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.


Geophone sensorThe 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
stiffness.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
  • Cracks
  • 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 Measurement
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
Where:
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)
Where:
Fm = frequency at point of measurement
V = Velocity


Typical mobility response

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
Where:
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


Simulation of Pile Shaft OverbreakThe 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.

Impedance profile of shaft necking

Tdr Test AccuracyError 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.