Cbr Report 71691d

Page |1

CBR (California Bearing Ratio) TEST

• WHAT IS CBR? The California Bearing Ratio test, or CBR test as it is usually termed, is an empirical test first developed in California, USA, for estimating the bearing value of highway sub-bases and subgrades. This test was first developed by the California Division of Highways as a method for evaluating the stability of soil sub grade and other flexible pavement materials. This test can be performed in the laboratory on prepared samples or in-situ on location. It is important to appreciate that this test, being of an empirical nature, is valid only for the application for which it was developed, i.e. the design of highway base thicknesses. • WHY CBR TEST IS NEEDED? The CBR test is essentially an arbitrary strength test and hence it is mainly used to evaluate the strength of soil subgrade. It can not be used to evaluate the soil properties like cohesion, angle of internal friction , or shearing resistence. • DEFINATION OF CBR: The CBR or California Bearing Ratio is defined as the ratio of the force per unit area required to drive a cylindrical plunger of 50mm diameter at the rate of 1.25mm/min into a soil mass to that required to drive the same plunger at the same rate into a standard sample of crushed stone. • CBR TEST PROCEDURE: Tests are carried out on natural or compacted soils in water soaked or un-soaked conditions and the results so obtained are compared with the curves of standard test to have an idea of the soil strength of the sub grade soil. The test can be conducted according to the standards made by IS: 2720 (Part 16) 1979. The procedure is as follows-

Page |2

•

APPARTUS REQUIRED:-

Cylindrical mould: Cylindrical mould assembly of 2250cc capacity with base plate, stay rod and wing nut confirming to 4.1,4.3 and 4.4 of IS: 96691980. For any group of moulds, one extra baseplate is desirable because two plates are required when a mould is inverted during the preparation of the specimen. 1.

2.Disk: Spacer Disc confirming to 4.4 of IS: 9669-1980. 3. Measuring apparatus: Apparatus for measuring expansion of soil, consisting of adjustable stem and perforated plate, tripod, and dial micrometer (reading to 0.001 in.) confirming to 4.4 of IS: 9669-1980. 4. Weights: Masses, including one annular Surcharge mass and several slotted or split surcharge masses confirming to 4.4 of IS: 96691980. 5. Metal rammer: Metal rammer confirming to IS: 9189-1979. 6. Soaking tank. Soaking tank of sufficient size to accommodate several test moulds and of sufficient depth to ensure submergence of the sample. 7. Penetration plunger: Penetration plunger of 50mm dia confirming to 4.4 of IS: 96691980. 8. Loading device: Loading device, either a laboratory testing machine or screwjack and frame arrangement having a capacity of at least 5000kg and

Page |3

equipped with a movable head or base that travels at a uniform rate of 1.25mm / min for use in forcing the penetration plunger in to the specimen. 9. General laboratory equipment: General laboratory equipment specified in CRD-C 653. 10. Proving rings: Calibrated proving rings. 11. Micro meters: Dial micro meters two numbers reading to 0.01mm. 12. Steel plate: Steel plate of 25.4 cm in diameter, having a 5.2 cm diameter hole in the centre and weighing 4.5 kg. 13. I.S. Sieves: IS sieves 37.50 or 22.50 or 19mm and 4.75mm. 14. Miscellaneous apparatus: Miscellaneous apparatus such as mixing bowl, straight edge, scales, soaking tank, drying oven, filter paper, dishes and calibrated measuring jar.

Page |4

• COMPACTION OF SOIL SPECIMEN: There are two types of methods in compacting soil specimen in the CBR moulds i. Static Compaction method. ii. Dynamic Compaction method. i. STATIC COMPACTION METHOD:•

•

•

The material used in the above method shall 19mm sieve for fine grained soils and 37.50mm sieve for coarse materials up to 37.50mm. The material retained on 19mm sieve is replaced by an equal amount of material ing 19mm sieve and retained on 4.75mm sieve The material retained on 37.50mm sieve is replaced by an equal amount of material ing 37.50mm sieve and retained on 4.75mm sieve.

