Non-destructive Evaluation of Residual Tension of Ground Anchor Using Resonance Phenomenon

A new method of non-destructive evaluation for residual tension load of ground anchors was developed. If the tendon tension part of a ground anchor is approximated by a “string”, the frequency of its free vibration is determined by line density and tension length of the PC steel and the operating tension. Although the free vibration of tendon tension part cannot be directly excited because it is in the ground, it is confirmed by a physical model experiment that the free vibration can be excited by applying a power at the extra length of anchor head. A series of field experiments was conducted by using a small vibrator and an accelerometer. A swept-frequency vibration was applied to the extra length of anchor head, and the vibration waveform was measured at the same position. The observed waveform was analyzed by running-spectrum analysis in order to find a resonance frequency which is used to calculate the residual tension load.