What is a Cone Test?

A cone test, usually called a cone penetration test (CPT), is a type of soil test used to collect various types of data. The CPT collects data at regular intervals as it penetrates downward through the soil. Data collected from a cone test can be used to design piled foundations and to evaluate the seismic classification of the soil.

The cone test is so named because the equipment used to bore into the soil consists of a cylindrical metal rod with a conical, pointed tip. Other parts include a moving friction sleeve, a filter element for measuring porous pressure, and sensors to measure the various forces acting on the rod. The entire assemblage that goes into the soil is known as a cone penetrometer or a piezocone penetrometer. Penetrometers may be mechanical or electrical, and various diameters of cone may be used — common sizes include 0.55 inches (about 1.4 cm), 1.42 inches (about 3.6 cm) and 1.73 inches (about 4.4 cm).

A CPT rig is necessary to perform measurements using the cone penetrometer. The rig is a truck specially outfitted to perform cone tests. A hydraulic ram is mounted in the center of the flat bed of the truck to push the penetrometer down through the soil. The truck also contains instrumentation that records the force at the tip of the cone, the friction force on the friction sleeve, the pore pressure, and in some cases, shear wave velocity measurements. A computer in the rig records all the data collected during a test.

To perform a cone test, the hydraulic ram pushes the penetrometer through the soil at a constant rate, usually 0.79 inches per second (about 2 cm per second). The desired measurements are taken at equal increments of between 0.20 to 1.97 inches (about 0.5 and 5 cm). Sections of rod about 3.28 feet (1 meter) long are added as the penetrometer proceeds deeper into the soil. Once the penetrometer is removed from the soil, the hole is filled as dictated by local regulations.

Additional equipment is needed to perform a seismic evaluation with a cone test. Seismic tests require a special type of penetrometer called a seismic cone penetrometer (SCPT), which measures shear wave velocity. The setup for a seismic test also includes a plate that contains a triggering device. Test personnel strike the plate with sledge hammers to generate seismic waves. The computer used to collect data can function as an oscilloscope, a device that collects shear wave data.