Cone Penetrometer Test (CPT) Borings
The CPT test method uses strain-gauge load cells to measure the tip resistance on a conical steel tip and also the side shear developed behind the tip. The bearing pressure on the tip is a function of the soil’s shear strength components, density and confining stress. The side shear measured on the friction sleeve helps identify different soil types through a comparison with the tip pressure known as the friction ratio. The friction ratio increases in cohesive soils such as silt and clay and decreases in cohesion with soils such as sand, shell and gravel. Also measured during the testing is pore pressure below the water table. CPT data can also be converted to equivalent Standard Penetration Test (SPT) ‘N’ values.
CPT testing is completed by hydraulically advancing the rods using either a heavy ‘counterweight’ drill platform or a lighter platform anchored to the ground to provide the required resistance force. Data through the tip is transmitted through a wire or wireless microphone system to a computer nearly continuously throughout the depth of the boring. In general, the CPT system is limited in its depth of penetration through very dense sands and limestone or highly consolidated clays due to the required ‘push force’. In addition, the tip can be sheared and damaged or lost in these types of soils.
