Everything about The Cone Penetration Test totally explained
The
cone penetration test (CPT) is an
in situ testing 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. Based on this history it has also been called the "Dutch cone test". Today, the CPT is one of the most used and accepted insitu test methods for
soil investigation worldwide.
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.
History and development
The early applications of CPT mainly determined the soil geotechnical property of
bearing capacity. The original cone penetrometers involved simple mechanical measurements of the total penetration resistance to pushing a tool with a conical tip into the soil. Different methods were 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 (Begemann, 1965). Electronic measurements began in 1948 and improved further in the early 1970s (de Reister, 1971). Most modern electronic CPT 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 (the so-called U1 position), immediately behind the cone tip (the most common U2 position) or behind the friction sleeve (U3 position). 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.
Additional insitu testing parameters
In addition to the mechanical and electronic cones, a variety of other CPT-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 of 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, soil
conductivity/
resistivity and cameras for capturing video imagery are also increasingly advanced in conjunction with the CPT probe.
Standards and use
CPT for geotechnical applications was standardized in 1986 by
ASTM Standard D 3441 (ASTM, 2004). Later ASTM Standards have addressed the use of CPT for various
environmental site characterization and
groundwater monitoring activities. Particularly for geotechnical soil investigations, CPT is gaining popularity compared to
standard penetration testing as a method of geotechnical soil investigation by its increased accuracy, speed of deployment and reduced cost over other soil testing methods. The ability to advance additional insitu testing tools using the CPT direct push
drilling rig, including the seismic tools described below, is accelerating this process.
Further Information
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