Page |5

In this method the mass of wet soil is calculated 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 W100 Weight of water = Weight of wet soil - Weight of dry soil. m = Optimum moisture content obtained from the laboratory compaction test. • Oven dried soil sample of calculated weight is taken and is thoroughly mixed with water (OMC) as obtained from the above equation. • The empty weight of the mould with base plate, with extension collar removed (m1) is recorded. • The correct mass of the wet soil is placed in to the mould in five layers. • Each layer is compacted gently with the spacer disc. • A filter paper is placed on top of the soil followed by a 5cms displacer disc. • The mould is compacted 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. • The load is hold for about 30 seconds and then released. • 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. • The mould is removed from the compression testing machine. • The spacer disc is removed and the mould is weighed with compacted soil (m2). • The extension collar of the mould is replaced. • Two more specimens are prepared in the same procedure as described above. •

ii.

dynamic COMPACTION METHOD:-

• The material used in the above method shall 19mm sieve for fine grained soils and 37.50mm sieve for coarse materials up to 37.50mm.

Page |6

•

•

• • • • •

•

•

• • • •

The material retained on 19mm sieve is replaced by an equal amount of material ing 19mm sieve and retained on 4.75mm sieve. The material retained on 37.50mm sieve is replaced by an equal amount of material ing 37.50mm sieve and retained on 4.75mm sieve. Representative sample of soil weighing approximately 6kg is taken and is mixed thoroughly at OMC. The empty weight of the mould with base plate, with extension collar removed (m1) is recorded. The extension collar of the mould is replaced. A spacer disc is inserted over the base plate and a coarse filter paper is placed on the top of the spacer disc. The mould is placed on a solid base such as a concrete floor or plinth and the wet soil in to the mould is compacted in five layers of approximately equal mass each layer being given 56 blows with 4.90kg hammer equally is distributed and is dropped from a height of 450 mm above the soil. The amount of soil used should be sufficient to fill the mould, leaving not more than about 6mm to be struck off when the extension collar is removed. The extension collar is removed and the compacted soil is carefully levelled to the top of the mould by means of a straight edge. The spacer disc is removed by inverting the mould and the mould is weighed with compacted soil (m2). A filter paper is placed between the base plate and the inverted mould. The extension collar of the mould is removed. Two more specimens are prepared in the same procedure as described above.

•

SOAKING OF SOIL SPECIMEN:

•

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.

•

Page |7

• •

•

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 OF SOIL SPECIMEN:

• •

The initial height of specimenis determined (h) in mm. The expansion-measuring device is mounted along with the tripod on the edge of the mould and the initial dial gauge reading is recorded (ds).

Page |8

•

This set up is kept as such undisturbed for 96 hours noting down the readings every day against the time of reading. A constant water level is maintained through out the period of soaking. The final reading of the dial gauge is noted at the end of soaking period (dh). Calculations for Swelling:= df -ds Expansion ratio = ----------- x 100 h ds = Initial dial gauge reading in mm df = final dial gauge reading in mm h = initial height of specimen in mm

•

PENETRATION TEST:

•

After 96 hours of soaking the specimen is taken out from the water and the extension collar is removed. The excess water is drained off by placing the mould inclined for about 15 minutes and the mould is weighed. The mould is placed on the lower plate of the testing machine with top face exposed. To prevent upheaval of soil in to the hole of surcharge weights, a 2.5kg annular weight is placed on the soil surface prior to seating the penetration plunger after which the reminder of the surcharge weights are placed. The plunger is set under a load of 4 kg so that full is established between the surface of the specimen and the plunger. The stress and strain gauges are set to zero. The initial load applied to the plunger is considered as the zero load. The load is applied at the rate of 1.25 mm / min. 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 are taken. The plunger is raised and the mould is detached from the loading equipment. The sample of about 20 to 50gms of soil is collected from the top 30mm layer of specimen and determine the water content in accordance with IS: 2720 (Part 4) 1973.

• • •

• •

•

• • • • • •

Page |9

•

The specimen is examined carefully after the test is completed for the presence of any over size soil particles, which are likely to affect the results if they happen to be located directly below the penetration plunger, perforated disc, surcharge weights and filter paper.

