HOW TO CALCULATE CBR VALUE USING DCPT RESULT ON SITE?

Dynamic Cone Penetration Test or DCPT is a quick, simple and low cost test used by many agencies worldwide to measure the strength of compact materials of unbound granular and sub-grade layers with reasonable accuracy.

Nowadays DCPT is commonly used for the following purposes during construction.

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

Principle of DCPT

The basic principle involved in the operation of this instrument is to measure the resistance offered by the pavement layers to the penetration of a standard cone with a diameter of 20 mm (60 in).0 top angle) driven by a freely falling 8 kg hammer through a height of 575 mm. The amount of cone penetration (in mm) is generally reported in terms of average penetration per blow, DCPI60 (mm/blow).

pay attention: Higher value of DCPI60 Indicates a soft material and vice versa.

Follow the step by step procedure given below 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 process.


Process

  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 (ie the reading on the scale) from the current reading.
  3. Calculate the cumulative depth of penetration (in mm) for each blow by adding up all previous individual depths of penetration.
  4. Create a chart using statistics on the cumulative depth of penetration and the number of blows. (See Figure-1)
  5. This step is about identifying the depth of each layer. Identify the slope change of the entry curve. Each change in slope represents a change in the type of layer.
  6. After each slope change is identified, calculate the average penetration/shock for each slope change and 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(DCPI)60,

  1. To find the variation of the CBR value in depth, draw another quasi-log chart with CBR values ​​on the logarithmic scale as the abscissa (X axis) and the ordinate (Y axis) as the depth of penetration on the normal scale. (see fig-2).
number of blows DCP Scale Reading (cm) DCPI60 pen/blow(mm) Cumulative penetration (mm) CBR value (%) Cumulative Penetration (cm) Layer Thickness(mm) Average 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
1 1 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

pay attention:

  • In the example above the initial reading is 9.5.
  • Several equations exist to obtain the CBR value from the DCPT result. The equation we have used is commonly used. You can use any other equation if you want.
Figure-1 DCP Chart
Figure-1 DCP Chart
Figure-2 Variation with depth of CBR
Figure-2 Variation with depth of CBR

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

Also Read: Factors Affecting Dynamic Cone Entrance Test Result

Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.