One of the more significant operationally-executed measurements developed during the last few years is the evaluation of concrete pile integrity in pile foundations. Integrity could also be referred to as coherence or compactness. We began conducting the pile integrity measurement in 1999, and now we have a total of four machines at our disposal.
The most frequently used PIT method (the acoustic or knocking method) is based on an analysis of the response to force impulses applied to the pile head. The force impulses are induced by the knocking of a special hammer which enables the recording of the time behaviour of force. The pile response is registered by an oscillation rate sensor temporarily located on the pile head
The response times of incoming reflections from significant heterogeneities in a pile body can be read on the oscillation rate time behaviour record (picture). In this particular case, a reflection from a significant division is represented by a pile footing. Therefore, with regard to the known range of the velocity of seismic wave propagation in cured concrete, both the length of a pile and the potential extent for failures can be determined.
Significant information is provided by a frequency analysis of the observed pile head vibration (oscillation) record which is specific for particular piles and provides information about the geometric (length and diameter) and material characteristics of a pile. A comparison between the theoretical response and the observed measured curve enables the creation of a pile model, determining its diameter, the velocity of elastic transverse wave propagation, the density of the entire pile, and the extent of irregularity in case of impaired integrity.
In addition to the acoustic method, we also perform integrity tests through a transparent method (CHA). The principle of this method is to use a high-frequency emission from a UV signal generated by an emission probe and a simultaneous detection mechanism by a receiving probe. This method involves the placement of both probes in pre-built in tubes in the pile body, which enables the continuous observation of concrete quality in the whole length of the pile. A 3D tomographic model is available, in addition to the time behaviour records.
What can be detected through measurements of pile integrity:
Pile intactness with a virtually 100% exclusion of significant failures - the measured response is free of anomalies and the measured curve resembles the theoretical curve for the parameters concerned
Disruption of a pile body, more significant cracks
Significant reduction or increase in a pile diameter
Marked material changes in a pile body (impaired concrete)
Inferior concrete in the upper pile body
Larger cavities in a pile
What cannot be reliably indicated:
Small and gradual changes in a pile diameter
Minor gradual material changes in a pile body
Evaluation of the influence of possible integrity failures on performance (transfer of load)
Many piles determined by us to have integrity defects were examined through the use of a bore with confirming results. Thus, pile integrity measurement has proven itself to be a quick, cost-effective, and reliable field testing technique for both engineers and investors.