Helical Piles: Rupture after five years of installation-PPT Presentation

PPT Presentation

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The presentation examines repeated failures of helical-pile guy anchors on a 328-km transmission line in Brazil. Of the project’s 820 structures, 414 used helical-pile anchors. After installation and initial testing, piles began rupturing and moving during service, causing tower-mast misalignment and tensile stress on insulators, with potential risk to power-line operation. Some recorded displacements reached about 200 mm.<br /><br />A consultancy reviewed structural and geotechnical design, installation records, field observations, and soil data. Its analysis found that calculated geotechnical capacities at investigated sites were substantially below the required tension loads. Installation torque requirements also conflicted with the piles’ structural torsional limits. Structural checks identified concerns about shaft torque capacity, while differences in shaft dimensions and steel-strength assumptions affected the results. SPT records from different dates showed significant variations in soil resistance, and installation torque trends indicated that piles could approach structural limits before reliably demonstrating adequate capacity.<br /><br />Laboratory testing identified substantial collapse potential in sampled soils. The region’s wind-deposited silts and fine sands are generally dry, but heavy rainfall and wetting can weaken their structure and contribute to settlement or anchor movement. The presentation emphasizes that changing moisture conditions, soil variability, inadequate geotechnical verification, design assumptions, and possible misunderstandings of safety factors may all have contributed to failures.<br /><br />The proposed reinforcement uses two new helical piles connected by a transition beam to act as a guy anchor, without relying on the capacity of the existing pile. The authors stress the importance of engineering-geologist involvement, appropriate soil-collapse testing, adequate pile embedment and torsional design, and attention to rainfall-related moisture changes.

Keywords

helical-pile anchors

transmission line

anchor failure

geotechnical capacity

torsional limits

tower misalignment

soil collapse potential

SPT records

rainfall moisture changes

reinforcement design