Cable Anchors versus Solid Bar Anchors in Rockfall Mitigation Systems

摘要

This is a comparison of different types of anchors used in rockfall mitigation systems across the US. For years the standard had been using a solid bar drilled and grouted into rock to support the upper cable in all rockfall drape systems. We also saw solid bars used for the tie-back and lateral anchors in rockfall barrier installations in the beginning stages of constructing these barriers in the US. It was found during observation of the installed systems that, due to the impacts from the rockfall, the solid bar anchors used in rockfall barriers were not working as rocks were shearing off the top of the anchor bars as they rolled down the slopes prior to impacting the barriers resulting in post failures and reduced net heights, thus compromising the systems capacities before impact. We also found years ago that solid bar anchors for rockfall drapes were being compromised by using too small of an anchor rod. The forces that are transferred to the anchors in more in shear than the vertical pull-out direction that the anchors were tested in, thus reducing the capacities most engineers were requiring as the shear strengths in the solid bars were considerably less than the required pull-out strength. In recent years, there has been some testing to determine the difference in strengths between cable anchors and solid bar anchors (WSDOT field test). This testing clearly showed that cable anchors are superior to solid bar anchors. When testing the anchors in the same direction as the loads that are stressing the anchors the pull-out strength increased substantially in cable anchors versus the standard vertical pull-out tests usually performed. It also showed that the solid bar anchors failed at or near the shear strength of the bar which was much less than the required pullout strength for most anchoring systems designed using a solid bar anchor.