FAQ » PIT Test

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Q001

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.

 

Q002

Can Steel Piles be tested with the PIT?

There is limited applicability of PIT on steel piles, due to

1.    their generally high L/D ratio:
2.    their generally high surface area to volume ratio: This characteristic causes the input wave to be quickly dampened by the surrounding soil.

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.

Q003

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

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).

 

Q004

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:

1.    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.
2.    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.
3.    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.
4.    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.
5.    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.
6.    Piles with splices. Splices can reflect the wave, and it may be impossible to evaluate the integrity of the pile below the first splice.
7.    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.
8.    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).
9.    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).
10. 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.
11. 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.

Q005

How accurate is the Pile Integrity Tester (PIT) when used for determining foundation lengths?

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.

Q006

Can the results of PIT tests be confirmed by comparing them with the piling records?

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.

 

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