Foundation Stability of Wood-Pole H-Frame Structures for Transmission Lines [includes discussion]

Two cases of 138-kv H-frame line failures, one during a wind and ice storm, and one during a tornado, gave rise to an investigation and a series of tests of pole foundations and structures. The inadequacy of native soil backfill before reconsolidation during the early life of a line, especially in wet soil, was demonstrated. It was shown that a partial backfill of crushed-stone aggregate, properly proportioned with respect to fine, intermediate, and coarse particle sizes, would provide adequate foundation strength immediately after installation, at moderate cost. Such a foundation was found to be adequate to develop the full strength of an X-braced H-frame structure, with small deflection and uplift movement. Data are given on loadings, developed strengths, and deflections of structures for various foundation conditions. Tests were made in rock setting with different depths, and with both crushed-stone aggregate and weak concrete backfill. These showed that with a depth in rock two thirds of that normally used, either method of backfill develops adequate strength for H-frame structures, with substantial economy. Fiber strengths were calculated from data obtained from the poles broken during the tests. The developed fiber strengths average below American Standards Association (ASA) ratings.