HOW TO CALCULATE CBR VALUE USING DCPT RESULT ON SITE?

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DCPT (Dynamic Cone Penetration Test)

Dynamic cone penetration test or DCPT is a quick, simple and low cost test which is extensively used by many agencies around the globe to measure the strength of compacted material of unbound granular and sub grade layers with reasonable accuracy.

Now a days DCPT is commonly used during construction for the following purposes.

  • To control quality of construction
  • To determine in situ CBR value
  • To evaluate quality of flexible pavement

Principle of DCPT

The basic principle involved in the operation of this apparatus is the measuring of the resistance offered by the pavement layers to the penetration of a standard cone with a diameter of 20 mm (with 600 apex angle) driven by an 8 kg hammer freely falling through a height of 575 mm. the amount of penetration (in mm) of the cone is generally reported in terms of the average penetration per blow, DCPI60 (mm/blow).

Note: Greater the value of DCPI60 indicates a softer material and vice versa.

Calculation of CBR Value from DCPT Result

Follow the below mentioned step by step procedure to calculate CBR value using DCPT result. To get a clear understanding of the calculation, an example is also given at the end of the procedure.

Procedure

  1. Record the initial reading on the dynamic cone penetrometer scale.
  2. Calculate the value of penetration (in mm) for each blow by subtracting the previous recorded reading (i.e. reading on scale) from the current reading.
  3. Calculate the cumulative depth of penetration (in mm) for each blow by adding all the previous individual depth of penetrations.
  4. Draw a chart using the data of cumulative depth of penetration and number of blows. (See Fig-1)
  5. This step is about identifying the depth of each layer. Identify the slope change of the penetration curve. Every change in slope represents a change in layer type.
  6. After identifying each slope change, calculate the average penetration/blow for each slope change and calculate the thickness of each layer.
  7. Use the following equation and determine the corresponding CBR value of each layer.

log10CBR = 2.48 – 1.057 log10(DCPI60)

  1. To know the variation of the CBR value across the depth, draw another semi-log chart with CBR values on logarithmic scale as abscissa (x axis) and depth of penetration on ordinary scale as ordinate (y axis). (See Fig-2).

Example Calculation of CBR value

No. of blows DCP scale reading (cm) DCPI60 pen/blow (mm) Cumulative penetration (mm) CBR value (%) Cumulative penetration (cm) Layer thickness (mm) Avg CBR (%)
0 9.5 0 0 0 73 44.9
1 10.1 6 6 45.4 0.6
2 10.7 6 12 45.4 1.2
3 11.3 6 18 45.4 1.8
4 11.9 6 24 45.4 2.4
5 12.5 6 30 45.4 3
6 13.1 6 36 45.4 3.6
7 13.7 6 42 45.4 4.2
8 14.3 6 48 45.4 4.8
9 14.9 6 54 45.4 5.4
10 15.5 6 60 45.4 6
11 16.1 6 66 45.4 6.6
12 16.8 7 73 38.6 7.3
13 19 22 95 11.5 9.5 178 11.4
14 21.3 23 118 11.0 11.8
15 23.5 22 140 11.5 14
16 25.8 23 163 11.0 16.3
17 27.8 20 183 12.7 18.3
18 30 22 205 11.5 20.5
19 32.3 23 228 11.0 22.8
20 34.6 23 251 11.0 25.1
21 38.2 36 287 6.8 28.7 646 6.9
22 41.8 36 323 6.8 32.3
23 45.4 36 359 6.8 35.9
24 49.0 36 395 6.8 39.5
25 52.6 36 431 6.8 43.1
26 56.1 35 466 7.0 46.6
27 59.8 37 503 6.6 50.3
28 63.4 36 539 6.8 53.9
29 66.9 35 574 7.0 57.4
30 70.5 36 610 6.8 61
31 74.1 36 646 6.8 64.6
32 77.7 36 682 6.8 68.2
33 81.3 36 718 6.8 71.8
34 84.8 35 753 7.0 75.3
35 88.4 36 789 6.8 78.9
36 92 36 825 6.8 82.5
37 95.6 36 861 6.8 86.1
38 99.2 36 897 6.8 89.7

