Maintenance, checks and calibration
A.1 Maintenance and checks
A.1.1 Linearity of push rods
Before the test is carried out, the linearity of the push rods should be checked by one of the following methods:
- holding the rod vertically and rotating it. If the rod appears to wobble, the straightness is not acceptable;
- rolling the rods on a plane surface. If the rod appears to wobble, the straightness is not acceptable;
- sliding a straight hollow tube which is slightly longer than the rod over the rod. If the rod can pass through the tube without jamming, the straightness is acceptable.
If any indications of bending appear, the use of the rods should be suspended.
Other methods of checking of the straightness may be used.
NOTE In application class 4 there is no inclinometer, so it is more important to control the straightness of the rods.
A.1.2 Wear of the cone penetrometer
The wear of the cone and the friction sleeve shall be checked regularly (see Table A.1) to ensure that the geometry satisfies the tolerances (see 4.4 and 4.5). A standard geometrical pattern similar to a new or unused cone penetrometer may be used in this control.
A.1.3 Gaps and seals
The seals and gaps between the different parts of the cone penetrometer shall be checked regularly (see Table A.1). In particular, the seals should be checked for intruding soil particles and cleaned. The penetrometer shall be cleaned before storage.
A.1.4 Pore pressure measuring system
If pore pressure measurements are carried out, the filter should have sufficient permeability for satisfactory response (see 4.6.1). The pore pressure system should be completely saturated before the penetration starts, and this saturation should be maintained until the cone penetrometer reaches the groundwater surface or saturated soil. For maintenance intervals, see Table A.1.
Before each test the filter needs to be checked visually for damage, wear and clogging. Preferably before each test, the filter should be replaced and the saturation procedure should be carried out.
A.1.5 Maintenance procedures
For maintenance and calibration of the equipment, the check scheme in Table A.1 shall be followed, along with the manufacturer's manual for the particular equipment.
|Checking routine||Start of test||End of test||Every sixth month|
|Vertically of thrust machine||x||–||–|
|Gaps and seals||x||x||–|
|a and at intervals during long term testing, see A.2.1.|
A.2.1 General procedures
A new cone penetrometer shall be calibrated with respect to:
- the net area ratios, used for correction of measured cone resistance and sleeve friction;
- influence of internal friction — restriction to movement of the individual parts;
- possible interference effects (electrical cross talk, etc.);
- ambient temperature effects.
The calibrations and checks are specific to each cone penetrometer. They will show variations during a penetrometer's life caused by small changes in the function and geometry of the cone penetrometer. In such cases, a re-calibration of the cone penetrometer should be carried out. Calibration should be carried out regularly, at least every six months. If it appears from a track record that no significant deviations are registered, a longer period between calibrations can be applicable. Depending on application class requirements and zero drift at zero load, more frequent calibration can be required. During long term testing, calibration may be carried out more frequently.
The calibrations should include the whole measurement system, i.e. mounted transducers, data acquisition system, cables, etc. Preferably calibration is performed as "system calibration", i.e. carried out using the same data acquisition system, including cables, as in the field test, representing a check of possible inherent errors of the system. During the fieldwork, regular function controls of the equipment should be carried out. These should be carried out at least once per location and/or once per day. Furthermore, a function control and possibly a re-calibration should be carried out if the operator suspects overloading of the load sensors (loss of calibration).
In general, the requirements of ISO 10012 should be followed.
A.2.2 Calibration of cone resistance and sleeve friction
Incrementally axially loading and unloading the cone and the friction sleeve calibrate the cone resistance and sleeve friction. The calibrations of cone resistance and sleeve friction can be carried out separately, but the other sensors should be checked individually to ensure that the applied load does not influence them. The calibration is carried out for various measuring ranges, with special emphasis on those ranges relevant for the forthcoming tests.
A new calibration should be carried out after a cone penetration test has been performed under difficult conditions if a significant zero shift has been recorded, for instance if the cone penetrometer has been loaded close to or over its maximum capacity.
When loading the friction sleeve alone, a specially adapted calibration unit substitutes the cone. This unit is designed to transfer the axial forces to the lower end area of the friction sleeve.
When a new cone penetrometer is calibrated, the sensors should be subjected to 15 to 20 repeated loading cycles up to the maximum load, before the actual calibration is carried out.
NOTE The requirement for separate calibration procedures for cone and friction sleeve is usually not required for subtraction cone penetrometers.
A.2.3 Calibration of pore pressure and net area ratio
The calibration of the pore pressure measuring system, and determination of pore pressure effects on the cone resistance, sleeve friction and the net area ratio, a, should be carried out in a specially designed pressure chamber (e.g. Figure A.1). The pressure chamber should be constructed so that the lower part of the penetrometer can be mounted in the chamber and be sealed above the friction sleeve. The enclosed part of the cone penetrometer should then be subjected to an incrementally increasing chamber pressure, and cone resistance, sleeve friction and pore pressure are recorded. In this way a calibration curve for the pore pressure transducer is obtained and the net area ratio can be determined from the response curves for cone resistance and sleeve friction. The pressure chamber is also well suited to check the response of the pore pressure sensor to cyclic pressure variations.
- 1 clamp
- 2 "O" ring seal
- 3 pressure line
A.2.4 Calibration of ambient temperature effects
The cone penetrometer shall be calibrated for ambient temperature effects at various temperature levels, for example by placing the cone penetrometer in water reservoirs at different temperatures. The sensor signals shall be recorded until the values stabilize. From these results a measure for changes in zero readings per °C is obtained and an impression is gained of the time needed for temperature stabilization in the field performance. This is important information for a proper preparation of the test equipment before the penetration test starts. The above applies to ambient temperatures only and not to transient temperatures.
A.2.5 Calibration of penetration length sensor
The depth sensor should be calibrated at least every sixth month and after repair.
A.2.6 Calibration of the inclinometer
The inclinometers in the cone penetrometer shall be calibrated over the measuring range with the vertical in two orthogonal directions. Calibration should be carried out every 1° for application classes 1 and 2 or 2° for application class 3. From these results an impression is gained of the linearity of the sensor.