Site Investigation

Cone penetration testing

Cone penetration testing (CPT) involves driving a steel cone vertically into the ground using either a four or six wheeled drive truck or a semi-tracked or crawler mounted rig. The cone is pushed into the ground at a constant rate of penetration of 20mm per second.

During the penetration, measurements are recorded of the cone resistance, the side friction against the shaft and in the case of the piezocone test, the pore water pressure generated by the penetration of the cone into the ground.

The measurements are recorded using an electric current and the frequencies of the readings are presented as a graphical interpretation of the variation of the measured parameters with increasing depth. In some soil types depths of up to 100m can be reached. Typically a number of shallower probes are specified and between 100 and 150m of probing can be carried out during a normal working shift, depending upon access limitations and favourable ground conditions.

The purpose of the CPT is to determine the sub-surface stratigraphy and to enable an interpretation of the materials encountered. The results are used to obtain parameters for geotechnical design purposes. The advantage that CPT testing has over conventional soil sampling is high production and minimal soil disturbance. However, the CPT does not provide a physical sample of the soil encountered and the testing cannot be carried out through rock layers.