Tieback anchors are now used routinely to stabilize landslides affecting transportation infrastructure in the USA and abroad. It is generally understood by design engineers that friction loss over the unbonded length of a tieback anchor is inherent to the system, and such loss is permitted as long as it does not exceed a certain threshold, typically 20 percent of the post-tensioning load. However, recent experience from projects utilizing tieback anchors with unbonded lengths in excess of 150 feet has demonstrated that this criterion may be difficult to achieve, particularly in certain ground conditions. Furthermore, anchor friction loss, even if below the 20-percent threshold, has important implications for the stability of the slope that are often ignored in practice. This paper presents the results of a study in which friction loss is quantified from load test data for projects in a variety of ground conditions by means of a "wobble coefficient." Factors that may influence the wobble coefficient are examined, and recommendations for addressing this issue in practice are presented. A recent instrumented load-test program for a landslide stabilization project in Southern California is examined, demonstrating that with appropriate monitoring controls, uncertainty with regard to friction loss can be minimized.
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