5.4 Preparation of the test

For cone penetrometers with measurement of pore pressure, the filter element and other parts of the pore pressure system shall be saturated with a liquid before field use. Appropriate measures should be taken to preserve saturation during the test.

Usually de-aired, distilled water is used when testing is carried out in saturated soils. If performing penetration tests in unsaturated soils, dry crust, dilative soils (e.g. dense sands) or when the groundwater table is located at large depths, the filter should be saturated with glycerine or a similar fluid.

Saturation of the cone penetrometer before penetration starts or during operation in a predrilled hole can be maintained by applying a rubber membrane around the filter. 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.

NOTE 1 See Annex E in case a slot filter is used.

NOTE 2 When penetrating coarse materials, predrilling may be used in parts of the profile if the penetration stops in dense, coarse or stone-rich layers. Predrilling may be used in coarse top layers, sometimes in combination with casings to avoid collapse of the borehole. In soft or loose soils, predrilling can be used to penetrate the crust to reach the groundwater table. Predrilling can be done by ramming a dummy-rod of (45 to 50) mm diameter through the dense layer to provide an open hole and reduce the penetration resistance.

The zero readings of the cone resistance, the penetration length the sleeve friction and, if applicable, the pore pressure and inclination of the cone penetrometer relative to the vertical axis shall be recorded.

In order to obtain the required accuracy, depending on application class, it can be necessary to ensure that cone temperature is at or near ground temperature before commencing testing. Zero readings of all sensors should be taken with the cone penetrometer unloaded and temperature-stabilized ideally at ground temperature. Required accuracies are given in Table 2. Annex A contains maintenance, checks and calibration procedures.

NOTE 3 When the cone penetrometer is lowered into the ground or in the water, small temperature gradients will occur if the air temperature is different from the ground temperature, which will influence the sensors. Therefore, it is important that the penetrometer is left to come to equilibrium so that the temperature gradients can be reduced to zero before the penetration starts. Usually, the largest gradients will occur after 2 min to 3 min. The cone penetrometer will usually be completely temperature-stabilized after 10 min to 15 min.

5.5 Pushing of the cone penetrometer

During the penetration test, the cone penetrometer shall be pushed into the ground at a constant rate of penetration of (20 ± 5) mm/s. The rate shall be checked by recording time and penetration length.

NOTE 1 The penetration is regarded as continuous even if the penetration is stopped regularly for a new stroke or mounting of a new push rod. Some thrust machines can carry out true continuous penetration without any stops and this can be an advantage, particularly in layered silt and clay deposits.

NOTE 2 The penetration is regarded as discontinuous if larger stops are introduced, such as dissipation tests (see 5.9) or due to unforeseen malfunctions of the equipment.

NOTE 3 In dense sands and gravels, penetration rates less than the "standard" can be acceptable, to prevent damage to the cone and cone rods.

5.6 Use of friction reducer

The use of a friction reducer (see 3.1.14) is permissible. The cone penetrometer and, if relevant, the push rod shall have the same diameter for at least 400 mm measured from the base of the cone before the introduction of the friction reducer if applicable.

5.7 Frequency of logging parameters

The minimum logging frequency of parameters shall be in accordance with Table 2. Logging shall include (clock)time for application classes 1 and 2 of Table 2.

The logging interval for the various measured values can also be chosen depending on the detail required in the profile, e.g. detection of thin layers. Usually the same reading interval is used for registration of cone resistance, sleeve friction and pore pressure.

If the values are measured more frequently than the required reporting intervals according to Table 2, then the average value calculated can be reported. Other methods, however, can also be applicable. The method used shall be reported.

5.8 Registration of penetration length

The level of the base of the conical part of the cone shall be determined according to the requirements in Table 2, relative to the ground level or another fixed reference system (not the thrust machine). The resolution of the penetration length measurement shall be at least 10 mm.

The penetration length shall also be checked and recorded at least every 5 m for tests according to application class 1 of Table 2. For other application classes, the penetration length shall be checked and recorded at the end of the test. Penetration length shall be checked without using the depth sensor.

The penetration of the cone penetrometer and the push rods shall be terminated when one of the following events occurs.

  • The desired penetration length or penetration depth has been reached, see also EN 1997-2:2007, 4.2.2 (3)P.
  • The inclination of the cone penetrometer relative to the vertical axis exceeds the measuring range, or 15°.
  • The agreed maximum thrust or maximum capacity of the cone penetrometer or measuring systems is reached.

Possible damage to the equipment can also be a valid reason to end the test.

NOTE The measured parameters for a cone penetrometer with a large inclination can deviate from the values that would have been measured if the cone penetrometer was vertical.

The penetration depth should be calculated from penetration length and inclination measurements as in Annex B.

ISO 22476-1:2012 Field testing — Part 1: Electrical cone and piezocone penetration test