What is a Dynamic Cone Penetrometer (DCP)?

Definition

The dynamic cone penetrometer (DCP) is a portable device that measures how far a cone sinks into soil or unbound pavement layers per hammer blow. The operator drives a cone tip into the ground with a sliding hammer and records the depth after each blow or set of blows. The result is the DCP index in millimetres per blow. A low index means strong material.

Equipment

A typical DCP has an 8 kg hammer that falls 22.6 in (about 575 mm) onto a 16 mm drive rod. The rod ends in a 60 degree cone with a base diameter of 20 mm. ASTM D6951 also allows a 4.6 kg hammer for soft ground. Drive rod extensions increase the depth. The typical test depth is down to 1000 mm.

From index to strength

A plot of the DCP index against depth shows layer boundaries as changes in slope. The FAA uses this to find the thickness and stiffness of the base, subbase and subgrade.

The index converts to California bearing ratio (CBR) with an empirical equation. For most soils, CBR = 292 / DCP^1.12, with the index in millimetres per blow. Soil-specific equations exist for CL and CH soils. For FAARFIELD pavement designs, the FAA says to use DCP CBR values with laboratory or in-place CBR values. They describe the stiffness of subgrade soils.

Limits

• ASTM D6951 says that the 8 kg DCP cannot be used in highly stabilised or cemented materials. It also cannot be used in granular material with a large share of aggregate over 50 mm (2 in).
• A field CBR from a DCP does not normally match the laboratory or soaked CBR of the same material.
• The test is minimally destructive and gives a result at one point.

Use on airfields

FAA AC 150/5320-6G lists the DCP with the falling weight deflectometer, the heavy weight deflectometer and ground penetrating radar as tests for the subsurface. It says to run DCP tests on each soil layer during soil borings, or after taking cores of an existing pavement. The standard is ASTM D6951/D6951M.