Transient Dynamic Response (TDR) test is a rapid method of assessing the integrity of both pre-cast and cast in situ concrete piles. It is a natural evolution of the Steady State Vibration test first developed and applied to foundation testing by J Paquet in 1966. At that time a heavy (25Kg) vibrator was used to excite the pile at a range of frequencies. Since that time there have been dramatic improvements and miniaturization of the equipment, the most significant single step coming in 1982 when it was found that identical results could be obtained using a transient impulse on the pile top, using a small hand held hammer acting through a load cell in place of the heavy vibrator. Advances in micro processing meant that the time domain signal could be readily converted to frequency using the Fast Fourier Transform. This technique is now known as the Transient Dynamic Response and testing now only takes about 30 seconds per pile compared to about 15 minutes in 1965. It is now considered by many engineers to be the most appropriate test method for checking bored cast foundations.
Equipment used is lightweight and portable and is very rapid in operation. Analysis of results can be carried out instantly on site to confirm the length of the foundation and depth of any defects if they exist. The TDR system also has a powerful software analysis program, to enable more detailed analysis of changes in pile section and the influence of soil. It can also be used to predict the expected test result before even visiting site! The required preparation is minimal and in normal conditions up to 60 piles per day can easily be tested, increasing to 200 where access is very good.
The method is based on measuring the frequency and amplitude response of a pile known as impulse. This response, known as Mechanical Admittance (or mobility), contains all the information necessary to check pile integrity and to analyse soil influences. At higher frequencies the resonating harmonics of the pile are detected, whereas at low frequency the response is generally linear allowing measurement of pile-head
What will it tell you ?
The TDR method of assessing piles is able to analyse acoustic anomalies corresponding to the following :
- Pile Toe Level
- Shaft restraints
- Over break
- Reductions in section
- Zones of poor quality concrete
For further information on the advantages and limitation of this technique and other low strain methods of assessing piled foundation, we recommend reading CIRIA report 144 titled Integrity testing in piling practice published in 1997.
How does it work ?
After ensuring that the concrete in the pile head is visually free of loose material and contaminants, a geophone sensor is placed in contact with the pile head, which is struck axially using the force response hammer. The response of both transducers is measured simultaneously, and these signals, velocity and force, are digitally processed and displayed on the test unit.
When a pile top is struck with the hammer a longitudinal wave travels down the shaft it can be likened to a snake swallowing an egg. When the wave reaches the base of the pile it is reflected back up to the top. By assuming a wave speed velocity it is possible to calculate the pile length. Reflections can also be obtained from acoustic anomalies within the pile shaft. At low frequency the response is generally linear allowing measurement of the dynamic pile head stiffness.
Length measurements are calculated from the distance between resonating peaks, produced by the pile toe or acoustic anomalies along the shaft. Lateral soil restraints, overbreak, changes in shaft section, cracks and zones of poor quality concrete can all produce various types of acoustic anomaly which can be detected.
Length, L = C/2df
C = velocity of longitudinal waves in concrete
df = distance between two resonating peaks
Dynamic Pile Head Stiffness
The dynamic pile head stiffness is measured at low frequencies, when the pile head and surrounding soil are moving as one unit and is the reciprocal of the slope of the initial part of the curve.
Stiffness, E = 2 pi fm/(V/fm)
Fm = frequency at point of measurement
V = Velocity
Mobility (inverse of impedance)
Concrete density or conversely the cross-sectional area of the pile (if concrete strength is known) can be calculated from the mean Mobility (N) of the resonating part of the curve using the following formula.
Mobility, N = 1 / pCA
p = concrete density
C = velocity of longitudinal waves in concrete
A = pile cross sectional area
Pile Head Preparation
In order the obtain the very best data possible when testing a pile, it is essential that the pile head is prepared properly prior to testing. Without good data any interpretation carried out will be meaningless. It is essential that the measurement transducers are mounted in the correct position and on sound concrete. The essentials of pile head preparation for integrity testing are given below :
a) Piles should if possible be tested at the cut-off level and trimmed to sound concrete. Any weak, broken concrete that sounds hollow should be removed and the pile top left roughly horizontal over the complete cross section.
b) Reinforcing bars should be bent slightly away if practicable and the helical removed to allow for a good swing of the test hammer.
c) Two areas should be prepared for the transducers, one for the hammer in the centre of the pile and the other for the geophone close to the pile perimeter. The areas should be approximately 100 mm in diameter and prepared as flat and level as possible using a scabbler, scutch hammer or a hammer and chisel, then brushed free of debris with a wire brush. If at first you are unable to obtain a valid result, it is always advisable to re-prepare the pile and carry out a re-test, as cracking in the pile head is not always apparent but can affect the test result significantly.
Simulation of test results
The Simulation software is a finite element programme that simulates the frequency response of a real concrete pile by defining it and the surrounding soil in up to 10 segments. For each segment, the following information can be input: length, diameter, concrete wave propagation velocity, concrete density, soil shear wave velocity, soil density and base soil details. With the TPAP simulation it is possible to super impose the simulated result onto a real frequency response curve. Soil and Concrete parameters can be changed using sliders and the simulation alters instantaneously in response. Simulation of Pile Shaft Over break
The operator is able to carry out curve matching to simulate the probable cause of any anomalies. Simulations are generally carried out on pile test results that have shown an intermediate response and enables a high degree of confidence in the interpretation. Impedance Profiling The impedance profile method of analysing pile integrity results combines the mobility curve obtained at the top of the pile and details of soils surrounding the pile to produce a specific reflectogram of the pile shaft, and a profile of the variation of local characteristic impedance, as a function of depth. The local impedance is well related to the mechanical properties of the concrete crosssection. To create an impedance profile, the nominal pile concrete properties and dimensions are input, together with the known soil conditions. The resulting impedance profile enables the operator to check for reductions in pile impedance, which could be caused by bands of poor quality concrete, of necking and increases in pile impedance, probably caused by increases in pile section, or overbreak. Tdr Test Accuracy Error calculations for TDR test responses are highly complex, due to the many factors involved. The accuracy of response curves is influenced by equipment accuracy and operator accuracy. As a guide, the accuracy of mobility and frequency measurements are shown in the table below for a standard test with black hammer tip to 1000Hz: Background vibration on site can also influence the accuracy of results. This is however usually apparent to the site operator, who can take the necessary action to remove the source. Pile head preparation is the single most important factor which influences test results. A poorly prepared pile will not inhibit the accuracy of the result it will not, however, properly represent the body of the pile.
Tdr Test Accuracy
Error calculations for TDR test responses are highly complex, due to the many factors involved. The accuracy of response curves is influenced by equipment accuracy and operator accuracy. As a guide, the accuracy of mobility and frequency measurements are shown in the table below for a standard test with black hammer tip to 1000Hz:
Background vibration on site can also influence the accuracy of results. This is however usually apparent to the site operator, who can take the necessary action to remove the source.
Pile head preparation is the single most important factor which influences test results. A poorly prepared pile will not inhibit the accuracy of the result it will not, however, properly represent the body of the pile.