DCP - Max Depth of Penetration 3

[ONENGINEER]

Online searches show DCP is usually driven to 1 m (3-4 ft. DCP results up to 8 ft was seen in an engineering report

In practice to what depth can reliable results be obtained with DCP, assuming sufficient manpower available - working mainly on clay soils). Thanks for your advice in advance.

 

[BigH]

Please define your DCP details. Hammer weight, drop, cone size, etc.

 

[EireChch]

You can do a DCP down to 2m depth, then hand auger the 2m and DCP another 2m. You could repeat forever if you have enough rods but at some point there is loss of energy and other issues with rods, bh collapse etc which will affect results. I personally would not go more than 6-7m.

 

[Mad Mike]

We drive our DPL probes (10kg hammer, dropping 450mm to drive a 25mm diameter, 60deg. cone) down to 8m routinely. Not sure how the energy losses compare to your DCP, which I'm assuming is the lighter hammer, smaller cone diameter, what we in South Africa call the CBR-DCP.

Consider why you would want to use the DCP probe to depth, then try it out on site and see how the numbers compare with your observations of stiffness in a trial pit. Sometimes the energy losses don't mess up the data as badly as you might expect. If you want accurate data use EireChch method, but it will soon become tedious enough to justify a heavier probe.

The standard 1m testing depth is aligned with its purpose of testing road pavement layers; but I don't see any problems in going deeper- these data shouldn't be used in isolation for sensitive design work, so the information is still useful provided it's supported by direct observation in trial excavations.

My standard practice is- light DCP for road layerworks; heavier DPL for probing the deeper subsoils. If I had to thumb-suck a maximum depth for the DCP, it would be about 8ft!!!

All the best.
Mike

 

[Ron]

EireChch is fundamentally correct, but the energy loss with a small dropped weight of a typical hand-held DCP will give you erroneous results with more depth....which is likely why BigH asked for the additional info...

I've done many, many DCP tests, but never more than the depth of a typical hand auger...about 7 to 8 feet.

 

[BigH]

Further to Ron's comment, the reason, as I have mentioned before in other threads, is that DCP means differnt things to different people. There is the TRL DCP, there is the Mackintosh Probe DCP, there is, what we call in Canada, the Pentest which is a 60 degree cone put on the end of an A rod and driven using a 140 lb hammer dropping 30 inches (same energy as the SPT). And, there are other DCPs. Which is why it is important to fully describe what you mean. The Pentest can be driven quite far obviously.

https://theconstructor.org/geotechnical/mackintosh-probe-test/3801/

(4 kg hammer)

https://www.geo-integrity.co.uk/2018/04/06/in-situ-testing-using-the-trl-dcp/

(8 kg hammer)

http://www.novahgk.com/wp-content/uploads/2017/07/Canadian-Foundation-Engineering-Manual.pdf

(see page 44 2nd paragraph, for Canada's DCP - Pentest)

 

[geotechguy1]

You can hand auger the meter you DCP'd and then add another 1m rod to it. I suppose at some point you would want to start adding corrections for the free rod length like you would in an SPT but in practice I think unless you've got some very skilled hand auger operators you'll probably stop being able to hand auger before you really need a correction

 

[EDB9]

You can go further than 3-4’ easily. Typically I’ll call out to go down to 10’ or until AR. Anything deeper than that becomes less accurate in my experience.