CBR Test of Soil Procedure, Calculation & Graph is required to get the Bearing capacity of soil, soaked or un-soaked condition. Here we described CBR test procedure along with calculation and graph as per IS Code.
What is CBR Test
The CBR test ( California Bearing Ratio test ) is a penetration test which evaluates the strength of flexible pavements in soaked or un-soaked condition. CBR test is the most widely used method for the checking and design of road and pavement.
The test in which a ratio expressed in percentage of force per unit area required to penetrate a soil mass with a circular plunger of 50 mm diameter at the rate of 1.25 mm per minute to that required for related penetration in a standard material.
The CBR value is usually determined for penetration on 2.5 mm and 5 mm. Where the ratio at 5 mm is consistently higher than that at 2.5 mm, the ratio at 5 mm is used.
CBR Test IS Code
The IS Code used for CBR test of soil is IS 2720 part 16.
Apparatus and Tools Required for CBR Test
- CBR testing machine with a movable head ( minimum capacity 5000 kg )
- Moulds with base plate
- Spacer disc
- Surcharge weight
- Metal rammer ( 4.89 kg )
- Expansion measuring apparatus
- Penetration plunger
- Dial gauge 2 nos – reading 0.01 mm
- IS Sieves 47.5 mm and 19.0 mm
- Hot air oven
- Straight edge
- Non-absorbent tray
- Miscellaneous like filter paper, dishes, calibrated measuring jar etc.
Precautions Before Proceeding CBR Test
- Proctor test of the same soil must be done as we described before to get the Maximum dry density and Optimum moisture content value.
- Before proceeding the test ensure that all CBR moulds are clean and nuts are properly tightened with base plate and collar.
Taking Soil Sample for CBR Test
Take the soil sample and sieve it by using 19.0 mm IS sieve. Mostly CBR test shall be done for subgrade layer in earthwork so obviously coarser than 19.0 mm particle can be found.
To replace the coarser particles from your sample add the same weight of material which passed through 19.0 mm and retained on 4.75 mm.
Undisturbed Sample for CBR Test
We can collect undisturbed specimens by fitting to the mould, the steel cutting edge (150 mm internal diameter) and pushing the mould gently into the ground.
In this process, we dig the soil from the outside of the mould to push it down until the mould gets full of soil. When the mould is sufficiently full of soil, then remove the mould by digging under the mould.
Now trim the top and bottom surfaces of soil to give a perfect shape for the test.
If the mould cannot be pressed in soil, then collect the soil sample by digging at a circumference greater than the size of the mould and bring the soil lumps outside of the ground.
We can calculate the density of soil by taking the soil weight and mould volume ( if the mould fully filled with soil ) otherwise measure the weight and volume of soil lump after trimming.
The density of soil = Weight of soil / Volume of soil
Specimen Compaction for CBR Test
There is two way to compact soil sample in CBR mould.
- Statically Compaction
- Dynamically Compaction
In this method, we take a fixed weight of soil and water content and then we compact the mixed soil in CBR mould
by displacer disc using a compression machine.
Here’s how we are describing it step by step.
1. First, calculate the standard specimen volume in the mould ( CBR mould volume). For this, take a CBR mould
without a collar and put a displacer disc in it then measure the depth of mould ( from top of displacer disc to top of mould ).
Then measure dia of the mould.
Volume of mould = Λr2h
where the value of pi Λ is 22.7,
r is radius ( dia/2 ),
h is height or depth of mould.
2. Take the soil and water weight as per your maximum dry density and optimum moisture content.
Suppose if maximum dry density value is 1.90 g/cc and optimum moisture content is 12.0 %
then required soil quantity is :
weight = volume * density
= 2250 * 1.90 = 4275 gm ( if mould volume 2250 cc ).
3. Water weight = soil weight * OMC %
= 4275 * 12 % = 513 gm
4. Mix the soil and water in a non-absorbent tray properly by breaking the lumps while mixing.
5. After mixing take a CBR mould and place a cleaned displacer disc in the mould.
6. Place a filter paper on displacer disc in the mould.
7. Fill the total mixed soil in the mould and a displacer disc after soil. If need press the displacer disc by hand or hammer.
8. Take minimum of 50 gram soil sample from mixed soil each time when you are going to fill the mould.
9. Don’t forget to place a filter paper on the top of the soil.
10. Place this set in a compressing testing machine and start compressing it.
11. Compress the soil in the mould until the top label of displacer disc and mould’s collar reaches equal. (watch my video )
12. Repeat the same procedure for another two CBR moulds. Total three required.
- In dynamically compaction method we take generally 5.5 kg to 6.0 kg soil sample by sieving as we described above.
- Water weight shall be OMC % of the soil weight. If soil weight is 6000 gm then water weight = 6000 * 12.0 % = 720 gram.
- Mix soil and water properly in a non-absorbent tray.
- Take a properly tighten CBR mould with extension collar.
