Cone Penetration TestinCone Penetration Testing

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Cone Penetration Testing (CPT)

Photo by Scott Haefner, USGS. Figure by Tom Holzer, USGS. Cone Penetration Testing (CPT) is used to identify subsurface conditions in the upper 100 ft of the subsurface. The USGS CPT uses a 23-ton truck to push a “cone” into the ground. The weight of the truck is partially supported by both the tip of the cone and the sleeve of the cone. The “tip resistance” is determined by the force required to push the tip of the cone and the “sleeve friction” is determined by the force required to push the sleeve through the soil. The “friction ratio” is the ratio between sleeve friction and tip resistance, measured as a percentage. Soil type and thereby resistance to liquefaction can be inferred from these measurements. Data from the USGS cone, which includes a seismometer, can also be used to predict how local shallow soil conditions can modify shaking. The capacity of local soil conditions to modify shaking is inversely proportional to the shear-wave velocity near the surface, which can be computed with data recorded with the seismometer. Seismic energy is created manually with a sledgehammer or automatically by a compressed air driven hammer. We measure the time it takes for the seismic energy to travel from the land surface, through the ground, to the

seismometer mounted in the cone. The distance to the seismometer divided by the travel time of the shear-wave is approximately the average shear-wave velocity. Output from the cone as it penetrates the soil is digitally recorded by a computer and is collectively known as a sounding. 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. Based on this history 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.

Contents
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1 History and development 2 Additional in situ testing parameters 3 Standards and use 4 References

History and development

The result of a cone penetration test: resistance and friction on the left, friction ratio (%) on the right.

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.[1] Electronic measurements began in 1948 and improved further in the early 1970s.[2] 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. 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.

Additional in situ 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 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[3], 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 CPT deployed tool used in 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 (UXB) or duds. 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. There are two manufacturers of cone penetrometer equipment in the United States. They are Hogentogler and the Vertek[4] Division of Applied Research Associates.

Standards and use
CPT for geotechnical applications was standardized in 1986 by ASTM Standard D 3441 (ASTM, 2004). ISSMGE provides international standards on CPT and 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.Its increased accuracy, speed of deployment, more continuous soil profile and reduced cost over other soil testing methods. 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.

References
1. ^ Begemann, H. K. S, 1965, "The Friction Jacket Cone as an Aid in Determining the Soil Profile"; Proceedings, 6th ICSMFE, Montreal, Quebec, Canada, Vol I, pp.17-20. 2. ^ De Reister, J., 1971, "Electric Penetrometer for Site Investigations"; Journal of SMFE Division, ASCE, Vol. 97, SM-2, pp. 457-472. 3. ^ X-ray fluorescence

4. ^ vertekcpt.com
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"Cone Penetration Testing in Geotechnical Practice"; T. Lunne, P.K. Robertson and J.J.M. Powell. Blackie Academic & Professional. London. Meigh, A.C., 1987 "Cone Penetration Testing - Methods and Interpretation", CIRIA, Butterworths. ASTM, 2004, "Standard Method of Deep Quasi-Static Cone and Friction-Cone Penetration Tests of Soil"; ASTM Standard D 3441, ASTM International, West Conshohocken, PA, 7 pp. ASTM D-5778 "Standard Test Method for Performing Electronic Friction Cone and Piezocone Penetration Testing of Soils". International Reference Test Procedure for CPT and CPTU - International Society of Soil Mechanics and Geotechnical Engineering (ISSMGE) (http://www.issmge.org) Mayne, Paul; Auxt, Jay A.; Mitchell, James K.; Yilmaz, Recep (October 4-5, 1995). "U.S. National Report on CPT". Proceedings, International Symposium on Cone Penetration Testing, Vol. 1 (CPT '95). Linköping, Sweden:

In-situ tests












A Standard Penetration Test (SPT) is an in-situ dynamic penetration test designed to provide information on the properties of soil, while also collecting a disturbed soil sample for grain-size analysis and soil classification. Dynamic Cone Penetrometer (DCP) is a insitu test in which a weight is manually lifted and dropped on a cone which penetrates the ground. the number of mm per hit are recorded and this is used to estimate certain soil properties. This is a simple test method and usually needs backing up with lab data to get a good correlation. A Cone Penetration Test (CPT) is performed using an instrumented probe with a conical tip, pushed into the soil hydraulically at a constant rate. A basic CPT instrument reports tip resistance and shear resistance along the cylindrical barrel. CPT data has been correlated to soil properties. Sometimes instruments other than the basic CPT probe are used, including: CPTu - Piezocone Penetrometer. This probe is advanced using the same equipment as a regular CPT probe, but the probe has an additional instrument which measures the groundwater pressure as the probe is advanced. SCPTu - Seismic Piezocone Penetrometer. This probe is advanced using the same equipment as a CPT or CPTu probe, but the probe is also equipped with either geophones or accelerometers to detect shear waves and/or pressure waves produced by a source at the surface. Full Flow Penetrometers - T-bar, Ball, and Plate: These probes are used in extremely soft clay soils (such as sea-floor deposits) and are advanced in the same manner as the CPT. As their names imply, the T-bar is a cylindrical bar attached at right angles to the drill string forming what look likes a T, the ball is a large sphere, and the plate is flat circular plate. In soft clays, soil flows around the probe similar to a viscous fluid. The pressure due to overburden stress and pore water pressure is equal on all sides of the probes (unlike with CPT's), so no correction is necessary, reducing a source of error and increasing accuracy. Especially desired in soft soils due to the very low loads on the measuring sensors. Full flow probes can also be cycled up and down to measure remolded soil resistance. Ultimately the geotechnical professional can use the measured penetration resistance to estimate undrained and remolded shear strengths.

