Case Studies of Vibration Method to Evaluate Residual Tensile Force of Ground Anchors

In the maintenance of slopes and infrastructures stabilized by ground anchors, it is important to evaluate the residual tensile force of the anchors. Generally, a lift-off test is used to measure the residual tensile force. The test is performed by placing a hydraulic jack on the extra part of the PC steel wire at the anchor head and loading a tensile force. The measurement principle of the test is clear, and the results are reliable. However, the test has the following problems: (1) anchors have the risk of breaking or ejection, so safety equipment is needed to protect operators from the ejection of anchors, (2) the cost of the test is expensive. To solve these problems, our group developed a non-destructive and non-loading test method to determine the residual tensile force by measuring the resonant frequency of the free part of the wire. We named the new test method “non-lift test”, and we are working toward the practical application of this method and are conducting measurements to determine its applicability. We conducted measurement tests on several standards of anchors. As the results, it is found that the discrepancy between the residual tensile forces obtained by lift-off and non-lift tests is less than 10%.