GeoZone Applied Geology and Geotechnics

GeoZone - Pile Integrity Testing - Frequency Response Method

Any structure founded on a piled foundation relies on the integrity of the installed piles. However, by their very nature it is almost impossible to determine visually whether the pile has been installed to the correct depth or whether any flaws exist within the pile. GeoZone has a strategic partnership with the only pile integrity testing company in the country and the service on offer is considered by all role players in the civil engineering field to add significant value to piling contracts, particularly as such testing provides a rapid and inexpensive means of screening the structural integrity of all piles on a site. The alternative to integrity testing is pile load tests on a few selected piles – a drawn out, expensive and disruptive process.

A good example of poor pile installation came to light during one of these Pile Integrity tests. The results showed acoustic anomalies in twenty five percent of the piles and subsequent physical exposure of the piles to the depths indicated by the acoustic anomalies confirmed the results of the testing. The adjacent photographs shows the severe ‘necking' of the piles which had occurred during installation. The structural defects in the piles shown in the accompanying photographs were confirmed when exposed by excavating around the pile shafts.

Two methods are used for testing piles, namely "Frequency Response and Sonic Echo Testing" more commonly known as "Pile Tapping", and "Sonic Logging"

Frequency Response Response: This non-intrusive test can be implemented on all types of individual cast in situ piles of 250-mm diameter and greater. The impedance of vibratory waves along the length of the pile is measured. The waves are propagated from the top of the pile using an impulse hammer (refer to Figure 1) whilst simultaneously measuring its vibratory response.

Time Domain signal processing is referred to as the Sonic Echo Test, while Frequency Domain processing is called Frequency Response Test . The Frequency Response graph provides the most comprehensive integrity information whilst the sonic echo tests are only used to complement the information obtained from the frequency-response tests on selected piles.

The depth limit of the test is typically 20 to 30 times the pile diameter and is usually dependent on the surrounding soil conditions. Once the data has been gathered, the results are downloaded back in our offices with subsequent computing and analysis. Changes in impedance are related to physical features of the pile, taking into account piling methods, material properties and subsoil conditions and these results are then reported to the client with recommendations if necessary.