Flat Plate Dilatometer Test (DMT) is a flat plate probe often advanced using CPT rigs, but can also be advanced from conventional drill rigs. A diaphragm on the plate applies a lateral force to the soil materials and measures the strain induced for various levels of applied stress at the desired depth interval. In-situ gas tests can be carried out in the boreholes on completion and in probe holes made in the sides of the trial pits as part of the site investigation. Testing is normally with a portable meter, which measures the methane content as its percentage volume in air. The corresponding oxygen and carbon dioxide concentrations are also measured. A more accurate method used to monitor over the longer term, consists of gas monitoring standpipes should be installed in boreholes. These typically comprise slotted uPVC pipework surrounded by single sized gravel. The top 0.5m to 1.0m of pipework is usually not slotted and is surrounded by bentonite pellets to seal the borehole. Valves are fitted and the installations protected by lockable stopcock covers normally fitted flush with the ground. Monitoring is again with a portable meter and is usually done on a fortnightly or monthly basis.

Laboratory tests
A wide variety of laboratory tests can be performed on soils to measure a wide variety of soil properties. Some soil properties are intrinsic to the composition of the soil matrix and are not affected by sample disturbance, while other properties depend on the structure of the soil as well as its composition, and can only be effectively tested on relatively undisturbed samples. Some soil tests measure direct properties of the soil, while others measure "index properties" which provide useful information about the soil without directly measuring the property desired.
Atterberg limits The Atterberg limits define the boundaries of several states of consistency for plastic soils. The boundaries are defined by the amount of water a soil needs to be at one of those boundaries. The boundaries are called the plastic limit and the liquid limit, and the difference between them is called the plasticity index. The shrinkage limit is also a part of the Atterberg limits. The results of this test can be used to help predict other engineering properties.[3] California bearing ratio ASTM D 1883. A test to determine the aptitude of a soil or aggregate sample as a road subgrade. A plunger is pushed into a compacted sample, and its resistance is measured. This test was developed by Caltrans, but it is no longer used in the Caltrans pavement design method. It is still used as a cheap method to estimate the resilient modulus.[4][5] Direct shear test ASTM D3080. The direct shear test determines the consolidated, drained strength properties of a sample. A constant strain rate is applied to a single shear plane under a normal load, and the load response is measured. If this test is performed with different normal loads, the common shear strength parameters can be determined.[6]

Expansion Index test This test uses a remolded soil sample to determine the Expansion Index (EI), an empirical value required by building design codes, at a water content of 50% for expansive soils, like expansive clays.[7] Hydraulic conductivity tests There are several tests available to determine a soil's hydraulic conductivity. They include the constant head, falling head, and constant flow methods. The soil samples tested can be any type include remolded, undisturbed, and compacted samples.[8] Oedometer test This can be used to determine consolidation (ASTM D2435) and swelling (ASTM D4546) parameters. Particle-size analysis This is done to determine the soil gradation. Coarser particles are separated in the sieve analysis portion, and the finer particles are analyzed with a hydrometer. The distinction between coarse and fine particles is usually made at 75 μm. The sieve analysis shakes the sample through progressively smaller meshes to determine its gradation. The hydrometer analysis uses the rate of sedimentation to determine particle gradation.[9] R-Value test California Test 301 This test measures the lateral response of a compacted sample of soil or aggregate to a vertically applied pressure under specific conditions. This test is used by Caltrans for pavement design, replacing the California bearing ratio test. Soil compaction tests Standard Proctor (ASTM D698), Modified Proctor (ASTM D1557), and California Test 216. These tests are used to determine the maximum unit weight and optimal water content a soil can achieve for a given compaction effort. Soil suction tests ASTM D5298. Triaxial shear tests This is a type of test that is used to determine the shear strength properties of a soil. It can simulate the confining pressure a soil would see deep into the ground. It can also simulate drained and undrained conditions.

Unconfined compression test ASTM D2166. This test compresses a soil sample to measure its strength. The modifier "unconfined" contrasts this test to the triaxial shear test. Water content This test provides the water content of the soil, normally expressed as a percentage of the weight of water to the dry weight of the soil.

Geophysical exploration
Main article: exploration geophysics

Geophysical methods are used in geotechnical investigations to evaluate a site's behavior in a seismic event. By measuring a soil's shear wave velocity, the dynamic response of that soil can be estimated.[10] There are a number of methods used to determine a site's shear wave velocity:
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Crosshole method Downhole method (with a seismic CPT or a substitute device) Surface wave reflection or refraction Suspension logging (also known as P-S logging or Oyo logging) Spectral analysis of surface waves (SASW) Modal Analysis of Surface waves (MASW) Reflection microtremor (ReMi)

See also
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Geotechnical engineering Engineering geology Geotechnics Soil mechanics

Notes and references
1. ^ ASTM D1586-08a Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel 2. ^ ASTM D1587 -08 Standard Practice for Thin-Walled Tube Sampling of Soils for Geotechnical 3. ^ "D4318-10 Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils". ASTM International. Retrieved 2011-01-16. 4. ^ "D1883-07e2 Standard Test Method for CBR (California Bearing Ratio) of LaboratoryCompacted Soils". ASTM International. Retrieved 2011-01-16. 5. ^ "CALIFORNIA BEARING RATIO (CBR) AND ROAD PAVEMENT DESIGN". The Idiots' Guide to Highways Maintenance. Retrieved 2007-02-07. 6. ^ "D3080-04 Standard Test Method for Direct Shear Test of Soils Under Consolidated Drained Conditions". ASTM International. Retrieved 2007-02-07.

7. ^ "D4829-08a Standard Test Method for Expan

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