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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slope (i.e., when inertia effects are negligible, and pile-soil system behaves similarly to a spring) represents a measure of the initial pile stiffness (Equation 2). 2 f E V F  = (2)

Moreover, at higher frequencies the V/F ratio vs. frequency graph in Figure 2 is characterized by several peaks which represent resonant responses. These peaks are equally spaced of  f (Equation 3).

2 c V

f  =

(3)

L

Therefore, also with this method the pile length or the distance between the pile head and a relevant defect can be assessed once assumed a reasonable value for V c .

Fig. 2. IR method: Mechanical admittance vs. frequency graph

As previously described the execution of both IE and IR method are possible if the pile head is accessible and pile material properties (especially V c ) are reasonably known. Thus, in case of test to be performed on the foundation of existing structures with piles connected to a pile cap the application of these methods is practically not possible and a combination of two methods (IE and PS) is recommended (Davis, 1995). A possible solution is to modify the IE method in the so-called dual sensors Impulse-Echo testing (Hertlein and Davis, 2007). This approach involves exposing one side of the pile for a minimum length of 1.2 meters below the connecting structure and two sensors (accelerometers) are applied along the pile shaft with a spacing of approximately 1 meter, then, a sledgehammer blow is applied at the pile cap (Figure 3). The upper sensor represents the trigger for the acquisition apparatus and the two sensors register the velocity time histories with a sampling rate of at least 100 kHz. The assessment of V c relies on analyzing the time difference between the arrival of signals at the two sensors, both for the initial downward and the reflected upward signals, and this value will be applied to estimate the pile length or the distance to a relevant defect. As pointed out IE and IR methods are effective especially in the case of new foundations and for integrity check as they require the accessibility to the pile head. For the special case of existing structure PS test is more suitable in fact it was developed by the CEBTP exactly for this reason. It requires the execution of a borehole close and parallel to the foundation commonly lined with a PVC tube filled with water to guarantee acoustic coupling.