HOW TO CALCULATE CBR VALUE USING DCPT RESULT ON SITE?

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

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