Evaluation of Elongation Criteria and Friction Loss in Ground Anchors-PDF

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The report examines whether the widely used requirement that ground anchors achieve at least 80% of theoretical elastic elongation remains appropriate for anchors with long unbonded lengths. Using an analytical friction-loss model, the authors back-calculated a “wobble coefficient” from load-test data for 171 anchors across five projects.<br /><br />Results show a general tendency for friction loss to increase with unbonded length, making long anchors more likely to fall below the 80% criterion. Shallow inclination, drill-hole deviations caused by obstructions or variable geology, and forceful anchor installation may add curvature or damage seals, allowing grout to restrict strand movement. However, substantial variability in the data prevents firm conclusions about the influence of individual factors. Many anchors 100–200 feet long still met the criterion, so the authors do not recommend changing the acceptance standard at this time.<br /><br />Potential safeguards include using steeper anchor angles where feasible, considering two-stage grouting to straighten the unbonded section, and monitoring hole alignment. Instrumentation can directly measure friction loss and load reaching the bond zone.<br /><br />For slope-stability design, calculations should use the load expected to reach the bond zone—not simply the load applied at the anchor head. Anchors that fail the elongation criterion need not automatically be rejected if revised stability analyses, using measured loads and any supplemental anchors, demonstrate the required factor of safety. Further field data and research are needed.

Keywords

ground anchors

elastic elongation

unbonded length

friction loss

wobble coefficient

load-test data

anchor acceptance criterion

grout restriction

slope stability

bond zone load