What is the maximum size foundation that can be tested with the PIT?
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:
- Length of foundation effectively subjected to skin friction. The part of the pile or shaft above grade or in very soft soil has very little influence on the total length that can be tested.
- Amount of skin friction. Very high soil resistance can reduce the maximum length, while very low resistance throughout most of the shaft may allow longer foundation elements to be tested.
- Amount of end bearing. A clear toe reflection requires some elasticity of the soil at the toe, thus creating a tension reflection since the underlying soil stiffness is considerably weaker than the foundation. Very rigid soil or rock may result in a compression reflection or an unclear toe reflection.
- Hollow piles with relatively thin walls. These piles are difficult to test even in the best of conditions since a plane wave is never attained. Maximum length will be limited.
- Piles or shafts with variable cross-sections or material properties. Impedance variations reflect the wave and it may be difficult to assess the integrity of the foundation below the first major change.
- Piles with splices. Splices can reflect the wave, and it may be impossible to evaluate the integrity of the pile below the first splice.
- Foundations with cracks. Even cracks that would not be detrimental to the usefulness of the foundation may cause strong reflections so that a clear toe reflection would not be detected.
- Very long large diameter piles or shafts. The attenuation of the wave due to soil resistance and to internal damping may make it difficult to detect a toe reflection on foundations longer than 50 m (160 ft).
- Long timber piles. Because of the reflections caused by the tapered shape of those piles, a clear toe reflection may not be detected on timber piles longer than 15 m (50 ft).
- Quality of the electronics used. Low noise circuits and the use of high resolution (≤16 bits) A/D converter allow the PIT to go beyond the usual L/D limitation, so that values of 60 or more can be expected in many cases.
- Piles in structure. Reflections from existing structures might limit the maximum L/D ratio that can be effectively tested.
It should also be noted that sometimes the test will be able to assess the integrity of the foundation up to a certain depth. If it can be confirmed that no major defect is present in the upper part of the foundation, or in the upper zone of lateral movement, this is often very valuable information and sufficient to eliminate any major concern.