- Place a spacer disc in mould and a filter paper on spacer disc.
- Mould must be filled in five equal layers by giving 55 blows with a 4.89 kg rammer as we did in proctor test.
- Take a minimum of 50 gram soil sample from mixed soil each time when you are going to fill the mould.
- After completing the fifth layer remove extension collar and struck off extra soils from the top of the mould.
- Loose the nuts which attached mould with base plate and turn the filled mould vertically, means top surface will go down and put on the base plate and tighten the nuts. ( watch my video for better understanding )
- Remove spacer disc from the top of the filled mould.
- Take the weight of filled soil and mould.
- Attach extension collar.
- Add one filter paper on the top of the soil then insert one perforated disc.
- Then place two surcharge weights ( each 2.5 kg ) on the perforated disc. Now your first specimen is ready.
- Prepare another two specimens.
- Keep in water tank for four days for soaked CBR value.
Note: Now these days many of the project clients are asking for the CBR test by filling three moulds by 10 blows, 30 blows and 65 blows in 5 equal layers. ( dynamically compaction method )
CBR Test Procedure
Here are two ways to test the CBR.
Unsoaked Specimen Test Procedure
In the unsoaked specimen test, we test specimens without soaking in water just after testing. For testing procedure check soaked specimen test below.
Soaked Specimen Test Procedure
Follow these instructions step by step:
- Keep out the three specimens from water and remove the surcharge weights ( 2.5 kg * 2 ), perforated disc and filter paper.
- Allow it to drain water by keeping the specimens horizontally for at least 10 minutes.
- Take the weight of soil and mould.
- Then Place both surcharge weight (2.5 kg * 2) in the same mould.
- Place the first specimen with both surcharge weights in CBR testing machine plate under penetration plunger.
- Set both dial gauge load and penetration reading ( like in video below).
- Speed or rate of loading of the machine must be at 1.25 mm per minute.
- Set zero in both dial gauges before start the machine.
- Start the machine until the plunger touches the top surface of the soil in mould and reading has reached 4 kg in the load dial gauge. Stop the machine.
How to Read Dial Gauges
- Penetration gauge reading detail – half-round means 0.5 mm penetration, one round means 1.0 mm penetration.
- Load gauge reading detail – 100 points in one round, one point = 5.88 kg as per our dial gauge calibration.
- Start the machine and focus on both dial gauges simultaneously.
- Note down the load readings on penetration 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, 3.0 mm, 4.0 mm, 5.0 mm, 7.5 mm, 10.0 mm, 12.5 mm. ( Watch my video for better understanding, given below )
- After test do not forget to take the soil sample from each specimen which shall not be less than 50 gram.
|Penetration ( mm )||Load reading|
CBR Test Calculation
After 24 hours dry in a hot air oven take the soil sample out and allow it to be cool in atmosphere temperature. Take the soil sample weight near about after 10 minutes.
Calculate the load in kg for the penetration 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, 3.0 mm, 4.0 mm, 5.0 mm, 7.5 mm, 10.0 mm, 12.5 mm by multiplying by 5.88. ( 5.88 kg value we got from our load gauge calibration report, it can be different for your)
The main important load value is what taken on 2.5 mm and 5.0 mm penetration.
So lets calculate for these both
suppose we got reading in load dial gauge
on 2.5 mm penetration – 31
on 5.0 mm penetration – 52
so load value shall be on 2.5 mm penetration = 31 * 5.88 = 182.28 kg
and for 5.0 mm penetration = 52 * 5.88 = 305.76 kg
by this method you can calculate load value for each reading.
|Penetration ( mm )||Load reading||Load in kg ( reading * 5.88 )|
Now we calculate the CBR value:
CBR value on 2.5 mm penetration = 182.28 / 1370 * 100 = 13.31
CBR value on 5.0 mm penetration = 305.76 / 2055 * 100 = 14.88
For each specimen we consider CBR value the higher value between 2.5 mm and 5.0 mm penetration.
Calculate the average of three higher value from three specimens and that is the CBR value. Graph shall be drawn for each specimens separately. (See below)
If you are Casting CBR moulds in this way using 4.89 kg rammer –
First mould – 10 blows 5 layers
The second mould – 30 blows 5 layer
The third mould – 65 blows 5 layer
Then there is no need to calculate the average of three specimens higher CBR value. Graph shall be drawn separately for each specimen and then final graph in which we draw graph dry density vs CBR value.
CBR Test Report
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CBR Test Graph
If we use 55 blows for each specimen then three graphs will be drawn separately.
If we use 10 blows, 30 blows and 65 blows for each specimen then four graphs will be drawn separately.
Graph for 10 Blows
Graph for 30 Blows
Graph for 65 Blows
In this graph, we have drawn CBR vs Dry density and we took CBR value on 97 % of dry density.
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Our Practical Video
watch CBR related videos for calculation and report.
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