California Bearing Test

CALIFORNIA BEARING RATIO 2013

California bearing ratio test NAME OF GROUP C : I.

AZRI IZZAT BIN MOHD AZAMI

14DKA12F1061

II.

AHMAD AFFANDI BIN DULAH

14DKA12F1062

III.

MUHAMMAD ZULKARNAIN BIN SAINI

14DKA12F1063

IV.

AHMAD FIQRI BIN HAMDAN

14DKA12F1065

V.

MUHAMAD ZAZALI BIN PAIMAN

14DKA12F1066

VI.

MOHD SHAFIQ BIN MOHD TAHIR

14DKA12F1067

VII.

SITI NURJIHAN BINTI SELAMAT

14DKA12F1069

AIMAN BIN MOHD SAID

14DKA12F1070

MUHAMMAD SYUKRI BIN ABDUL SAMAD

14DKA12F1071

MOHD NORARIF BIN ARIPIN

14DKA12F1072

VIII. IX. X.

1

CALIFORNIA BEARING RATIO 2013 CONTENT NO.

CONTENT

PAGE

1.

INTRODUCTION

3–5

2.

THEORY

6

3.

OBJECTIVE

7

4.

APPARATUS

8 - 11

5.

PROCEDURE

12 – 16

6.

DATA AND ANALYSIS

17 - 18

7.

QUESTION

19 - 20

8.

DISCUSSION

21 - 23

9.

CONCLUSION

24

10.

REFERENCE AND APPENDIX

25 - 27

2

CALIFORNIA BEARING RATIO 2013 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 be in 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 the resistance 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 a standard 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.

3

CALIFORNIA BEARING RATIO 2013 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 the road 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 time of 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 wetting up 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 been exposed 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.

4

CALIFORNIA BEARING RATIO 2013 

Natural soil moisture content, after drainage, is the correct moisture content for determining 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 optimum strength/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 of the road to prevent the strength/CBR decreasing through weakening of the foundation by a rising water table.

5

CALIFORNIA BEARING RATIO 2013 OBJECTIVE 

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



The objective of the California Bearing Ratio test is to determine the CBR value for a soil under 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.

6

CALIFORNIA BEARING RATIO 2013 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 can’t 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 can’t 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

2.5

5.0

7.5

10.0

12.5

1370

2055

2630

3180

3600

70

105

134

162

183

(mm) Standard Load (kg) Unit Standard load (kg/

7

)

CALIFORNIA BEARING RATIO 2013 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

3.0

5.0

10.0

Not significant

(%) Maximum permissible variation (%)

8

CALIFORNIA BEARING RATIO 2013 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, the test on undisturbed soil specimen is not generally done. B. Disturbed or Remoulded soil Specimen: The test on demand or Remoulded soil specimen is 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 cm height.

9

CALIFORNIA BEARING RATIO 2013 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. But the 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.

10

CALIFORNIA BEARING RATIO 2013 PROCEDURE 

Normally 3 specimens each of about 7 kg must be compacted so that their compacted densities 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 of CBR.



Draw the graph between the age CBR and Dry Density, and find CBR at required degree of compaction.

11

CALIFORNIA BEARING RATIO 2013 APPARATUS NO

PICTURE

DESCRIPTON

1. CBR Mould

2. Perforated Base Plate

3. Extension Collar 150mm

12


4.

Spacer Disc

4.

Annular Surcharge Weight

5. Slotted Surcharge Weight

13


6. Perforated Plate

7. Metal Tripod

8. Cutting Collar

14


9. Rammer 4.89kg

10. Proving Ring with dial gauge

11. California bearing test

15

CALIFORNIA BEARING RATIO 2013 DATA Static Compaction 

In this method calculate the mass of wet soil at required moisture content to give a desired 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.

16


In some soil types where a certain amount of rebound occurs, 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.



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 at OMC.



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






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 wet soil 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 about 6mm to be struck off when the extension collar is removed.



Remove the extension collar and carefully level the compacted soil to the top of the mould 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.






Prepare two more specimens in the same procedure as described above.

17


In both the cases of compaction, if the sample is to be soaked, take representative samples 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 500 for granular soils.



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



Place the weights to produce a surcharge equal to the weight of base material and pavement 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 to the 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 day against 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).

18

CALIFORNIA BEARING RATIO 2013 Penetration Test 

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



Drain off the excess water by placing the mould inclined for about 15 minutes and weight the 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 surface of 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 any oversize soil particles, which are likely to affect the results if they happen to be located directly below the penetration plunger.

19


20


21


22

CALIFORNIA BEARING RATIO 2013 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).

23

CALIFORNIA BEARING RATIO 2013 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.

24

CALIFORNIA BEARING RATIO 2013 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.

25

CALIFORNIA BEARING RATIO 2013 REFERENCE

1. Craig, R.F. 2004. Craig’s 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 Idiot’s Guide to Highways Maintenance [online]. Available from:http://www.highwaysmaintenance.com/cbrtext.htm [Accessed on 11 Nov 2004]

26


APPENDIX

27

California Bearing Test

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