Transmission Line Foundation Design Using Inclined Earth Anchors-PPT Presentation

PPT Presentation

Pdf Summary

The presentation describes the use of inclined earth anchors to strengthen transmission-line foundations where large wire loads and weak soils make conventional drilled shafts inadequate or excessively large.<br /><br />Heavy-angle and dead-end monopoles can generate far greater foundation forces than tangent poles. For example, a 345-kV, 100-foot heavy-angle pole may produce a factored moment of about 20,900 ft-kips, compared with about 2,600 ft-kips for a tangent pole. Loads arise from conditions such as ice, wind, and unbalanced stringing or broken wires. Where weak soil overlies stiff clay or shale, a drilled pier alone may also deflect beyond utility limits.<br /><br />The proposed system combines a conventional drilled pier with inclined, high-strength steel earth anchors. Anchors extend from a corbel on the foundation into stronger soil or bedrock, helping resist the large lateral loads and control foundation deflection. The approach is suited to heavy-angle and dead-end structures, constrained rights-of-way, monopole designs, and weak-soil sites. Anchors can remain within the right-of-way and require a relatively small footprint.<br /><br />Construction involves installing the drilled pier and reinforcing, forming and pouring the foundation with corbels, then casing and inserting the coated steel anchors. The anchors are pressure-grouted, fitted with bearing plates, proof-tested to the design tension, and locked off with nuts. Protective grease is added for corrosion resistance. One example uses an 80-foot anchor designed for a 140,000-pound tension load. The completed foundation is intended to provide a 100-year service life.<br /><br />Although used selectively—18 tieback foundations among 2,219 foundations on one recent project—the system offers an alternative to oversized drilled shafts when loads, soil conditions, or site constraints demand additional foundation capacity.

Keywords

inclined earth anchors

transmission-line foundations

heavy-angle monopoles

dead-end structures

weak soils

drilled pier

lateral loads

corrosion resistance

proof-tested anchors

foundation deflection