Integrity testing of pile foundations using axial vibration

Construction of cast-in-place concrete piles can be susceptible to problems which in severe cases can produce failure of the foundation under applied load. This type of foundation is formed by drilling a substantial hole into the ground, placing steel reinforcement and filling with wet concrete. Several well-documented cases exist where concrete has not completely filled the hole and substantial voids have formed within the pile. Receptance theory has been developed to enable the nature, size and position of a finite defect within a suspect pile to be identified. The technique requires three axial resonant frequencies to be obtained from in situ vibration tests (after the concrete has hardened), and nominal dimensions and material properties of the pile. The application of the theory is demonstrated using data relating to a pile with a known defect. The theory described in this paper ignores the effect of the surrounding soil: guidance is given so that this can be included by further development of the theory.