Calculation of penetration depth
For application classes 1 and 2, the depth of cone penetration tests shall be corrected for inclination by the equation:
z is the penetration depth, in m;
l is the penetration length, in m;
Cinc is the correction factor for the effect of the inclination of the cone penetrometer relative to the vertical axis.
Equations for the calculation of the correction factor Cinc for the influence of the inclination of the cone penetrometer relative to the vertical axis, on the penetration depth:
a) for a non-directional inclinometer:
α is the measured total angle between the vertical axis and the axis of the cone penetrometer, in degrees;
b) for a bi-axial inclinometer:
β1 is the angle between the vertical axis and the projection of the axis of the cone penetrometer on a vertical plane, in degrees;
β2 is the angle between the vertical axis and the projection of the axis of the cone penetrometer on a vertical plane that is perpendicular to the plane of angle β1, in degrees.
Correction of sleeve friction for water pressure
The corrected sleeve friction can be determined from:
ft is the corrected sleeve friction, in MPa;
fs is the measured sleeve friction, in MPa;
As is the area of friction sleeve, in mm2;
Asb is the cross-sectional area of the bottom of the friction sleeve, in mm2;
Ast is the cross-sectional area of the top of the friction sleeve, in mm2;
u2 is the pore pressure measured between the friction sleeve and the cone, in MPa;
u3 is the pore pressure measured above the friction sleeve, in MPa.
This correction requires values of u2 and u3 and these parameters should both be measured if this correction is to be made.
NOTE u3 can be estimated from u2 using correlations given by SGI report 42 .
These corrections are most important in fine-grained soils where the excess pore pressure during penetration can be significant. Corrected values of the test results should be used for interpretation and classification purposes.
Preparation of the piezocone
Usually, de-aired, distilled water is used when testing is carried out in saturated soils. When performing penetration tests in unsaturated soils, dry crust and dilative soils (like dense sands), the filter should be saturated with de-aired glycerine or a similar fluid, which makes it easier to maintain saturation throughout the test. When de-aired water is used, the filters should be boiled for at least 15 min. The filter should be cooled in the water, before being stored in a sealed container. A larger volume of de-aired water should also be prepared. This water is necessary when mounting before use. Boiling of filters might not be acceptable for some types of filters (like high density polyethylene). If glycerine is used, the dry filters are placed directly in the liquid and treated with vacuum for approximately 24 h. A larger volume of liquid should be treated similarly and stored in a sealed container. The transducer chamber is usually saturated with the same fluid as used for the filter. This can be done by direct injection of fluid into the chamber, or by treatment of the dismantled cone penetrometer in a vacuum chamber. The vacuum should be applied until no air bubbles escape from the cone penetrometer (approximately 15 min to 30 min). The final mounting of filter and seals should be carried out with the penetrometer submerged in the saturation fluid. After mounting, the fitting of the filter should be checked. The height of the filter should be sufficient so that the filter is not loose, but small enough so that the filter can be rotated by the fingertips. This prevents excessive stresses in the joint around the filter, and also reduces any influence on the measurements. After mounting the filter, it is good practice to cover the filter element with a rubber membrane, which will burst when the penetrometer comes into contact with the soil. Other alternatives are also possible.
NOTE During saturation and mounting of the rubber membrane, the penetrometer will be subjected to small stresses, so that the sensors can show values different from zero.
D.2 Slot filter
In this system, the pore pressure is measured by an open system with a 0,3 mm slot immediately behind the conical part. Hence the porous filter element between the soil and the pressure chamber becomes redundant. The slot communicates with the pressure chamber through several channels. De-aired water, antifreeze liquid or other liquid can be used to saturate the pressure chamber, whereas the channels are saturated with gelatine, or a similar liquid.
The use of a slot filter can reduce the time required for preparation of the cone penetrometer. In addition, this pore pressure system maintains its saturation better when passing through unsaturated zones in the soil. A pressure sensor, similar to conventional porous filter piezocones, records the pressure changes in the saturated system. As for other cone penetrometers, the requirements for sufficient saturation are the same, so that adequate pore pressure response is obtained during penetration.
Uncertainties in cone penetrometer testing
Sources of uncertainties in CPT/CPTU testing include but are not limited to:
- ambient and transient temperature effects;
- incorrect calibration parameters, e.g. loss of calibration due to bending or damage;
- lack of or poor saturation;
- improper transfer of loads due to dirt in gaps and seals;
- error in the data acquisition system;
- deviation of the geometry of the cone;
- zero shifts.
Even if the requirements of this part of ISO 22476 are met, uncertainties in the measurements can occur when conducting a CPT, mainly caused by temperature effects in the cone penetrometer during testing.
These temperature effects are:
- ambient temperature; condition when the temperature of the cone penetrometer is changed at a constant temperature without temperature gradients through the penetrometer body. For these circumstances, a compensation system can be applied;
- transient temperatures; temperature change of the cone penetrometer (like heating due to frictional forces on the cone penetrometer) with gradients through the penetrometer body that cannot be compensated.
Compensation of the cone penetrometer for ambient temperature effects can be achieved. The ambient temperature effects can also be avoided by adjusting the penetrometer body to the temperature in the ground. The transient temperature effects cannot be compensated for; special equipment and procedures can reduce these effects. These measures can consist in, for example, letting high temperatures in the cone penetrometer dissipate before penetrating from a dense sand layer into a soft clay layer.
In dense to very dense layers, temperature gradients in the cone penetrometer of approximately 1° C per MPa cone resistance can occur, with uncertain gradients in the penetrometer body.
For special projects with CPTs in soft to very soft clays with special equipment, procedures and temperature measurement in the cone penetrometer (not necessary if all testing is in soft clay), application class 1 can be achieved.
For offshore operations with downhole equipment determination of zero load output of the cone penetrometer, the uncertainty can amount to 100 kPa to 200 kPa, depending on test depth and mud conditions in the drill string.
Metrological confirmation applicable to a cone penetration test should be according to ISO 10012.
A zero drift during the test can be an indication of not achieving the desired application class. If the zero drift exceeds the accuracy boundaries of the application class, the results will be assigned to a lower application class.
An uncertainty statement resulting from an uncertainty analysis can be presented. In this uncertainty analysis, uncertainties can be presented in accordance with WECC DOC. 19-1990  and ISO 10012.
 ISO 14688-2, Geotechnical investigation and testing — Identification and classification of soil — Part 2: Principles for a classification
 ISO 22475-1 Geotechnical investigation and testing — Sampling by drilling and excavation methods and groundwater measurements — Part 1: Technical execution
 EN 1997-1, Eurocode 7: Geotechnical design — Part 1: General rules
 EN 1997-2, Eurocode 7: Geotechnical design — Part 2: Ground investigation and testing
 WECC DOC. 19-1990, Guidelines for the expression of the uncertainty of measurement in calibrations
 T. Lunne, P.K. Robertson and J.J.M. Powell, Cone penetration testing in geotechnical practice, Blackie Academic/Routledge Publishing. New York, 1997
 SGI Report 42, R. Larsson and M. Mulabdic, Piezocone tests in clay, 1991
 NORSOK G-001, revision 2, October 2004, Marine soil investigations