsoaked CBR.

Take other samples and apply different blows and repeat the whole process. After four days, measure the swell reading and find %age swell. Remove the mould from the tank and allow water to drain. Then place the specimen under the penetration piston and place surcharge load of 10lb. Apply the load and note the penetration load values. Draw the graphs between the penetration (in) and penetration load (in) and find the value

ofCBR.

Draw the graph between the age CBRand Dry Density, and find CBRatrequired degree of compaction.


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APPARATUS

NO PICTURE DESCRIPTON

1.

CBR Mould

2.

Perforated Base Plate

3.

Extension Collar 150mm


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4. Spacer Disc

4. Annular Surcharge Weight

5.

Slotted Surcharge Weight


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6.

Perforated Plate

7.

Metal Tripod

8.

Cutting Collar


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9.

Rammer 4.89kg

10.

Proving Ring with dial gauge

11.

California bearing test


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DATA

Static Compaction

In this method calculate the mass of wet soil at required moisture content to give adesired density when compacted in a standard test mould as given below

Volume of mould = 2250cc.

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

m

Weight of wet soil =1+ ---------- x W

100

Weight of water = Weight of wet soil - Weight of dry soil.

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

Take oven dried soil sample of calculated weight and thoroughly mix with water (OMC)as obtained from the above equation.

Record the empty weight of the mould with base plate, with extension collar removed(m1).

Place the correct mass of the wet soil in to the mould in five layers. Gently compact each layer with the spacer disc. Place the correct mass of the wet soil in to the mould in five layers. Gently compact each layer with the spacer disc. Place a filter paper on top of the soil followed by a 5cms displacer disc. Compact the mould by pressing it in between the platens of the compression testing

machine until the top of the spacer disc comes flush with the top of the mould.

Held the load for about 30 seconds and then release.


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In some soil types where a certain amount of rebound occurs, it may be necessary toreapply load to force the displacer disc slightly below the top of the mould so that on

rebound the right volume is obtained.

Remove the mould from the compression testing machine. Remove the spacer disc and weigh the mould with compacted soil (m2). Replace the extension collar of the mould. Prepare two more specimens in the same procedure as described above.

Dynamic Compaction

Take representative sample of soil weighing approximately 6kg and mix thoroughly atOMC.

Record the empty weight of the mould with base plate, with extension collar removed(m1).

Replace the extension collar of the mould. Insert a spacer disc over the base plate and place a coarse filter paper on the top of the

spacer disc.

Place the mould on a solid base such as a concrete floor or plinth and compact the wetsoil in to the mould in five layers of approximately equal mass each layer being given 56

blows with 4.90kg hammer equally distributed and dropped from a height of 450 mm

above the soil.

The amount of soil used shall be sufficient to fill the mould, leaving not more than about6mm to be struck off when the extension collar is removed.

Remove the extension collar and carefully level the compacted soil to the top of themould by means of a straight edge.

Remove the spacer disc by inverting the mould and weigh the mould with compacted soil(m2).

Place a filter paper between the base plate and the inverted mould. Replace the extension collar of the mould. Prepare two more specimens in the same procedure as described above.


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In both the cases of compaction, if the sample is to be soaked, take representativesamples of the material at the beginning of compaction and another sample of remaining

material after compaction for the determination of moisture content.

Each sample shall weigh not less than 100g for fine-grained soils and not less than 500for granular soils.

Place the adjustable stem and perforated plate on the compacted soil specimen in themould.

Place the weights to produce a surcharge equal to the weight of base material andpavement to the nearest 2.5kg on the perforated plate.

Immerse the whole mould and weights in a tank of water allowing free access of water tothe top and bottom of specimen for 96 hours

Test for Swelling

This test is optional and may be omitted if not necessary. 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 every dayagainst the time of reading.

Maintain a constant water level throughout the period of soaking. Note the final reading of the dial gauge at the end of soaking period (dh).


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Penetration Test

After 96 hours of soaking take out the specimen from the water and remove the extensioncollar, perforated disc, surcharge weights and filter paper.

Drain off the excess water by placing the mould inclined for about 15 minutes and weightthe mould.

Place the mould on the lower plate of the testing machine with top face exposed To prevent upheaval of soil in to the hole of surcharge weights, place 2.5kg annular

weights on the soil surface prior to seating the penetration plunger after which place the

reminder of the surcharge weights.

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

Set the stress and strain gauges to zero. Consider the initial load applied to the plunger as the zero load. Apply the load at the rate of 1.25 mm / min. Take the readings of the load at penetration of 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4, 5, 7.5, 10

and 12.5.

