Encyclopedia of Anchoring (CA06114E)

soil seems homogeneous and the alignment is maintained properly, try a smaller or higher-strength anchor. Remember that installation torque is an indication of soil strength, so if the torque is higher than expected, the soil must be stronger than expected and a smaller anchor should develop the load. Another problem sometimes encountered is anchor “spinout”, or rotation without axial penetration. As an anchor is rotated in the soil, the inclined plane of each helix works against the soil producing a thrust which tends to move it axially. Under perfect conditions, it will advance one pitch length per revolution and soil disturbance will be minimal. If the anchor advances more or less than one pitch per revolution, something has to give. Either the soil gets churned (likely), or the helix gets bent or torn off (not so likely). Spinout can result from several different conditions requiring different corrective actions. You’ll have to rely on observations of installation conditions and anchor damage to guide you. Take the easy case first. If the operator fails to follow the anchor so that the digger holds back on the anchor instead of leading it, the anchor may be unable to advance at the proper rate. The soil loses strength due to the resultant churning and becomes unable to work effectively against the helical plate. Because neither the soil nor the digger is now providing the thrust, the anchor ceases to penetrate. If you’re lucky, simply applying down pressure to the anchor will get it started again. Sometimes, however, the undisturbed soil below the anchor is so strong that the down pressure is not enough to restart the anchor. It may be that you can back the anchor out because the soil above it will be weaker. If not, all you can do is abandon the anchor and start over. Either way, the next step is to move over a few feet and try again, being sure to maintain crowd this time. Unfortunately maintaining down pressure is not a cure all. In glacial tills and other obstruction-laden soils, too much crowd can cause spinout. An anchor should be allowed to work its way through such soils with minimal down pressure. If you try to force it through, chances are fairly good that the helix leading edge will get bent (destroying its helical form and the attendant thrust) or torn off. You may or may not be able to retrieve the anchor, depending on how badly it is damaged, but you will have to replace it regardless. You might try again, using extra care to maintain just enough down pressure to keep it penetrating. If you fail again, try an anchor with smaller or stronger (i.e., thicker or higher-strength material) helices. Or, consider CHANCE Tough One® anchors. They’re designed for difficult soils.

Also remember it’s in these obstruction

laden soils that the curvilinear

leading edge really shines thanks to its tendency to guide the anchor around obstructions without hanging up and its greater resistance to bending. So if you’re using anchors with straight leading edges and a curvilinear equivalent is available, try it. Even if the operator maintains good control of

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crowd, keeping the anchor advancing at one pitch length per revolution, unforeseen soil conditions can still lead to spinout. The installation may be progressing nicely with little or no down pressure required when the anchor can unexpectedly encounter a hard stratum or even a large rock or other obstacle and the resistance to penetration shoots up becoming greater than the combined crowd and thrust. The anchor may spin out before the operator can react and then refuse to start advancing again. In such cases you can usually retrieve the anchor. Look for abrasion or gouging on lower surfaces to confirm the problem, then move over and try again. This time be prepared to apply heavy down pressure on the anchor at the first sign of extra resistance. If it still doesn’t go, try using more, smaller helices or, if the problem area isn’t too thick, predrilling through it. In the latter case, be sure the upper helix is driven at least five times its diameter deeper than you predrilled.

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