Engineering and Construction

Helical anchors have been introduced previously. There are two possible methods that can be used to determine the ultimate bearing capacity of a helical anchor, as follows (A. B. Chance Co., 1989):

1. Bearing plus cylindrical shear method. This method is based on determining the ultimate bearing capacity of the projected area of the lowest helix. In addition, there is frictional resistance of a cylinder of soil with a diameter equal to the average diameter of the remaining helices and a length equal to the distance from the top helix to the bottom helix.

2. Bearing capacity method. This method assumes that the ultimate bearing capacity of the anchor is equal to the sum of the capacities of individual helices. Thus each helix bearing capacity is calculated and then summed up in order to obtain the total ultimate bearing capacity.

It has been stated that when the helices are spaced quite close (such as 6 in. apart), the Bearing Plus Cylindrical Shear Method is applicable. However, when the helix spacing is great (such as 10 ft apart) and all the helices are in the bearing strata, then the Bearing Capacity Method should be used (A. B. Chance Co., 1989). The allowable axial load capacity is typically determined by applying a factor of safety of 3 to the ultimate capacity calculated by using either method outlined above.

The allowable axial load capacity must not exceed the “safe limit” specified for a particular type of helical anchor that is selected for use. In addition, the shaft of the helical anchor must be strong enough to resist the applied axial load and withstand the estimated torque required to install the anchor. Because helical anchors are rather flexible, they typically have a much lower capacity when resisting lateral loads.