Encyclopedia of Anchoring (CA06114E)

Yet another way an installation attempt may fail is refusal where the torque required for continued penetration exceeds the capacity of the digger, but not that of the anchor. In such cases everything comes to a halt. As with the previous cases, there is not a single, universal fix for this circumstance. The next step depends on the torque at refusal, whether higher torque is available (by bringing in another machine for instance), the depth at refusal, possible soil stratification, and anchor availability. Again, take the easy case first. If the top helix is at least five diameters (that is, a distance equal to five times its own diameter) below the ground surface, three diameters into the current soil stratum, and three diameters below the level of seasonal change in soil properties, and the installation torque was above the minimum required to achieve the desired load capacity during the final three diameters of penetration, consider leaving well enough alone even though the rod or shaft may be sticking further out of the ground than desired. Replacing the top rod or shaft section, even if it requires some digging, may be better than the other alternatives. If on the other hand, the installation does not meet all of the above criteria, things can get pretty sticky. If the torque you attained was 75% or less of the anchor’s torque rating, bringing in higher-torque equipment is worth considering. Otherwise you run the risk of bringing in the new equipment and getting another foot or two of penetration, then having to shut down to keep from over-torquing the anchor. If bringing in a higher-torque machine is not feasible, consider predrilling. Particularly on round-shaft anchors, predrilling a hole slightly larger than the shaft size can significantly reduce installation torque with little affect on axial capacity. This approach is not as useful with anchors subjected to lateral loads, however, because lateral capacity and stiffness may be reduced.

If none of the previous suggestions does the trick, contact your CHANCE representative. In some limited situations we may be able to recommend a larger anchor which can develop the required load capacity at a lower torque. Otherwise, we will help you select an alternate type of anchor for the job.

Even if you manage to get past all the previously mentioned pitfalls, there is still more criterion for success. The anchor has to hold the load you designed it for.

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Operator and groundworker working together are critical to a successful anchor installation. PISA ® 4 anchor shown below.

Again, if you do predrill, be sure to drive the top helix at least five diameters below the predrilled depth.

If the soil contains obstructions, it is possible that the anchor just got “hung up” on something. Often it is possible to back up and then work your way past the obstruction. In this case, as in the previous one of anchor breakage, it is better to use more, smaller helices than fewer, larger ones, and a curved leading edge on the anchor can be very useful.

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