Encyclopedia of Anchoring (CA06114E)

Installation torque can be an excellent indication of anchor load capacity, if you follow the rules. Otherwise, it can be misleading. Basically, it is an indication of the effort necessary to compress and shear the soil around the anchor to allow penetration. Load capacity, of course, is also a measure of the effort necessary to compress and shear the soil, so it should come as no surprise that the former can be used to predict the latter. One major difference between the two which must be taken into account, though, is that only the soil in the immediate vicinity of the helices affects their installation torque, while their load capacity is affected by a much larger volume of soil located either above, for tension anchors, or below, for compression anchors. This is why we recommend that installation torque values over the final three diameters of penetration be averaged to determine load capacity. If only the final torque is used, a tension anchor which has just passed from a softer layer to a harder one will be overestimated because the softer soil above will not affect the helices torque but will affect their load capacity. Averaging torque is also important for compression anchors, but because compression capacity is affected by soil which lies below the anchor and has not had the opportunity to affect installation torque, unexpected results may still come. An anchor may fail to hold a given load for one of two reasons: Either the anchor fails structurally or the soil fails around it. Sometimes an anchor failure is accompanied by a sudden movement of the shaft or rod and/or some audible indication. Other times it is not. Soil failures on the other hand, are usually recognizable by gradual movement of the shaft and absence of any audible indication (one exception being soil failure when the upper helix is less than three diameters deep, which is usually characterized by eruption of the soil at the surface). One structural failure mode which occurs occasionally is bending of the helix under tension of compression loading. When this occurs, the answer is to use anchors with more helices to share the load or stronger ones to withstand the high stresses. With PISA® anchors, the rod is often the weak link. Failure may occur by fracture, thread stripping, or, if the application is controlled by deflection, rod yield. If a stronger rod is available, use it. If not, consider using an SS anchor.

between the hole and the end if the bolt strength happens to be significantly above minimum.

Soil failure can be cured by using more or larger helices to spread the load out over a larger volume of soil. Soil failure and helix bending usually give the same indications at the surface, so it becomes necessary to recover the anchor and observe the helices to differentiate between them. However, it may not be possible to unscrew the anchor in such cases because the disturbed soil or bent helix cannot generate the necessary axial thrust. In such cases, use an anchor with more helices because this will cure either problem. Remember, our experience indicates that 95 out of 100 CHANCE screw anchors are smoothly and successfully installed. The techniques we’ve shared with you can help you diagnose and solve any anchoring problems that you encounter and move you closer to the goal of a successful anchor installation.

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With SS anchors, the weak link is normally the coupling bolt, although one does occasionally see the shaft split on its axis

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