Coring and Dynamic Cone Penetrometer (DCP)
Introduction
The DCP test is carried out normally on top of the unbound roadbase layer immediately after coring to determine the thickness and structural properties of the unbound layers. It can also be performed on top of soil or subgrade to determine the CBR (California Bearing Ratio) value
Methodology
The TRRL DCP set consist of 16mm diameter hardened steel rod with 8 kg hammer dropping through a height of 575mm. The rod is fitted with 60 degrees one with 20mm larger diameter. With the road held vertically, the hammer is lifted up to the top of the rod and released to hit the anvil. The amount the cone penetrates into the unbound base then recorded using a ruler attached to the road.
As a rule, for hard surfaces, reading are taken after ten blows. As the cone penetrates deeper through the softer road sub-base and subgrade the number of blows are decreased to 5,3 and finally 1, accordingly.
Output
The penetration (in mm) of the cone is plotted against the number of blows and the slope of the resultant curve in mm/blow is measure of the penetration resistance and hence, strength of the layer. The turning point of the slope indicates a change in strength and therefore a change in material. This point can be approximately located by selecting the best straight line of the two slopes. The point of intersection indicates the depth at which there is a change in material strength and properties. The pavement layer thickness can be estimated by measuring the depth of the turning point.
If there is no measurable penetration after 30 consecutive blows it is assumed that the DCP will not penetrate the material. Under this circumstance a hole will be drilled through the layer using an electric drill before the lower layer(s) can be tasted in the normal way. Appropriate remarks will be recorded so that the result will be analyzed accordingly. The CBR of the subgrade is obtained from the relationship:
CBR = 269/DCP
where DCP is the rate of penetration (mm/blow). A minimum of three people to operate the DCP.