Cone penetration test
The test method consists of pushing an instrumented cone tip first into the ground at a controlled rate (usually 2 centimeters/second).
The resolution of the CPT in delineating stratigraphic layers is related to the size of the cone tip, with typical cone tips having a cross-sectional area of either 10 or 15 cm², corresponding to diameters of 3.6 and 4.4 cm.
In contrast to the classical mechanical CPT, the more advanced electrical CPT cones or Piezocones (CPTU) are equipped with electrical sensors and measuring elements, thus providing true in-situ measurements of the desired parameters. As such the superior accuracy and resolution of all obtained signals are the standard for today’s CPT.
In addition to the improved accuracy, electrical cones allow for additional measurements by means of extra sensors, such as pore pressure (U1 position), inclination, temperature etc.
The U.S. Geological Survey Cone Penetration Testing (CPT) truck is a fast and inexpensive way to conduct shallow subsurface exploration. Detailed data are available immediately, permitting on-the-fly mapping of stratigraphy and other subsurface features. CPT is a useful tool in geologic-hazard, hydrologic and environmental studies. This rapid and cost-effective approach is particularly advantageous in urban environments because no drill spoils are produced. The CPT truck is available on a reimbursable basis to all USGS staff and cooperators.
When you use this version you need to have a wire from probe up to logger unit on ground surface. Advantages are that data is continuously sent to surface and that you need no batteries. It may also be an advantage if you want to piggy back units from different vendors.
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.
Dutch Sounding Methods employed by this equipment provides essential information for foundation engineers, reduces cost of expensive boring and sampling and ensures precise control for sounding speed (Engine Driven Model)
Lunne et al (1997) suggested an approximate method to estimate soil permeability (hydraulic conductivity) from the CPT via the Soil Behavior Type (SBT) chart using non-normalized cone resistance and friction ratio. This was based on the basic observation that sands typically have high permeability and clays low permeability and that the CPT can be used to estimate soil type and hence, soil permeability.
The CPT technology is not that old. The first CPT apparatus dates from 1932, the so-called Barentsen-apparatus. This device was named after civil servant Mr. Pieter Barentsen, who was the first to perform a CPT test for a road in the vicinity of Gouda. He did this by pushing a 10 cm2 cone manually into the ground using his own body-weight.
Advantages of CPT testing to test borings include continuous data for soil profiles, improved measurements of critical geotechnical design parameters, and ability to obtain site specific shear wave velocity data for improved seismic site design and liquefaction potential. The Vertek VTK cone system includes a real-time data logging system with 5 ton and 10 ton seismic piezocones (SCPTu). SCPTu cones are equipped with tri-axial seismic modules to provide downhole S and P shear wave velocity testing.
The cone is first pushed into the soil to the desired depth (initial position) and then a force is applied to the inner rods that moves the cone downward into the extended position. The force required to move the cone into the extended position divided by the horizontally projected area of the cone is defined as the cone resistance (qc). By continual repetition of the two-step process, the cone resistance data is obtained at increments of depth. A continuous record of the cone resistance versus depth can be obtained by using the electric cone, where the cone is pushed into the soil at a rate of 10 to 20 mm/sec.
Truly robust crawler-based CPT penetrometer pushing rig with rubber tracks, Hatz 3-cylinder diesel engine incorporated in a sound insulating enclosure, load-sensing hydraulic pump, a 200 kN CPT penetrometer pusher, unique anchoring system on reaction arm. Practically all functionalities are controlled by means of a wireless remote control.
As part of In Situ's continued research into Cone Penetration Testing we have teamed up with the Civil Engineering department at the University of Ireland, Galway and Keller. The work is a pile design project and our part of the project was to help characterise the proposed test site in Keller's yard near Coventry. The work involved carrying out a number of CPT as well as seismic tests.
In the Cone Penetration Test (CPT), a cone on the end of a series of rods is pushed into the ground at a constant rate of 2 cm /sec. and continuous or intermittent measurements are made of the resistance to penetration of the cone. Measurements are also made of either the combined resistance to penetration of the cone and outer surface of a sleeve or the resistance of a surface sleeve.