Encyclopedia of Anchoring (CA06114E)

HOW TO SOLVE ANCHOR PROBLEMS

Know what to look for During the rare time you do have a problem installing power installed screw anchors (PISA® ) you can turn the bad experience into a benefit that will help you avoid similar troubles in the future. The secret is analyzing exactly what happened when you encounter a problem. Look at tooling or for anchor damage caused during the installation. The damage can tell you a great deal about what went wrong, so the improper action can be avoided in the future. One of the most common problems, particularly with PISA® anchors, is an anchor shaft fracture during installation. Because driving effort is transmitted from the anchor installing wrench to the anchor helix via the anchor shaft, if shaft stress exceeds the shaft’s ultimate strength, the anchor shaft will fracture. Fractures are not pleasant occurrences, but they tell you a great deal about what happened, and what you need to do to prevent it from happening again. If you experience a fracture, recover at least part of the anchor shaft and observe the fracture surface. This is usually quite easily done; just withdraw the anchor rod, and you should find the top part of the anchor shaft still attached.

installation maintaining proper down pressure at all times. Fracture problems can occur with the Square Shaft (SS) anchor if the anchor shaft is not pinned into the wrench, couplings are not properly bolted up, or they are subjected to gross misalignment. Such misalignment might be caused by leading off after encountering a hard stratum at an oblique angle or obstructions in the ground.

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If the fracture surface intersects the drilled hole in the shaft (see below left), insufficient wrench engagement was the problem.

In either of the first two cases, the shaft tends to work its way out of the wrench or coupling. Once it gets far enough out that the drilled section is loaded torsionally, it will break well below its rated strength (see below) In the third case, the bending moments at the joints cause gradual “belling out” of the coupling (see below) again leading to torsional loading of the drilled section and failure below rating. In all cases, the fracture surface will intersect the drilled hole. In the latter two cases, failure will usually occur below the ground line and only visual inspection of the coupling will show the difference. To avoid recurrence, make sure that the coupling bolt goes through both coupling and shaft, rather than passing above the shaft end. Pin the top shaft into the wrench during installation, and avoid misalignment along the anchor shaft or between anchor and Kelly bar.

The PISA® anchor/wrench system is designed so the wrench, when properly engaged, bridges the hollow section of the anchor shaft preventing it from having to carry any significant torsional load. If the wrench does not engage the anchor shaft sufficiently to bridge its hollow section, that section becomes the weak link in the system and fracture will occur well below the anchor’s rated torsional strength. To avoid a recurrence, you may use the same type and size anchor, but be sure to screw rods all the way into the anchor and couplings; lock the anchor assembly into the wrench with the locking dogs; and follow the anchor during

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