Case Study - Statistical Analysis of Helical Pile Tests in a Power Transmission Line Project-Paper

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The study evaluates helical piles used for transmission-tower foundations on a 592 km, dual-circuit 500 kV project in southeastern Brazil. The route crosses varied ground conditions, including weak, compressible soils, and areas with sand and rock. Helical piles were selected for certain structures because they are quick to install, adaptable to different soils, and suitable for locations with access constraints.<br /><br />The authors analyzed 12 static load tests—six in tension and six in compression—at six representative towers. They used descriptive statistics and compared measured displacements and test loads with preliminary design criteria. Tests followed Brazilian standard NBR 16903, with progressive loading over two cycles.<br /><br />Displacements increased during the second cycle in most tests, contrary to the expected reduction. The authors attribute this pattern to low soil stiffness and high compressibility. Nevertheless, all measured displacements remained within the specified limits: 50 mm in tension and 40 mm in compression. Test loads exceeded the corresponding allowable design loads by about 5–20%, indicating a margin above the design estimates. The results supported retaining the preliminary design parameters.<br /><br />The authors conclude that helical piles performed reliably under the tested conditions and are a technically robust, potentially economical and environmentally favorable option for transmission-line foundations. They also emphasize the value of in-situ testing and statistical evaluation in accounting for variable ground conditions. The findings are promising, though they derive from a limited set of validation tests; continued testing and research across a wider range of soils and structural conditions are recommended.

Keywords

helical piles

transmission-tower foundations

static load tests

Brazilian standard NBR 16903

tension and compression

ground conditions

soil compressibility

preliminary design criteria

displacement limits

transmission-line foundations