Ever wondered how to use Dynamic Cone Penetrometer Test to generate a solution to your problem in a project?
The Dynamic Cone Penetrometer Test is one of those test methods in the construction sector which can help calm down that triggering work pressure in any engineering project. However, it is only useful if done correctly.
But what is the Dynamic Cone Penetrometer Test, though? Why do we need it? And how do we go about doing it properly?
Let’s check it out!
What is Dynamic Cone Penetrometer Test?
The Dynamic Cone Penetrometer (DCP) Test is a geotechnical test conducted to understand the underlying soil strength of a soil by measuring the penetration of the device into the soil after each hammer blow.
The need to understand soil strength is very essential to have a tailored justification for every action taken for the project design. For design guide on Crop Water Requirement , check here.
Because you and I, are both aware that geotechnical engineering is like “Abraham” in the Holy Bible, the first man chosen by God for a role in the plan of redemption, as in its the genesis of a successful infrastructural work. So, let’s all agree it can be kind of a red flag if performed poorly.
How do we go about it properly?
Being it a road or any other foundation structure, before you begin with your design, you must first determine the strength of the existing ground, referred to as subgrade for a road pavement or just foundation for other structures.
It however adopts two approaches for its measurement;
- Blows/100 mm depth. This profiling approach is usually performed for deep foundation structures like bridge footings, dam headworks foundation investigation, and others.
When readings are observed after every 300mm depth, it is then referred to as Standard Penetration Test (SPT).
- Penetration/blow is the strength approach.
With a recorded number of blows in 300 mm depth, a number N- referred to as the Standard penetration number is used to compute the shear strength of the investigated material. Which can then guide us in our design.
Here penetration achieved in a single blow is noted. The test values from this approach can be correlated to CBR to assess the strength of a road subgrade, i.e. existing ground.
What is CBR?
California Bearing Ratio (CBR) is a test conducted on a geotechnical material to be used as a road layer.
It is the laboratory determination of the strength of a compacted sample dynamically loaded by metal rammers.
This test is needed because all strength of flexible road pavements is expressed in terms of CBR Values. However, the results can only be available after a period of 4 days.
A coin has two sides, and so does the DCP Test. So what are the good and bad sides of the DCP Test?.
| Pros | Cons |
|---|---|
| Causes the least amount of damage to the pavement. | It’s difficult to know whether the moisture and density conditions at the time of the test are typical of those predicted under the finished pavement. |
| A typical test takes only a few minutes; therefore, the instrument provides a quick and efficient way to acquire data. | The test can’t be performed on loose or cohesive soils |
How should the DCP test be conducted?
With the below required set of equipment , the test is conducted as follows,
Equipment
The whole set of DCP equipment includes Cone fastened at the bottom of a vertical rod, Steel ruler, hammer (8 kg mass), Rods, Base, and Jointer.
Operation Procedure.
The various pieces of the DCP machine are assembled to create the right configuration in order to conduct the test.
The DCP is then vertically set up on somewhat flat ground to be tested.
Before the hammer is lifted and released, the initial reading on the vertical scale rule is recorded.
To impart a standard impact to the cone and drive it into the earth, a hammer is repeatedly lifted and lowered upon a coupling at mid-height of the rod.
The depth of penetration of the cone is measured using a vertical scale alongside the rod. As the cone is driven into the soil, the penetration rate is recorded and then utilized to calculate the strength of the soil through which it is going.
This is repeated until the desired depth has been obtained.
Continuous measurements are usually taken down to a depth of around 850 mm or 2m when extension shafts are employed throughout the testing process. The boundaries of the layers are recognized and the thickness of the layers determined when a change in strength is detected during the test
A change in penetration rate implies a change in strength between the soil layers.
For a road pavement, the correlation to obtain the CBR Values can be done using the below formula.
LOG(CBR)=2.48-1.057LOG (mm/blow)
Where mm is the penetration depth for every blow.
Using the aforesaid formula, CBR values for the pavement layer at various depths can be generated.
Hint:
It’s always a good idea to favor the side of the road with the most rutting areas, which is usually in the outer wheel path, while choosing trial pits to test.
Conclusion
Based on my experience on road pavement design, I feel like the DCP test still gives a reliable result on one condition that it’s executed properly.
But when there is time, the Laboratory CBR Test should be prioritized because it caters to changes in moisture content which is something not considered in DCP Test, yet it’s the practical scenario on the ground.
For other infrastructural works like dams and bridges, I think DCP is still the top dog of the game.
There’s a huge difference between simply carrying out the test , and analysis & interpretation of the results.
To make this transition successful, you’ll need to adopt a more proactive and focused approach because some things can only be done when you are aggressive.
Have you tried Dynamic Cone Penetrometer Test yet? Let me know about your experience and views.
