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

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)