8. PILE INSTALLATION AND CONSTRUCTION CONTROL
8.5.2 Types of Non-destructive Integrity Tests
8.5.2.4 Echo (seismic or sonic integrity) test
The test is suitable for bored piles and precast concrete piles. The principle of echo tests is based on the detection of a reflected echo or longitudinal wave returning from some depth down the pile. The measured time of travel of the vibration wave together with an assumed propagation velocity enable the acoustic length to be determined. The test is normally carried out at least seven days after casting of the concrete.
There are two generic time domain echo type tests, namely sonic echo and pulse echo.
Reference may be made to Ellway (1987) and Reiding et al (1984) for a summary of the principles of operation and interpretation of the tests. Forde et al (1985) also described the improvements in time domain analysis of echo traces through the use of an auto-correlation function to detect reflections in the velocity-time signal.
In the echo test, the pile is struck by a hammer and the resulting vibration signal (e.g.
velocity) is measured at the pile head by means of a geophone or an accelerometer. In general, longer pulses are used to detect defects at greater depths whilst shorter pulses are used for possible defects at shallow depths. After digital filtering of extraneously low and high frequency oscillations, the signals can be range-amplified to magnify the response.
Random noise can also be reduced by signal-averaging techniques. Identification of reflection time and determination of echo phase can be done using signal processing techniques including auto-correlation and cross-correlation methods.
Examples of typical test results are given in Figure 8.8. The phase of the reflected wave provides a means of discriminating reflections from large bulbs or severe necks (or cracks), which constitute fixed and free surfaces respectively.
Figure 8.8 – Examples of Sonic Integrity Test Results (Based on Ellway, 1987)
Velocity (m/s)
Time (ms) (b) Echo from free surface
Straight pile, length as expected and free end condition
Velocity (m/s)
Time (ms) (c) Echo from fixed surface
Straight pile, length as expected and fixed end (e.g. pile founded on rock) (a) No Echo
Velocity (m/s)
Time (ms)
High length/depth ratio and/or high shaft resistance, no reflection at toe
Velocity (m/s)
Time (ms) (d) Echo from intermediate surface
Locally increased pile impedance
Velocity (m/s)
Time (ms) (e) Echo from intermediate surface
Locally decreased pile impedance
Velocity (m/s)
Time (ms) (f) Overshoot and ringing caused by imperfect
deconvolution
Irregular profile – irregular reflection Pile geometry
The limitations of the test may be summarised as follows :
(a) Multiple reflections from mechanical joints or severe cracks may limit the propagation of the stress wave. The test may not be suitable for prefabricated piles with many jointed sections (Hannigan et al, 1998).
(b) Reflections from surfaces of intermediate stiffness such as small bulbs or necks can cause frequency-dependent phase distortions of the signal making interpretation more difficult.
(c) In the case of anomalies near the pile head, the response can be distorted to such an extent as to give rise to problems of signal filtering.
(d) The penetration of the signal into the pile is limited by shaft resistance. A high shaft resistance will reduce pile length that can be tested. Under normal circumstances, it is generally unlikely that a reflection can be detected for a pile with a length to diameter ratio of greater than 30 or at depth greater than 20 m (O'Neill & Reese, 1999). The accuracy in determining the pile length depends on the accuracy of the prediction of speed of wave propagation.
Wave speed variation of 10% is not uncommon (Hannigan et al, 1998).
(e) Site vibrations (e.g. from construction plant) could affect the signal. This effect may be minimised by analysing repeated hammer blows and by signal averaging.
(f) It is capable of identifying well-defined cracks, particularly near the pile head. However, the signal is less clear for diagonal cracks.
(g) It is insensitive to changes in concrete quality as an average sonic velocity for concrete has to be assumed in the interpretation. Any inclusion needs to be significant enough to cause a reflection of the signal and this depends more on its dynamic and acoustic properties than on its strength.
(h) The long wave length generated from a hammer blow makes it difficult to detect defects of small thickness.
Samman & O'Neill (1997) reported that a defect of less than 25 mm cannot be reliably identified.
Both the echo tests and vibration tests involve excitation of the pile head and measurement of the dynamic response to vibration. In principle, a single signal of a hammer
blow can be analysed both in the time and frequency domains. There is an attempt to combine the results to produce a trace referred to as an impedance log, which provides a vertical section through the pile (Paquet, 1992). However, this should be treated with caution as the number of variables involved are such that the impedance log may not be unique and precise.