Non-axisymmetric response of piles in low-strain integrity testing

Low-strain testing of piles is routinely performed by striking the pile head with a small hand-held hammer, usually aiming for the pile centre, and measuring the vertical velocity response by means of a receiver placed also on the pile head. In practice, however, it is impossible for the operator to perfectly align the point of impact with the pile centre, as existing solutions for the interpretation of the velocity measurements assume. In the general case where the impact load is eccentric, the wave field is no longer axisymmetric with respect to the pile axis and waves resulting from the hammer impact can propagate along the vertical, radial and circumferential directions. This is taken into consideration in the analytical solution presented in this note, which provides the non-axisymmetric dynamic response of a pile to an impact load applied at an arbitrary position on the pile head. Arithmetic examples are used to depict the three-dimensional non-axisymmetric vibration characteristics of the soil-pile system, with emphasis on the effect of the eccentricity of the impact load on the optimal position of the receiver, the identification of which is essential for obtaining meaningful results from low-strain integrity tests.