The cone penetration test (CPT) is a method used to determine the geotechnical engineering properties of soils and delineating soil stratigraphy. It was initially developed in the 1950s at the Dutch Laboratory for Soil Mechanics in Delft to investigate soft soils. As a result it has also been called the "Dutch cone test". Today, the CPT is one of the most used and accepted in soil methods for soil investigation worldwide.
The test method consists of pushing an instrumented cone, with the tip facing down, into the ground at a controlled rate (controlled between 1.5 -2.5 cm/s accepted). 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.
The early uses of CPT predominantly determined just the soil geotechnical property of bearing capacity. The original cone penetrometers were strictly mechanical devices that took measurements of the total cone penetration resistance to pushing a tool with a conical tip into the soil. New methods were later employed to separate the total measured resistance into components generated by the conical tip (the "tip friction") and friction generated by the rod string. A friction sleeve was added to quantify this component of the friction and aid in determining soil cohesive strength in the 1960s. Electronic measurements began in 1948 and improved further in the early 1970s. Most modern electronic Cone Penetration Testing cones now also employ a pressure transducer with a filter to gather pore water pressure data. The filter is usually located either on the cone tip ( U1 configuration), immediately behind the cone tip (U2 configuration) or behind the friction sleeve (U3 configuration). Pore water pressure data aids determining stratigraphy and is primarily used to correct tip friction values for those effects. CPT testing which also gathers this piezometer data is called CPTu testing. CPT and CPTu testing equipment generally advances the cone using hydraulic rams mounted on either a heavily ballasted vehicle or using screwed-in anchors as a counter-force. One advantage of CPT over the Standard Penetration Test (SPT) is a more continuous profile of soil parameters, with CPTU data recorded typically at 2cm intervals.
In addition to the mechanical and electronic cones, a variety of other Cone Penetration Test deployed tools have been developed over the years to provide additional subsurface information. One common tool advanced during CPT testing is a geophone set to gather seismic shear wave and compression wave velocities. This data helps determine the shear modulus and Poisson's ratio at intervals through the soil column for soil liquefaction analysis and low-strain soil strength analysis. Engineers use the shear wave velocity and shear modulus to determine the soil's behavior under low-strain and vibratory loads. Additional tools such as laser-induced fluorescence, X-ray fluorescence, soil conductivity/resistivity, pH, temperature and membrane interface probe and cameras for capturing video imagery are also increasingly advanced in conjunction with the CPT probe.
An additional Cone Penetration Test tool deployed tool used in the United States, Britain, Netherlands, Germany, Belgium and France is a piezocone combined with a tri-axial magnetometer. This is used to attempt to ensure that tests, boreholes, and piles, do not encounter unexploded ordnance (UXO). The magnetometer in the cone detects ferrous materials of 50 kg or larger within a radius of up to about 2 m distance from the probe depending on the material, orientation and soil conditions.
Standardization of CPT for geotechnical applications occurred in 1986 by ASTM Standard D 3441 (ASTM, 2004) while ISSMGE provides international standards for CPT & CPTu. Later ASTM Standards have addressed the use of CPT for various environmental site characterization and groundwater monitoring activities.For geotechnical soil investigations, CPT is more popular compared to SPT as a method of geotechnical soil investigation as its increased accuracy, speed of deployment, and reduced cost provide many advantages. The ability to advance additional in situ testing tools using the CPT direct push drilling rig, including the seismic tools described above, are accelerating this process.
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