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 theUS. For years the standard had been using a solid bar drilled and grouted into rock to support theupper cable in all rockfall drape systems. We also saw solid bars used for the tie-back and lateralanchors in rockfall barrier installations in the beginning stages of constructing these barriers inthe US.It was found during observation of the installed systems that, due to the impacts from therockfall, the solid bar anchors used in rockfall barriers were not working as rocks were shearingoff the top of the anchor bars as they rolled down the slopes prior to impacting the barriersresulting in post failures and reduced net heights, thus compromising the systems capacitiesbefore impact. We also found years ago that solid bar anchors for rockfall drapes were beingcompromised by using too small of an anchor rod. The forces that are transferred to the anchorsin more in shear than the vertical pull-out direction that the anchors were tested in, thus reducingthe capacities most engineers were requiring as the shear strengths in the solid bars wereconsiderably less than the required pull-out strength.In recent years, there has been some testing to determine the difference in strengths betweencable anchors and solid bar anchors (WSDOT field test). This testing clearly showed that cableanchors are superior to solid bar anchors. When testing the anchors in the same direction as theloads that are stressing the anchors the pull-out strength increased substantially in cable anchorsversus the standard vertical pull-out tests usually performed. It also showed that the solid baranchors failed at or near the shear strength of the bar which was much less than the required pulloutstrength for most anchoring systems designed using a solid bar anchor.