It is now 30 years since the first pile integrity tests were carried out and those years have seen dramatic developments in both the test equipment and the ability to 'read' or interpret the results.
In the early 1970s the standard set of vibration equipment weighed in at around a quarter
of a tonne, testing took about 30 minutes per pile and costs were in the order of £70 per pile. Although interpretation techniques were rather limited and very laborious, there was great enthusiasm for this new test that used sound waves travelling in the pile to determine its integrity.
Ten years later it seemed that this type of testing went through a period of doldrums. The initial enthusiasm had worn off and many users became disillusioned and sceptical about the benefits of integrity testing.
This was largely due to the practitioners themselves, overselling the test and promising more than could be delivered. There is no doubt that piling contractors suffered by having their workmanship pronounced defective, often wrongly, by the test houses.
Now, as we near the millennium, it seems that integrity testing has finally come of age. The equipment is now pocket sized, battery powered and several hundreds of piles can be tested in one day. Test results can be sent from site by internal modem and mobile phone to the processing headquarters in seconds.
More importantly, with the recent publication of CIRIA report 114 Integrity testing in piling practice there is a wide understanding of just what the capabilities and limitations are.
State of the art
What is the current state of art? What can the user reasonably expect from an integrity test?
First, there is a need to distinguish between two test methods currently in use. The first and simplest is a straightforward echo type test where the pile is tapped with a plastic hammer and the response of the pile to that impact is measured with an accelerometer. The result is plotted along a time scale (often wrongly plotted as length) and if the pile is not too long and slender, and if the
soil is not too stiff, then a reflection or echo from the toe will be visible. Pile shaft anomalies such as bulbs and cracks may be detected.
The other type of test, which uses a more rigorous application of the physics involved, uses two separate transducers, one of which is built into the hammer to measure the impact force and the other to measure the response of the pile to that impact. This allows a single unique 'signature' of the pile to be produced and this is usually in the form of a plot of mobility against frequency.
In simple terms, this mobility plot quantifies the pile acoustically in both the X and Y directions, ie diameter and depth. A good pile integrity test system will have both of the above-mentioned capabilities and for the most reliable interpretation it is necessary to be able to view the results in both the time and frequency domain.
It is also essential to measure the hammer impact force as well as the pile response.
A recently released system developed by the authors has all of these capabilities as well as a powerful data processing and interpretation package. The system is known as the TDR-2 and the software as T-PAP.
The site hardware is a lightweight, user-friendly 'blue box' that allows several hundred piles to be tested per day. The system automatically gathers five sets of data (from five hammer blows) and processed them to reject the worst two. Thus three sets of data per pile are stored. The results are displayed on screen so that a preliminary interpretation can be carried out on site if required.
One of the big advantages of this system is in having an in-built modem. Coupled to a mobile phone, data can be sent from site to any host computer within seconds of having completed the site testing.
The interpretation of test data used to be a time consuming and laborious task. It was also more of an art form than a science and different engineers could come up with different explanations for the same test result. In the T-PAP programme, the aim has been to minimise the subjective element of interpretation and instead to rely on two computer interpretation aids.
The first is a simulator whereby a pile/soil system is created and the theoretical response of this system matched against a real pile response. Although this technique has been available for many years, a single calculation used to take 30 minutes and was thus too time consuming to be of real benefit. In this latest version (SIMUL) everything takes place in real time and an iterative 'match' can be obtained in minutes rather than days.
An example of SIMUL from a specially built test pile at Blyth in Northumbria is shown. The very close match between real and simulated results can be seen.
The second and more recent innovation is IMPRO, a technique devised by the late Jean Paquet. The philosophy behind the technique is that any real result is influenced by two factors: the properties of the pile itself and the properties of the soil surrounding and in contact with the pile.
By quantifying the soil properties it is possible to remove the effect of these from the response, leaving only information relating to the acoustic properties of the pile.
The result is an impedance profile, an example on the same pile at Blyth is shown. It is important to note that the profile shown represents pile impedance (density x wave speed velocity x area) and is not intended to represent a visualisation of the shaft. The process is rapid, reliable and the most useful interpretation technique available, and should form part of any interpretation process.
