What is the minimum pile length that the PIT can test?
The PIT performs a low strain dynamic test based on the sonic pulse echo or transient response theory. The test is most suited for determining the integrity of relatively long foundation elements in the buried, hard to visually inspect, lower portions of the pile. At the base of this theory is the one-dimensional stress propagation theory, which applies to slender rods. The slender rods refer to foundations with relatively large L/D ratios (L = Pile Length and D = Diameter of Pile). Piles with very large L/D ratios may present difficulties when testing. For more information click here.
Pile Dynamics, Inc. (PDI) supplies hammers to use with the PIT that are optimized and tested for relatively long foundation elements. The PIT is not recommended for testing short piles (less than 2 m) without modification. 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 gram (1 lb) hammer may generate frequency contents up to 1400 Hz. Its most efficient frequency range generates up to approximately 1000 Hz. This corresponds to a foundation length of 2 m (assuming a wave speed of 4000 m/s). The chosen hammer should test piles or shafts no shorter than the corresponding length.
Heavier hammers produce lower frequency pulses making them better for testing longer foundation elements. For example, when testing a 1 m long pile the pulse's frequency content as generated by the hammer impact must exceed 2000 Hz. This type of pile; would require a special hammer or another impact device, such as one with less weight and a harder tip or a steel ball bearing.
Often, engineers use frequency analysis to analyze short piles. An Acoustic Concrete Tester (ACT) may be helpful in assessing the integrity of pile caps or slabs of less than 1 m thickness. Regardless, an engineer can easily determine the integrity of a very short pile by visual examination (e.g. simple excavation).