Note:

  • In the example given above the initial reading is 9.5.
  • There exist many equations to derive CBR value from DCPT result. The equation that we have used is commonly used. If you want you can use any other equations.
Fig-1 DCP chart
Fig-1 DCP chart
Fig-2 Variation of CBR with Depth
Fig-2 Variation of CBR with Depth

To down load the excel sheet for the above calculation click here.

Also Read: Factors affecting Dynamic Cone Penetration Test result

25 thoughts on “HOW TO CALCULATE CBR VALUE USING DCPT RESULT ON SITE?”

  7. Hello!
    I have repeatedly tried to download the excel sheet CBR calculation. Everytime I get a message “Google Drive…You need access…request access”. I click “Request access” and get a new message saying that I’ll receive an email when the file has been shared with me. Then I check my email, and I’ve received an email saying that there has been a new login on my Google account. Then I check make the security check, and that’s it. I don’t receive any file. Please help me out on this.
    Regards, Robert

    Reply

  8. Thank you, the spreadsheet is very useful yet simple; however, penetration index is to be modified by the user according to his requirement. Formula developed by TRRL has been used, which is reliable and applicable for wide ranges of soil types (all).

    Reply

  9. I have used 1, 2, 3, or 5 blows according to the changing rate of penetration in site. In the spreadsheet your have shared, there is provision only for single blow, CBR%=10^(2.48-(1.057*LOG10(penetration per blow, mm))). Please give the idea or share the spreadsheet for large variable number of blows.

    Reply

  12. Thanks very much because you have helped me with how to deal with dcpt results to come out with cbr values but now another thing is to get the total bearing capacity of the soil. Please help me because i have a task to design a foundation. Thank you.

    Reply

    • Hi, very useful information, however, it looks like it was done on one spot. Doing it on the coal stock pile which covers 500 metres long X 200 metres breadth, one has to do it 24 times. Then does that suggest that for an individual testing I have to do a separate culculation? I hope you will grasp my question.

      Reply

  14. Good day Sir, hope everything is fine.
    I´m having some trouble to calculate DCP penetration rate and estimated CBR.
    I would like to request if you can give me some exemples.
    I have 5 blows to a depth of 74mm
    then 10 blows to a depth of 152 mm

    Reply

    • I would like to know how to fill in the design data summary table. Penetration rate column, Material class and estimated CBR. I need fomulars to fill in these columns I have just mentioned please.

      Reply

  17. I would like to have some information on the calculations on using a (dcp) dynamic penetrometer

    number blows versus depth of rod per Cantimeter to determire the percentage of strength of soil

    so say for instance you do 5 blows and measure in goes down 2.25 cm you do a set of 5 more blows and measure it goes down 3.5 cm you do another set of 5 blows and measure and it goes down 1.5 inches what % of strength is this determine kg strength of 150 ? would it hold a 100,000 kgpiece of equipment up ? thanks

    DCP formola culculation of site Data to how can we take result

    Reply

    • Hi boss,very sorry I need to know the procedure of calculating the CRB value from the dynamic cone penetrometer test
      I seen the formula to your comment which is crb%=10^{2.48-1.057(log10(penetration per blows in mm))),how to use it

      Reply

    • I would like to have some information on the calculations on using a (dcp) dynamic penetrometer

      number blows versus depth of rod per inches to determire the percentage of strength of soil

      so say for instance you do 5 blows and measure in goes down 2.25 inches you do a set of 5 more blows and measure it goes down 3.5 inches you do another set of 5 blows and measure and it goes down 1.5 inches what % of strength is this determine psi strength of 150 ? would it hold a 100,000 pound piece of equipment up ? thanks

      Reply

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