Site Investigation

Dynamic probing

Dynamic probing involves driving a steel cone vertically into the ground using a sliding hammer and recording the number of blows for each 100mm of penetration. The results obtained from the dynamic probe tests can be correlated to the standard penetration test (SPT).

The cone diameter and angle will depend on the chosen method. For example, the super heavy dynamic probe (DPSH) has a cone diameter of approximately 50mm and a cone angle of 90 degrees.

The cone is attached to a 1.0m section steel rod with a diameter of approximately 35mm and graduation markings at 100mm intervals. Depending on the chosen method, the drop height can be adjusted. The super heavy method is typically the preferred choice, with a drop weight of 63.5kg and a drop height of approximately 750mm.

The cone can be fixed to the rods and recovered at the end of the test; or can be disposable and detaches from the rods at the end of the test when the rods are retracted. It is usual to conduct a torque reading of the rods after each metre of penetration using a torque wrench to estimate the skin friction effects on the rods. These torque readings are normally only conducted when using the disposable cone. A further rod is then added, and the process is continued until the probe refuses to penetrate the ground or the test reaches the target depth.

The depth of penetration achievable is dependent on the density of the material encountered. For a typical project with dynamic probes driven to depths between 5.0m and 10.0m at least 50m of probing would typically be achieved during a single working shift.

The test was designed to provide a rapid and economic method of assessing the penetration resistance of the ground and can be roughly equated to equivalent SPT ā€˜Nā€™ values. The test provides a continuous profile of ground resistance with depth, rather than at the discrete depths of a conventional SPT test. A log of the dynamic probe test results is provided in tabular and graphical form, providing a visual interpretation of the different soil strata encountered.

Dynamic probing is able to locate boundaries between strata of differing density and driving resistance. The dynamic probe results can be used to establish the sub-surface presence of obstructions like old foundations or buried objects. The results are also used to identify soft areas and voids or cavities within the soil, such as dissolution features in chalk or mine workings.