In common with all types of indirect test, pile integrity testing has inherent limitations which should be recognised by the engineer and taken into consideration. One of those obvious limitations is that on long, slender piles, the full pile length may not be 'seen' by the test. This is particularly true in stiff, clay soils. The usual depth limitation in say London Clay is about 30 diameters.
In countries such as the Netherlands, where piles are mainly driven through very soft soil layers, attenuation of signals is less and it is possible to obtain reflected signals from anything up to 60 pile diameters (the simple echo type test was developed primarily for this market).
Another difficulty, inherent in all hammer based systems, is that anomalies very close to the pile head (other than full width cracks or discontinuities) are hard to detect. This is due to the fact that it is difficult to generate sufficiently high frequencies (or short enough wave lengths) to enable clear detection within the upper metre of shaft.
In general and subject to depth limitations, low strain pile integrity testing methods should be able to detect the following features: The pile toe, cracks, joints, significant sudden increases or decreases in cross section, soil layer changes, large inclusions and significant changes in material properties.
However, the following features are unlikely to be detected by low-strain pile integrity testing methods: small inclusions, gradually increasing or decreasing pile diameters, curved piles, local loss of cover, gradual changes in material properties and debris at the pile toe.
Low strain integrity testing methods such as TDR can be used on many types of pile, including precast driven, driven cast-in-situ, bored cast- insitu, continuous flight auger and mini piles.
Testing is quick, relatively inexpensive and it is common practice for every pile on site
to be assessed in this way. In the view of the authors, integrity testing should not generally be the final arbiter of good and bad but should be seen as a single part of the overall quality control procedures on foundation contracts.
Used in this way, integrity assessment can help identify construction problems at an early stage to the benefit of all concerned.
Low strain testing cannot be used with any confidence on diaphragm walls, secant walls and piles under existing structures or connected by pile caps, for which alternative methods should be considered.
These are briefly described below:
Sonic coring is an ultrasonic method used for assessing large diameter piles, diaphragm and secant walls. It does not have any depth limitation and can accurately locate the depth of any acoustic anomaly.
The method involves installing 50mm steel tubes in the pile during construction (or base grouting tubes can be used). The time is measured for an ultrasonic signal to traverse the concrete between adjacent tubes and an ultrasonic profile is built up over the full pile depth,the signal transit time
depending upon the path lengthand the quality of concrete between the tubes. All medium and large diameter piles underthe various Canary Wharf phases were assessed by this method.
Parallel seismic testing is the only method of assessing the integrity of piles under existing structures.
The method involves installing a 50mm plastic tube alongside the pile shaft, and measuring the response time for an impact signal to travel from the pile head to a hydrophone placed in the tube at gradually increasing depths.
The level of the pile toe is indicated by a change in the gradient of the signal arrival time versus depth slope. TDR and echo testing on such piles via slots cut into the exposed pile shaft do not work due to the confusing return reflections from the restraint at the pile head and at depth.
It should be recognised that integrity testing will not give any information on pile performance under load. This can only be obtained by either carrying out a conventional static load test, or by using a dynamic load test such as SIMBAT, developed specifically for bored cast-insitu piles. This has become a widely accepted economic alternative to static loading.
Integrity testing is a powerful quality control tool for the engineer and the piling contractor. It can help to identify anomalies at an early stage and gives confidence in the piling method chosen.
Testing is rapid and recent developments in equipment such as the TDR- 2 ensure that the quality of results obtained are reliable. The latest interpretation software is a real step forward in providing accurate assessment of results.
The trend towards NAMAS accreditation by pile testing practitioners will ensure that equipment used is calibrated and traceable to national standards, and provides the engineer with independent assurance that the correct procedures are adhered to.
At the end of the day however, it is crucial that the engineer is aware of the limitations of the various integrity test methods and to choose the most appropriate to give the information that is required.
Dick Stain is managing director and Huw Williams is an engineer at Testconsult.