What is the minimum pile length that can be tested with the PIT?
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 (see question Q1031).
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).