39 Static cone penetration testing

39.1 General


This is probably the most widely used in-situ test worldwide and has an extensive database of published correlations for geotechnical parameters. The cone penetration test (CPT) consists of "pushing" a conical 60° cone into the ground at a constant rate and recording the pushing force required to do this. The cone penetration test was originally developed in the 1930s for use in recent alluvial soils. Its application, however, has since been widened to other materials, such as dense sands, stiff and very stiff clays, gravelly clays, chalk, other weaker rocks and many more (see Lunne et al., 1997 [74]). However, penetration is usually terminated when dense gravel, coarse gravel, a cobble or rock is encountered. The depth of penetration is limited by both the safe load that can be carried by the cone, and the push system together with the reaction force available for pushing the penetrometer into the ground. The latter is controlled by the capacity of the hydraulic equipment and the weight of CPT machine, or capacity of any anchorage included. CPT termination at depth above the scheduled depth (i.e. refusal) is likely to occur where the operator judges that excessive bending of the cone or section of the push rod string is taking place, largely based on information from the inbuilt inclinometer.

The most common forms of cone penetration test uses "electrical sensors with the cone" to measure the load on the cone; these are referred to as electrical CPT.

A build-up of skin friction along the rods, together with the load on the cone, can prevent further penetration, because the total thrust capacity of the machine is reached. The friction on the sounding tubes can be reduced by the use of a friction reducer. Where a penetration test is stopped for any of the above reasons, it might be possible to prebore and continue the test from the bottom of the prebored hole. If this procedure is adopted, it is necessary to provide lateral support to rods in the prebored hole. For softer strata over harder strata the CPT rig can install casing to support the cone rods through the soft strata, allowing penetration of the harder strata. It is important to use appropriate resolution and accuracy (see BS EN ISO 22476-1) together with all required corrections, e.g. for end area effects (for example, for very soft clays and silts, more sensitive cones could be used).

There have been extensive developments in the technology of the static cone penetration test with probes to measure a variety of other parameters such as electrical conductivity, temperature and the velocity of seismic waves (see Lunne et al, 1997 [74]). Probes and equipment for use on the seabed in water depths of up to 5000 m have been developed. Probes for carrying out chemical testing of soil and ground water are available for use in contaminated ground (see Robertson, Lunne and Powell, 1995 [79]).

The selection of equipment and test procedures should be carried out in accordance with BS EN ISO 22476-1 for electrical cone penetrometers and BS EN ISO 22476-12 if using mechanical cones (no longer used in the UK), as well as BS EN 1997-2 and NA to BS EN 1997-2. The testing should be undertaken by specialist contractors.

39.2 Electrical cone penetration test


The basic principle of the electrical CPT is that a cylindrical probe, fitted to the lower end of a string of hollow rods, is pushed into the ground at a slow uniform rate by a static thrust. The probe has a cone at its base, which is fitted with a sensor, so that its resistance to penetration can be measured. Most probes incorporate a friction sleeve, by which the local frictional resistance can be measured, and in addition often incorporate a piezometer for measuring the pore water pressure in the vicinity of the cone and sleeve (see Lunne et al., 1997 [74] and Meigh, 1987 [80]). The probes have electrical sensors, or when including a piezometer CPTU, which can permit near continuous recording throughout the test. The CPT and CPTU test are widely used in-situ tests.

Electrical cone or piezocone penetration equipment and method of testing should conform to BS EN ISO 22476-1, BS EN 1997-2 and NA to BS EN 1997-2. When undertaking piezocone tests, where penetration pore water pressure is measured, the probe should be fitted with a piezometer intake and pressure transducer.

Load sensors, pressure transducers and other instrumentation should be of suitable capacity and sensitivity for the ground conditions likely to be encountered, for example, in very soft clays and silts more sensitive cones with appropriate resolution accuracy, together with all the required corrections should be used. They should be calibrated regularly and proof of calibration should be available on-site (see BS EN ISO 22476-1). The test procedure should include a means to check that the probe in use has been correctly identified and is working satisfactorily.

NOTE 1 There are generally two alternative positions available for the piezometer element:

  • a) on the shoulder of the cone, just behind the cone; or
  • b) on the face of the cone.

The first of these, on the shoulder, is given as the preferred location in BS EN ISO 22476-1. It allows the measured values of cone resistance to be corrected for errors induced by pore water pressures acting on the surfaces of the penetrometer and this correction is a requirement in BS EN ISO 22476-1 when determining geotechnical parameters in fine grained soils. The generated pore water pressures vary with the geometry of the penetrometer (see BS EN ISO 22476-1).

The pore water pressure measurements can vary substantially, depending on the location of the intake. The response appears to depend on the material being tested and its over-consolidation ratio. The optimum location depends on the soils to be investigated and the purpose of the investigation (see Lunne et al., 1997 [74] and Meigh, 1987 [80]).

