DCPT or Dynamic Cone Penetration Test

The dynamic cone penetrometer is a portable equipment that can be used to evaluate different layer of a pavement such as unbound granular base, sub-base, sub-grade and also pipeline congested0 narrow trenches where testing with other equipment is difficult and not feasible due to cost, space and time constraints. Due to this, DCPT has gained in popularity in many parts of the world.

The basic principle involved in the operation of this apparatus is the measuring of the resistance offered by the pavement layers to the penetration of a standard cone with a diameter of 20 mm (with 60⁰ apex angle) driven by an 8 kg hammer freely falling through a height of 575 mm (IRC – SP:72 – 2007).

The amount of penetration (in mm) of the cone is generally reported in terms of the average penetration per blow as DCIP. This indicates relative shear strength of the material across the depth tested. During penetration of the cone, the shear strength of the material is mainly accrued due to resistance offered by the shear displacement of the soil particles.

Factors Affecting DCPT Result

1. Alignment of DCP rods

While testing, the bottom and top rods of the DCP should be straight and the cone should be seated freely in position on the test material. If the penetrating rod is tilted during resting, resistance around the rod (i.e. skin friction) will increase due to contact with confining pavement layers which causes a decrease in rate of penetration. Such situation may also occur when the DCP rod penetrates through collapsible granular material.

2. Depth of Testing

DCP test results are very sensitive to the depth of testing. When the bottom rod of the DCP used is longer than the standard penetrating rod, correction to the DCPI value should be applied because vertical confinement and skin friction around the rod increases resistance to the penetrating rod. The effect of the vertical confinement due to the asphaltic layer has been found to be significant when the granular layer is tested with DCP.

3. Damaged Cone Tip

If the cone tip of the DCP is damaged it will give erroneous test results.

4. Apex Angle of the Cone

The penetration rate will be significantly affected by change of cone apex angle from 30⁰ to 60⁰ since the upward frictional force on a cone surface with a 60⁰ apex angle will be greater. It has been found from experiments that the penetration rates obtained by the DCP with cone apex angle 30⁰ are 10% greater than those obtained with the cone having an apex angle of 60⁰.

5. Hammer Weight

The hammer weight should be exactly 8 kg. If the weight of hammer is less than that specified, then rate of penetration will decrease and vice-versa.

6. Lifting Height of Hammer

During DCP testing, for each blow, the hammer weight should be lifted to the top restraint plate and freely dropped. During testing, if the hammer is not lifted to the standard given height, the impulse force exerted by the cone will be reduced and the values of penetration, in turn, decrease.

7. Moisture Content

DCP test results are very sensitive to variations in moisture content present in the test materials. As the moisture content increases, the penetration rate of DCP also increases and vice versa. Due to this reason, DCP tests are conducted during the post-monsoon seasons when the granular and sub-grade layers become soft and their minimum strength are recorded.

8. Material Composition

DCPI varies with test material composition, soil class, coefficient of curvature, uniformity, density of the layer material and plasticity of the soil.

9. Intensity of Compaction

DCPI will be influenced by the intensity of compaction and confinement of granular and sub0grade layers.