Dynamic cone penetration test

Introduction

The standard penetration test and dynamic cone penetration test are almost similar except for the fact that no bore hole is advanced in this test. As per the name of the test: Dynamic cone penetration, a Cone is fixed at the end of a drill rod which penetrates the ground as it is driven into the ground surface. Two types of DCPT are common in practices which are: dry DCPT and Wet DCPT. Previously, we had posted Static cone penetration test or cone penetration test.

Dry Dynamic Cone Penetration Test

In dry DCP test, a cone of 50 mm diameter having apex angle of 60⁰ is fixed at the end of a drill rod. The number of blows required for every successive 300 mm penetration known as N_C value, are counted. The test is started from the surface to the required depth. After completion of the test the drill rods are withdrawn leaving the cone behind. Because of high skin friction between drill rods and the soil, much load is required to overcome the friction; the entire process becomes troublesome with addition of extra loads.  Hence the results are not much reliable.

Wet Dynamic Cone Penetration Test

For wet DCP test, a 62.5 mm cone is fixed at the end of a drill rod. This test corrects the limitations of dry DC test. To eliminate the skin friction between drill rods and the soil, special type of mud called drilling mud is used. During the DCP test, the drilling mud is forced under pressure through a hollow drill rod and the cone. The mud comes out from the cone along the drill rod and thus frictional resistance between the drill rods and the surrounding soil is eliminated. However, this test does not guarantee of the reliable results beyond a depth of 6 m.