P a g e | 10

•

CALCULATION OF CBR FROM LOAD PENETRATION CURVE:

Plot the load penetration curve in natural scale, load on Y axis and penetration on X – axis. • If the curve is uniformly convex upwards although the initial portion of the curve may be concave upwards due to surface irregularities make correction by drawing a tangent to the upper curve at the point of contraflexure. • Take the intersection point of the tangent and the X – axis as the origin. • Calculate the CBR values for penetration of 2.50mm and 5.00mm. • Corresponding to the penetration value at which CBR is to be desired, take the corrected load values from the load penetration curve and calculate the CBR from the equation PT x Cf California Bearing Ratio = ------------- x 100. PS •

PT = Corrected unit test load corresponding to the chosen penetration from load penetration curve PS = Total standard load for the same depth of penetration, which can be taken from the Table below Cf = Proving ring correction factor.

STANDARD LOADS AT SPECIFIED PENETRATIONS: Penetration depth (mm) 2.50 5.00 7.50 10.00 12.50

Unit Standard load Kgf/ cm2 70 105 134 162 183

Total Standard load (kgf) 1370 2055 2630 3180 3600

P a g e | 11

LOAD VS. PENETRATION CURVE:-

•

REPORT OF CBR TEST:-

P a g e | 12

The report of CBR test should include the following• Report the CBR value to the nearest second decimal. • Take the average of three test specimens as the CBR value of the test. • Generally, the CBR value at 2.50mm penetration will be greater than that at 5.00mm penetration and in such case take the value at 2.50mm as the CBR value. • If the CBR value corresponding to a penetration of 5.00mm exceeds that of 2.50mm,repeat the test. • If the identical results follow, take the value corresponding to 5.00mm as the CBR. A general format of CBR report is shown below-

P a g e | 13

P a g e | 14

• CORRECTIONS TO THE LOAD PENETRATION CURVE:-

P a g e | 15

Normally the curve obtained from the load penetration data is with a convexity upwards as for a particular specimen and the loads corresponding to 2.50mm and 5.00 mm penetration values. But sometimes for some test results a curve with initial upward concavity is obtained, indicating the necessity of correction for that specimen. In this case the corrected origin is established by drawing a tangent from the steepest point of the curve. The load values corresponding to 2.5mm and 5.0mm penetration values from the corrected origin are noted. The main causes for the initial concavity of the load penetration curve calling for the correction in origin are due toi. The bottom surface of the plunger or the top surface of the soil specimen not being truly horizontal, with the result the plunger surface not being in full with the top of the specimen initially. ii. The top layer of the specimen being too soft or irregular.

• PRECAUTIONS TO BE TAKEN DURING TEST:The following precautions should be taken while performing the testThe holes of the base plate and that of perforated disc should be cleaned thoroughly. • The surcharge weight should be aligned properly with the plunger so that the plunger penetrates freely in to the soil. • If the CBR value obtained from the test at 5.0mm penetration is higher than that of 2.5mm then the the test should be repeated for checking. If again it gives the same result then the higher value obtained at 5.0mm penetration is reported as the CBR value. • If the variation in CBR value between the three specimens is more than the prescribed limits, then the test should be repeated on adiitional three samples and the average of six samples is reported as the final CBR value. • Materials ing 20mm sieve is only used in the test. •

P a g e | 16

• APPLICATION OF CBR TEST:Due to the simplicity of the test procedure the CBR method is adopted globally for pavement design. The CBR tests were carried out by the California State Highway Department on existing pavement layers including sub-grade, sub base and base course. Based on the extensive CBR test data collected on pavement which behaved satisfactorily and those which failed, an empirical design chart was developed correlating the CBR value and the pavement thickness. The basis of the design chart is that a material with a given CBR required a certain thickness of pavement layer as cover. A higher load needs a thicker pavement layer to protect the subgrade. design curves correlating the CBR value with total pavement thickness cover were developed by the California State Highway Department for wheel loads of 3175 kg and 5443 kg representing light and heavy traffic. later the design curve for 4082 kg wheel load was obtained by interpolation for medium traffic. The design chart is shown in next page. The Indian Road Congress has recommended a CBR design chart for tentative use in India. Different curves A, B, C, D, E, F, & G have been given based on the volume of commercial vehicles. So the CBR test is very essential for pavement design.

P a g e | 17

P a g e | 18