Improved Design of Embedment Depths for Transmission Pole Foundations Subject to Lateral Loading

Abstract

Self-supported direct-embedded poles are widely used by the utility industry in the United States to support high-voltage transmission lines. For a typical transmission pole, the lateral loads caused by wind and ice loadings govern the design of the pole and foundation. Recent research findings reveal that the methods used in current practice do not yield consistently reliable pole foundation embedment depths in all soil types and for all possible pole classes, lengths, species, and pole loading scenarios. In order to generate improved design methods for transmission pole foundations, validated methods for analyzing laterally loaded piles have been incorporated into the current study. New reliable methods to design safe and cost-effective transmission pole foundations, incorporating both soil and pole properties, are proposed and recommendations are made in rigorous and simplified forms such that they can be easily adopted for use in the utility industry. This study found that the current methods for determining embedment depths for relatively rigid transmission poles significantly underestimate or overestimate the required embedment depths depending on the pole and soil properties by as much as ±60%. A field case study is presented, where direct-embedded round wood transmission poles fell over due to foundation failure, and the results of this case study are compared to the methods presented in this paper. The proposed methods are also compared with PLS-Caisson software predictions, and it is determined that the methods proposed in this paper are relatively easy to use and generate accurate and reliable results.