4.6 Filter element
4.6.1 General filter location
A filter position in or just behind the cylindrical extension of the cone is recommended like in Figure 2. Other filter locations can be accepted.
NOTE 1 Measurements at different filter locations in addition to the recommended ones can give valuable information about the soil conditions.
NOTE 2 The measured pore pressure is influenced by soil type, in situ pore pressure and filter location on the surface of the cone penetrometer. The pore pressure consists of two components: the original in situ pore pressure and the additional or excess pore pressure caused by the penetration of the cone penetrometer into the ground.
The filter should not influence the measured cone resistance or sleeve friction.
Tolerances on filter dimensions are tolerances at the start of a test.
The pore pressure measuring system shall be saturated at the start of the test.
The filter should remain saturated, even when the cone penetrometer is penetrating an unsaturated layer.
NOTE 3 This might not always be possible; in these circumstances other methods like prepushing, preboring or changing saturating fluid can be necessary.
Porous filters should have a pore size between 2 µm and 20 µm, matching a permeability between 10–4 m/s and 10–5 m/s. Filter materials that get clogged by fine particles should be avoided.
NOTE 4 The following types of material have been used with good experience in soft normally consolidated clay: sintered stainless steel or bronze, carborundum, ceramics, porous PVC and HDPE.
The cone penetrometer shall be designed in such a way that it is easy to replace the filter and that the liquid chamber is easy to saturate (see 5.4).
NOTE 5 With regard to the choice of saturating liquid, saturation of pore pressure measurement system, and use of slot filters, see Annex D.
Filters should be replaced before each test.
4.6.2 Pore pressure u1
The surface of the filter shall fit the shape of the cone: it shall not protrude more than 0,5 mm and shall not recess.
The deviation of the surface of the filter to the surface of the cone should be assessed visually. The filter element should be positioned in the middle third of the conical part.
4.6.3 Pore pressure u2
The filter element shall be placed in or just behind the cylindrical part of the cone. The diameter of the filter at the start of test shall correspond to the diameter of the cylindrical part of the cone and the friction sleeve, with a tolerance limit of 0,0 mm to +0,2 mm. The filter can be larger, but shall never be smaller than the diameter of the cylindrical part of the cone. The filter shall not have a larger diameter than the friction sleeve:
d2 – 0,2 mm ≤ dfil ≤ d2
dc ≤ dfil ≤ (dc + 0,2) mm
To correct for pore pressure effects on cone resistance, the filter element should be located in the gap between the cone and the friction sleeve. Since this is not possible in practice, the filter should be located in the cylindrical part of the cone as close as possible to the gap.
4.6.4 Pore pressure u3
The diameter of the filter shall correspond to the diameter of the friction sleeve with a tolerance limit of 0,0 mm to 0,2 mm, i.e. the diameter of the filter shall not be smaller than the diameter of the friction sleeve:
d2 ≤ dfil ≤ d2 + 0,2 mm
The filter element should be placed immediately above and as close as possible to the gap between the friction sleeve and the shaft of the cone penetrometer.
4.7 Gaps and soil seals
Gaps between the different parts of the cone penetrometer shall not exceed 5 mm in height. A soil seal shall protect the gap to prevent soil particles affecting the measurement.
The soil seal shall deform easily relative to the load cell and other elements in the penetrometer, to prevent the transfer of significant forces through the gap.
4.8 Push rods
The push rods shall have the same diameter as the cone for at least 400 mm measured from the base of the cone for cones with a base area of 1 000 mm2. For other size cones, this distance shall be scaled linearly in proportion to the diameter.
Friction along the push rods can be reduced by a local increase in the rod diameter (friction reducer). The friction can also be reduced by lubrication of the push rods, for instance by mud injection during the test. The injection point should be at least 400 mm above the base of the cone for cones with a base area of 1 000 mm2. For other size cones, this distance shall be scaled linearly in proportion to the diameter of the cone.
Above the ground level, the push rods should be guided by rollers, a casing or similar device to reduce the risk of buckling. The push rods may also be guided by a casing in water or soft strata to avoid buckling.
The push rods should be chosen with respect to the data signal transmission system chosen.
NOTE The push rods can also be used to support and/or protect parts of the measuring system. With acoustic transfer of CPT results, the rods are also used for transmission of data.
4.9 Measuring system
The resolution of the measuring system shall be better than one-third of the required accuracy applicable to the application class given in Table 2.
Uncertainties in the measuring system, including temperature effects, are described in Annex E.
4.9.2 Sensors for cone resistance and sleeve friction
The load sensor shall be compensated for possible eccentricity of axial forces. The sensor for recording the side friction force shall be constructed so that it measures the friction along the sleeve, and not the earth pressure against it.
NOTE Normally strain gauged load cells are used for recording cone resistance and sleeve friction.
4.9.3 Sensor for pore pressure
The sensor should not show significant deformation during loading. The sensor communicates with the porous filter on the surface of the cone penetrometer via a liquid-filled chamber. The measuring system should be as rigid as possible (see 5.4) to obtain a good response.
NOTE 1 The pore pressure sensor is normally a pressure transducer of the membrane type.
NOTE 2 This system measures the pore pressure in the surrounding soil during penetration.
4.9.4 Sensor for inclination
The inclinometer shall normally have a measuring range of at least ±15° relative to the vertical axis.
Other measuring ranges may be used, but these can affect the maximum penetration which can be achieved (see 4.9.5).
An inclination sensor is not required where the target penetration is less than 5 m and for application class 4.
4.9.5 Measuring system for penetration length
The measuring system shall include measurement of the penetration length.
If relevant, the measurement system for depth shall also include a procedure for correction of measurements if upward movements of the push rods occur relative to the depth sensor, caused by a decrease in force on the push rods.
4.9.6 Raw data
The test data may be recorded as raw data.
NOTE It is preferable to record the data as raw data so they are available for future processing. Raw data is the unprocessed data as the output of the sensors.
4.10 Thrust machine
The cone penetrometer shall be able to penetrate at a standard rate of penetration of 20 mm/s ± 5 mm/s, and the equipment shall be loaded or anchored to limit movements of the thrust machine relative to ground level while the penetration occurs. Hammering or rotation of the penetration rods during measurements is not allowed.
Required reaction (counterweight) for the thrust machine may be supplied by dead weight and/or soil anchors.
The pushing equipment shall give a stroke of at least 1 m. Other stroke lengths may be used in special circumstances.