Raise the plunger and detach the mould from the loading equipment. Collect the sample of about 20 to 50gms of soil from the top 30mm layer of specimen

and determine the water content in accordance with IS: 2720 (Part 4) 1973.

Examine the specimen carefully after the test is completed for the presence of anyoversize soil particles, which are likely to affect the results if they happen to be located

directly below the penetration plunger.


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DISCUSSION

California Bearing Ratio CBR may be a field (ASTM D 4429-93, 1998 or BS 1377 Part 9, 1990)

or a laboratory test (ASTM D 1883-94, 1998 or BS 1377 Part 4, 1990). For both field and

laboratory versions of the test a steel cylinder with a cross sectional area of 3 square inches

(diameter 1.95 inches or 49.65 mm) is pushed into the soil at a constant rate with a jack. Because

of the size of the cylinder the test is appropriate only to soil with a maximum particle size of

three quarters of an inch (20 mm) or less. In the laboratory test a steel mould is used to hold the

soil. The same mould is used for modified Proctor compaction tests.

The soil is placed in the mould and either compressed or compacted (for details refer to the test

standards: US and UK standards differ in sample preparation). The load and penetration are

recorded as the cylinder is pushed into the soil. Bearing values are calculated as the stress in

pounds per square inch (psi) when the penetration reaches one tenth of an inch and one fifth of

an inch. The test bearing values are divided by the bearing values deemed typical of compacted

crushed rock i.e. 1000 psi and 1500 psi for the two penetrations. The greater of these ratios when

expressed as a percentage is the CBR eg. for a soft clay with stress 50 psi at one tenth of an inch

penetration the CBR is 5. The test was conceived specifically for design of flexible pavements

(highway base course). The method of design is purely empirical. Design charts were developed

by the California Division of Highways with the benefit of their practical experience in flexible

pavement success and failure. For any value of CBR the charts predict a suitable thickness of

pavement. Charts have since been derived for airport runways (curves for different wheel loading

and tyre pressure) and for highways with different traffic volumes and axle weights).


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CONCLUSION

Mechanistic design methods utilizing elastic layer theories require the determination of the

elastic modulus. The elastic modulus for soil sub grades can be characterized by the resilient

modulus and can be obtained from the repeated load tests. Due to the time and skill required to

conduct these tests, approximate correlations between resilient modulus. Elastic model,

especially Duncan hyperbolic measured parameter is utilized.

The commonly used California Bearing test value is used to obtain a prediction of resilient

modulus. Plasticity models should be utilized when realistic evaluations of strains and

displacements are required. Elastic models, especially the Duncan hyperbolic model (Duncan

and Chang 1970) can suitably predict deformations at failure as long as the orientation of stresses

remain constant but have limited benefit when evaluating displacements at and after failure. In

addition, the hyperbolic model is of limited suitability if realistic evaluations of pore pressure are

required. Linear elastic models are of limited benefit as they do not accurately predict stresses or

strains in the sub grade soil.


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IMPORTANT OF CBR

Field in-place CBR tests are used for evaluation and design of flexible pavement components

such as base and sub base course and sub grades and for other applications (such as un surfaced

roads) for which CBR is the desired strength parameter. If the field CBR is to be used directly

for evaluation or design without consideration for variation due to change in water content, the

test should be conducted under one of the following conditions: when the degree of saturation

(percentage of voids filled with water) is 80 % or greater, when the material is coarse grained

and cohesionless so that it is not significantly affected by changes in water content, or when the

soil has not been modified by construction activities during the two years preceding the test. In

the last-named case, the water content does not actually become constant, but generally

fluctuates within a rather narrow range. Therefore, the field in-place test data may be used to

satisfactorily indicate the average load-carrying capacity.

Any construction activities, such as grading or compacting, carried out subsequent to the bearing

ratio test will probably invalidate the results of the test. Soils and flexible pavement components

at the same location may exhibit significantly different load deflection relationships. No method

presently exists to evaluate the precision of a group of non-repetitive plate load tests on soils and

flexible pavement components due to the variability of these materials.


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REFERENCE

1. Craig, R.F. 2004. Craigs Soil Mechanics. 7th ed. London: Spon. ISBN 0-415-32703-2

2. University of Abertay Dundee. Unknown. Subgrade and Unbound Pavement Foundation[Class information sheet provided by Mr. Ogwuda].

3. The Idiots Guide to Highways Maintenance [online]. Availablefrom:http://www.highwaysmaintenance.com/cbrtext.htm [Accessed on 11 Nov 2004]


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APPENDIX

California Bearing Test

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