PSI - Issue 62

Stefano Stacul et al. / Procedia Structural Integrity 62 (2024) 185–192 Stefano Stacul and Nunziante Squeglia / Structural Integrity Procedia 00 (2019) 000 – 000

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acquired via receiver transducers placed inside tubes located in (in CSL case) or adjacent (in PS case) to the pile shaft, while the source is a transmitter transducer placed in another tube located in the pile shaft (in CSL case) or the blow of a hammer applied at the pile-cap (in PS case). IE and IR measurements are affected by pile-soil interaction, damping in concrete piles, radiation damping (surrounding soil) and a great care should be taken in the preparation of the test in order to guarantee a good transmission of the stress wave generated at the pile-head (coupling) (Hertlein and Davis, 2007). IE (Paquet, 1968) is carried out applying a light hammer (< 1 kilogram) blow (with a plastic tip) at the pile head, this blow represents the source of a compressive stress wave that will propagate down the shaft of the pile and after encountering a reflecting surface (i.e., a pile defect or the pile base) will be reflected to the pile head in which an accelerometer or a velocity transducer will record the head response. Thus, the interpretation of the acquired data is based on the following simple formula: 2 c a L V t = (1) in case of concrete piles V c is commonly in the order of 4000 m/s, t a is the time interval shown in Figure 1, while L is distance between the pile head and the reflecting surface (Figure 1). Thus, if the pile material is known and a reasonable value of V c is assumed, then the interpretation of this test can permit to obtain an assessment of the pile length or at least the distance between the pile head and a relevant pile defect. A good practice is to repeat this test several times as the noise (i.e., random signals) can be attenuated by a factor proportional to the square root of the number of blows. Moreover, due to attenuation effects is common practice to apply an amplification function to the recorded signal, in which the amplification increases exponentially with increasing time.

Fig. 1. IE method: head response vs. time graph

IR method, in its original formulation (Paquet, 1968, Paquet and Briard, 1976), involved the use of a swept frequency vibrators and a velocity transducer at the pile head, and a meticulous preparation of the pile head for the application of the instruments was necessary (i.e., a smooth surface is required). Subsequently the use of an impulsive force (via a simple hammer) replaced the application of swept frequency vibrators, and the efforts needed for an accurate preparation of the pile head were reduced. In its current form (Swann, 1983; Stain and Davis, 1983) the method consists of applying an impulsive hammer blow at the pile head and the recorded force and velocity time-domain signals are converted, after applying a Fast Fourier Transform (FFT) algorithm, in the frequency domain. The signals are then elaborated to provide the response curve in terms of V/F ratio versus frequency. V/F ratio is the mechanical admittance (i.e., the ratio between the velocity, V, and the applied force, F). Typical results are in the form shown in Figure 2. In this Figure the inverse of the initial