The spacing, inclination, length and the load applied of each anchor depend on the local soil or rock conditions. The available drilling equipment and the structural capacity of the other support elements, such as wales, lagging or a concrete retaining wall, may dictate the capacity and configuration of anchors. A factor of safety of 1.5 to 2.5 should be utilized in anchor design.
For rock anchors, the shear stress on the rock socket perimeter is used to size the bond length. For soil anchors, the bond length is generally assumed on the basis of experience and site testing. Field testing should always be conducted to verify design assumptions
Pull out tests verify that the bond capacity of the THREADBAR® in grout exceeds the recommendations of ACI 318. The THREADBAR® grout interface does not control the bond length. Bond in cohesive soils can be considerably increased using the DYWIDAG Postgrouting System.
The stressing length depends on the assumed failure plane and/or the size of the rock or soil mass necessary to resist the anchor force. A minimum stressing length of 15 ft. is recommended so that small movements in the retaining system will not result in a major loss of prestress force.
DYWIDAG Anchor Installation
Selection of the drilling method depends on the number of anchors, the composition of the soil or rock, availability of equipment and the required diameter of the hole. The selection of the tools and techniques should be left to the discretion of the drilling contractor where practical. The depth of the bore hole should be based on sit tests. The diameter of the bore hole should exceed the maximum diameter of the anchor by at least 1. If centering devices are used, larger holes are required.
In stressing, an electrically powered hydraulic jack with built-in socket wrench tightens the anchor nut. The jack fits over a pull rod designed to thread onto the THREADBAR® extension protruding from the anchor nut. Elongation of the anchor can be measured directly or can be monitored by a counter on the jack. Hydraulic pressure is measured by a gauge on the hydraulic pump. Discrepancies of more than 10 % between elongation and gauge reading should be investigated. Lift off reading should be taken to determine the applied prestress force. Movement of the structural system must be considered.
For rock anchors, bore holes should be pressure tested to determine water leakage before the anchors are installed. Consolidation grouting, redrilling and retesting are required where water seepage is excessive. After the anchor is installed in the bore hole, the bond length is grouted. Rock anchors and anchors in cohesive soils are generally grouted without pressure. Soil anchors in loose granular material are pressure grouted while the drill casing or auger is withdrawn. DYWIDAG Postgrouting may be used for the installation of anchors in cohesive soils and non-cohesive soils. This technique permits additional grouting operations after the primary grout has cured. Using a series of valves in a preplaced grout pipe, grout can repeatedly be injected under high pressure. Regrouting displaces the previously injected grout and increases the anchor capacity.
Prior to the installation of any production anchors, test anchors should be installed to verify all design assumptions, including anchor length. Test anchors should be proof stressed to 80 % of the guaranteed ultimate strength of the DYWIDAG Threadbar. After 24 or more hours, readings may be required on selected anchors to determine creep behavior. All production anchors should also be proof stressed, but the load need not be held for an extended period.