Framework for Risk-Based Electric System Deep Foundation Design-Paper

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The paper proposes a risk-based framework for designing deep foundations for electric transmission systems, an industry where design practices vary widely and no unified foundation standard exists. Surveys show that many designers still rely on deterministic methods and global safety factors, while load assumptions, resistance factors, and allowable foundation movements differ among utilities.<br /><br />The framework treats both structural loads and foundation resistance as uncertain. Loads vary with weather, structural models, and material properties; resistance varies with soil conditions, design models, construction methods, and limited site data. Reliability-based design uses probability distributions and compatible load and resistance factors to achieve an appropriately low probability of failure, rather than relying on a single global factor of safety.<br /><br />Design must distinguish ultimate strength from serviceability. Foundation movement limits—such as settlement, deflection, and rotation—should be evaluated at appropriate load levels, reflecting the nonlinear load–deformation behavior of foundations. The paper notes that service loads and performance criteria are not consistently established in current practice.<br /><br />The framework also calls for evaluating environmental effects beyond loads transferred from structures, including scour, flooding, flowing ice, liquefaction, lateral spreading, and landslides. These events can add loads or reduce ground resistance, and their likelihood must be considered alongside structural loading rather than simply combined as simultaneous extremes.<br /><br />The authors recommend developing uniform guidance for both allowable-stress and reliability-based design; setting consistent reliability targets and resistance factors; improving geotechnical investigations and construction quality assurance; defining ultimate and service limit states; and establishing compatible load combinations for environmental events. They suggest that methods used in bridge engineering offer a useful model, while emphasizing the need for further research and adaptable guidance suited to the diversity of transmission systems.

Keywords

deep foundations

electric transmission systems

risk-based design

reliability-based design

load factors

resistance factors

geotechnical uncertainty

serviceability limits

environmental hazards

foundation standards