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    Rock Anchor Manual

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    I am designing rock anchors to resist seismic uplift of a steel water storage tank. The anchors will extend approximately 30-40 feet below grade into weathered granitic rock and I have several questions: Is there a minimum unbonded length for the anchors?

    I saw 10' referenced somewhere. Is the unbonded zone filled with grout after the anchors are tensioned? Is there a minimum grout cover between the steel anchor and the rock? How is corrosion dealt with? Is the grout cover increased to mitigate corrosion? Is an epoxy coating used? Is there testing that should be completed to evaluate corrosion potential?

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    FJ Series Rock Anchors. Tensile capacities of the Rod Series offered in this manual. Mechanical anchors are activated immediately. This feature is of major benefit to. Manual or pneumatic. The rock bolt anchor should be perpendicular to the rock face within. Rock bolts or anchors tested. Bond Length – The portion of the tieback which is bonded to the soil or rock and. (1990) “Ground Anchor Inspector’s Manual”, In Situ Soil Improvement.

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    Are the anchors generally tensioned to the design load, or some other load? Thanks for your responses RE: Rock Anchors (Structural) 22 Feb 07 23:35.

    A free reference for ground anchors available online is the FHWA manual Geotechnical Engineering Circular No. 4 - Ground Anchors and Anchored Systems, FHWA-IF-99-015. The unbonded length is usually 10 feet for bar stendons and 15 feet for strand tendons. This length prevents significant long-term reductions in the load. However, if rock is 30-40 feet deep, your unbonded length will probably extend down to the top of rock. The unbonded length is filled with grout, but there is a sleeve around the tendon to prevent the grout from bonding to the tendon in the unbonded length. The above manual contains the answers to your other questions. Mercedes Audio 20 User Manual there.

    RE: Rock Anchors (Geotechnical). Is there a minimum unbonded length for the anchors? I saw 10' referenced somewhere. Is the unbonded zone filled with grout after the anchors are tensioned? PTI call for 15' minimum unbonded length. Most anchors are grouted completely in one stage. Two-stage grouting is not done too often.

    The unbonded length usually has plastic sheathingand greasing to allow stressing without obtaining capacity in the unbonded zone. Is there a minimum grout cover between the steel anchor and the rock? How is corrosion dealt with? Is the grout cover increased to mitigate corrosion? Is an epoxy coating used? Is there testing that should be completed to evaluate corrosion potential?

    Tendons usually have a minimum of 0. Acura Body Repair Manual. 5 inches of grout cover. There are several levels of corrosion protection depending on the corrosivity of tyhe ground or rock. Epoxy coatings are available for both bar and strand tendons.

    I don't recommend it for strand tendons. Sometimes there are problems with epoxy coated strands. PTI and FHWA publications include information on soil testing. There are tests for ph, resistivity, sulfates, sulphides. Are the anchors generally tensioned to the design load, or some other load? According to PTI, anchors should be locked off at at least 50% of their guaranteed ultimate strength. I don't agree with this for several reasons.

    Usually anchors are locked of at between 75% and 110% of their design load. The lock-off load can depend on how much load you want to add to the foundation bearing pressure and how much elastic tendon stretch (structure movement?) you can live with if the tendon ever sees its full design load. Don't forget that anchor length depends not only on the unbonded length and the required bonded length, but also on the total length required for mass stability in uplift.

    The cone of soil or rock (sometimes buoyant weight) must be heavy enough with a factor of safety to prevent the uplift. Frequently, expecially in ground conditions with good bond, the mass stability length is greater than the total of the unbonded length and the minimum bonded length. RE: Rock Anchors (Geotechnical).

    I think that dik has a good point. Why are you tensioning the anchors. Why not just let them be passive anchors? You don't have to worry about lock-off loads, or relaxation with time. You need to have some 'movement' to engage the anchors but unless there is a strong need to minimize the movement that can occur during the engagement of the anchors, I think that the movement associated with passive anchors would be acceptable. See DM-7.02 page 170. I've seen some better representations but momentarily can't remember the source - but it was older than the FHWA manual listed.

    RE: Rock Anchors (Geotechnical) 27 Feb 07 09:20. Strand anchors should have sufficient lock-off load to properly seat the wedges into the anchor haed. According to PTI, anchors should be locked off at at least 50% of their guaranteed ultimate strength. I don't agree with this for several reasons. Usually anchors are locked of at between 75% and 110% of their design load. The lock-off load can depend on how much load you want to add to the foundation bearing pressure and how much elastic tendon stretch (structure movement?) you can live with if the tendon ever sees its full design load.