CPTs should not be carried out in isolation except where extensive experience of the site or similar materials is available. A number of exploratory holes should be put down adjacent to CPTs to ensure that correlations being used are valid for the particular strata being investigated; the number of these would be dependent on the variability of the ground, size of site and layout of investigation

NOTE 2 In use, the probe is advanced at a uniform rate of penetration by thrust on the sounding rods and the electric signals from the various measuring devices are normally carried by a cable threaded through the penetrometer rods to the surface. Other systems are available, which either transmit the data to the surface using acoustics, or store the test data in the probe so that it can be downloaded when the probe is recovered at the end of each test. Data from the test is displayed for immediate assessment, recorded automatically at selected intervals by computer or data logger for later processing. Sole dependence on data that is recorded electrically, which cannot be assessed during or immediately after test completion, is not recommended.

NOTE 3 Penetrometer tests can be deflected off line by some ground conditions, leading to significant errors in the reporting of vertical depths. BS EN ISO 22476-1 specifies the use of inclinometers when undertaking testing in certain ground conditions and always if the results are to be used to determine geotechnical parameters. BS EN ISO 22476-1 sets classes of equipment related to the purpose of the test.

NOTE 4 The electrical cone penetration test is relatively quick to carry out and the results can be made available immediately following the completion of the test; it is also usually cheaper by comparison with boring, sampling and laboratory testing. However, no direct inspection of the ground is carried out, and all information regarding material type and properties is derived from essentially empirical correlations with the behaviour of the cone during penetration. The descriptions of soil types are reported from CPT/CPTU tests. The soil descriptions presented on CPT logs traditionally follow the format of those on exploratory holes (see Section 6), these are, however, not based on actual particle size distribution or plasticity assessment but on correlation of soil type with cone behaviour. The soil types in commonly used correlations use non-UK descriptive terminology. Consistency and relative density terms are used in accordance with different definitions to those for soils in Section 6.

NOTE 5 The results of a test are presented as plots versus depth of cone resistance, local friction and friction ratio (local friction divided by cone resistance), together with pore water pressure if a piezocone has been used in accordance with BS EN ISO 22476-1. The frequency of data recording can be varied to suit the needs of the investigation. An assessment of these data can give a useful indication of the ground profile, together with many parameters such as strength, relative density and modulus of elasticity. Many correlations are now available for a wide range of soils (see Lunne et al., 1997 [74] and Meigh, 1987 [80]) and others are constantly under development. For any site, however, it is important to ensure that the correlations being used are valid for the particular strata being investigated. Electrical cone penetration test data can also be used directly for design purposes e.g. pile capacity or driveability.

NOTE 6 The electrical cone and piezocone penetration test is commonly used as a rapid and economical means of interpolating the ground profile between boreholes. It can accurately detect the presence of thin soil layers of less than 100 mm thickness, which can easily be undetected by conventional boring and sampling. The test can reliably identify variations in strength across a site and with depth. The results can then be used to plan a programme of selective boring sampling and laboratory testing in zones of special interest. When the piezocone is used, dissipation tests, which monitor the decay of the porewater pressure measured by the cone during a pause in driving, can facilitate the assessment of the coefficient of consolidation and its variation across the site and with depth.

When using any correlations for soil parameters from CPT/CPTU tests, it should always be ensured that the correlations being used are appropriate for the ground conditions being investigated. Ideally they should be validated by correlation to site-specific laboratory test data (see also BS EN 1997-2:2007 and NA to BS EN 1997-2). Any correlations used should be reported in the Ground Investigation report.

NOTE 7 Some penetration test equipment can take piston samples in soft or loose soils from specific horizons (e.g. Mostap sampling, see 25.5.3) and to operate the Delft continuous sampler (see 25.6). The samples are used for strata descriptions and conventional laboratory testing. The equipment can also be used to install piezometers and to carry out various in-situ tests including the push-in pressuremeter and sampling, and environmental tests (see Lunne et al., 1997 [74]).

39.3 Mechanical cone penetration test


The older type of mechanical cone penetrometer (known as CPTM) measures the cone and friction resistance by means of a system of internal rods, which thrust against an hydraulic load capsule set at ground surface. Its use is standardized in BS EN ISO 22476-12. Mechanical penetrometers are occasionally used in very isolated sites, where the more sophisticated electrical read-out systems are not readily applicable, and for doing preliminary probing to assess whether the ground conditions are suitable for the use of the more expensive electrical probe (the probe can experience severe damage when penetrating some types of grounds).

Two types of probe may be used: the mantle cone, which measures only cone resistance; and the friction jacket cone, which measures both cone resistance and local friction.

Probing should be carried out in accordance with BS EN ISO 22476-12, BS EN 1997-2 and NA to BS EN 1997-2.

NOTE 1 The probe is pushed to the required depth by thrust on the outer penetrometer rods, the cone and friction sleeve having been tipped to make them slide into the closed position. Thrust is then applied to the inner pressure rods and measured, usually by a hydraulic load cell. The cone advances ahead of the body of the probe and has sufficient travel to enable a measurement to be taken of its ultimate cone resistance. In the friction jacket cone, further travel of the cone makes it engage with the friction sleeve. Both cone and friction sleeve are now advanced and have sufficient travel to give a measurement of the combined resistance of the cone and friction sleeve. This procedure is repeated at regular intervals of depth, which are normally 0,2 m.

NOTE 2 In the UK the mechanical cone is now almost completely superseded by the electrical cone.

BS 5930:2015 Code of practice for ground investigations