DCP testing

What actually happens during a DCP test?

A standard cone is driven into the ground by repeated blows from a falling hammer, and the operator records how far it advances with each blow. An 8 kg hammer dropping 575 mm onto a 60 degree cone is the usual arrangement. The result is a penetration rate in millimetres per blow, where a smaller figure means a stiffer material. The equipment is handheld, so no plant or vehicle is needed at the position.

Is there a British Standard for DCP testing?

The DCP is not covered by a British Standard for soils, so testing usually follows a recognised method such as ASTM D6951 together with the equipment's own procedure. Where a CBR is inferred from the penetration rate, the correlation most often used in this country comes from published TRL research. Your specification should say which correlation is acceptable, and where it does not, that is worth agreeing with the designer before testing.

How deep can the test reach before it refuses?

Around a metre is typical with standard rods, and extensions can take it deeper, although adding rods changes the mass being driven and weakens any correlation. Dense granular layers, cobbles or a stiff crust can stop the cone well short of that, and refusal on a single stone is common in coarse fill. Where you need a reliable profile below about a metre, an intrusive investigation is the better tool.

How reliable is a CBR figure inferred from a DCP?

Treat it as an estimate rather than a measurement, because the CBR is inferred from the penetration rate through a correlation. The published relationships work best in homogeneous fine grained soils and become unreliable in mixed material, made ground or anything with significant stone content. Moisture condition matters too, since the same soil penetrates more easily when wet. Where a defensible CBR percentage is needed, a direct test is the stronger option.

Can a DCP show where one layer ends and the next begins?

Yes, and this is often the most useful thing it does. A clear change in the penetration rate marks a boundary between materials of different resistance, such as soft clay over dense gravel. The report plots cumulative blows against depth at each position, with the penetration rate for each interval, so layer changes and any refusal read straight off the profile. It shows where things change rather than what the material is, so pair it with an inspection pit.

When is DCP the right choice rather than an in-situ CBR or plate bearing test?

Use a DCP when access rules out the alternatives, for example inside a building, in a narrow trench, behind a structure or anywhere a vehicle and reaction machine cannot reach. It is light, quick and needs no plant, but it gives an approximate assessment rather than a direct measurement, so where an in-situ CBR or plate bearing test is possible those usually carry more weight. Describe the access and we will tell you what is feasible.

Does the moisture condition of the ground affect the result?

Yes. Wetter soil offers less resistance, so the same material shows a higher penetration rate after a period of rain and any inferred CBR comes out lower. That makes the timing of the test relevant to how the result should be read, particularly in fine grained soils. The state of the ground and the recent weather are recorded with the test, because the result reflects conditions on the day rather than a design condition.

Do we need a utilities search before you drive a cone into the ground?

Yes. The DCP drives a steel rod up to a metre into the ground, so a services search and a scan of each position are needed before testing, and the same applies to any buried structure, membrane or drainage you know about. Send us the services drawings with the proposed positions and we will agree which of them can safely be tested. Positions can usually be moved a short distance to keep clear.

Does the position need any preparation, and what is left behind afterwards?

The position needs a flat, firm surface at the level you want profiled, with loose material cleared so the cone starts from a known point. Where there is a bound surfacing, a small hole has to be cored or broken through first and reinstated afterwards. The test itself leaves a hole around 20 mm across, usually backfilled with the arisings, so reinstatement is minimal compared with a trial pit.

How long does a DCP profile take at each position?

A straightforward position takes around 15 to 20 minutes including setting up and recording, so a good number can be covered in a day where access is easy. Deeper profiles, hard layers and moving between positions inside a building or across a live site all slow it down. Because the equipment is carried by hand, the walking distance between positions matters more than it does for vehicle based methods.

Is DCP testing within your UKAS accredited scope?

Hixtra holds UKAS accreditation to ISO/IEC 17025 for the tests listed in its schedule of accreditation, number 8996. Accreditation is specific to the listed methods and scope rather than to every service we provide. Because DCP results are usually interpreted through a correlation, it is worth agreeing in advance exactly what the report will state and asking us to confirm the accreditation status of the testing you need.

What do you need to price up DCP testing?

Send the site postcode, the number and location of the positions, the depth you need to reach, your preferred dates and the specification if there is one. Say whether the positions are through a bound surface, because coring and reinstatement add to the work. Travel, the number of positions and any restricted access, permit or out of hours requirement are the main things that move the price.