FAQ » PIT Test
How often do the Pile Integrity Tester (PIT) Accelerometer and Instrumented Hammer need to be recalibrated?
Pile Dynamics recommends that the PIT Accelerometer and the PIT Instrumented Hammer be recalibrated every two years, regardless of how much they have been used during that period. It should be noted, however, that these calibrations are only important in case of simultaneous force and velocity measurements. When tests are performed with the accelerometer only (velocity only) the data is normalized and the actual velocity amplitude is not relevant.
It should be noted that Pile Dynamics has a policy of recalibrating any Accelerometer or Instrumented Hammer that is returned to its Maintenance Department for check-up or repair.
There is limited applicability of PIT on steel piles, due to
The PIT test is therefore mostly applicable to determine the integrity of concrete foundation elements. However, successful detections of defects on short (less than 15 m or 50 ft) H-piles in soft soils have been reported by others. PIT is not recommended for open steel pipe piles, unless they are filled with concrete. It should be noted that the PIT equipment and software allows changing the wave-speed and specific weight to those of steel.
The PIT performs a low strain dynamic test, based on the sonic pulse echo or transient response theory. It is mostly applicable to determine the integrity of relatively long foundation elements with buried, hard to visually inspect lower portions. In addition, the one dimensional stress propagation theory which is the base of this method applies to slender rods, i.e., foundations with a relatively large ratio of L/D (L = Pile Length and D = Diameter of Pile). Note, however, that a very large L/D may present difficulties for this type of test
The hammers supplied by Pile Dynamics, Inc. (PDI) for use with the PIT have been optimized and tested for relatively long foundation elements. PIT is not recommended for testing on short piles less than 2 m without modifications. For example, the frequency content of the wave generated by the hammer impact will determine the length limitations of PIT testing with that hammer. The 450 grams (1 lb) hammer may generate frequency contents up to 1400 Hz, with its most efficient frequency range up to about 1000 Hz. This corresponds to a foundation length of 2 m (assuming a wave speed of 4000 m/s), so this hammer should be used to test piles or shafts no shorter than this length. Pulses generated by heavier hammers, which generate lower frequencies, are better suited to test longer foundation elements, and not well suited for shorter piles. For a 1 m-long pile, the frequency content of the pulse generated by the hammer impact needs to exceed 2000 Hz, so a special hammer or other impact device is usually required, such as a hammer with less weight and harder tip, or a steel ball bearing. For such short piles, frequency analysis is often used to interpret the data.
It should be noted that to assess the integrity of pile caps or slabs of less than 1 m thickness, an Acoustic Concrete Tester (ACT) may be helpful. Also, for very short piles the integrity can be easily determined by visual examination (e.g. simple excavation).
The maximum size for successful PIT testing is usually specified in terms of a maximum Length (L) over Diameter (D) ratio. An L/D ratio of 30 has in the past been mentioned as being the upper limit for the PIT test, since for older electronics and for piles or shafts longer than 30 times their diameter the input is usually depleted by shaft soil resistance so that no clear toe reflection would be observable. The limit, however, can in actual practice be higher or lower than 30, depending on several factors, like:
The accuracy of foundation length is dependent on the accuracy of the wave speed. Since the wave speeds for good quality concrete vary between 3500 m/s (11,500 ft/s) and 4500 m/s (14,500 m/s) on low strain tests like those performed with the PIT (and the Acoustic Concrete Tester (ACT) - see Question 1018), if an average wave speed of 4000 m/s (13100 ft/s) is used then a maximum variation of plus or minus 12.5 % is possible.
Certainly comparing the PIT results with installation records is always advised. This is particularly true for augered cast-in-place or continuous flight auger (CFA) piles with installation records obtained with a magnetic flow-meter and with depth increments of 600 mm (2 ft) or less (such as Pile Dynamics’ Pile Installation Recorder would produce). In addition, one should always compare results of PIT with the soil profile. There may be systematic changes in the PIT records that correspond to soil layers. For example, if a loose sand overlays a stiff clay, the pile may have a bulge in the sand and then return to nominal dimension at the clay layer. In this case the relative reduction should not be interpreted as a defect since the nominal diameter is maintained.
It is also good to compare the PIT records with each other, to try to establish the normal result and then to determine those that substantially deviate, particularly if a reflection indicating a defect is observed (i.e. large reflection with the same sign as the input pulse).
In summary, all information available should be considered when determining final acceptance of foundations using the PIT method.