Guidance for Factoring Deep Foundation Structural Resistance for Landslide Stabilization and Excavation Support-PDF

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The 2023 Deep Foundations Institute report addresses how to include the structural resistance of passive deep foundations—such as piles and drilled shafts—in limit-equilibrium slope stability analyses for landslide stabilization and excavation support. It aims to resolve inconsistent practice around structural resistance factors, global factors of safety, and LRFD requirements.

The report recommends a distinct, coordinated design process. First, use lateral pile–soil interaction analyses, typically the p-y method, to identify the strength limit state that controls the foundation’s resistance. Possible controls include structural flexural or shear failure, soil flow-around, and lateral instability. The resistance entered into the slope analysis should be the mobilized shear force at the slide plane when the controlling limit state occurs—not automatically the pile’s full shear strength.

Apply the appropriate structural resistance factor while determining that limit state, then enter the resulting factored resistance into the slope stability analysis using <strong>Method A</strong>. Method A does not divide the structural resistance by the global stability factor of safety, allowing structural and geotechnical resistances to be factored separately. The analysis must still meet the project’s required global stability factor of safety, commonly about 1.3–1.5. <strong>Method B</strong>, by contrast, further reduces structural resistance by the global factor of safety and can lead to more costly designs.

Reliability analyses of three example projects show that additional factoring may reduce calculated failure probability, but the benefit varies. It can be negligible when no structural strength limit state is reached, or when the critical failure surface shifts away from the foundations. The report therefore does not recommend Method B as a routine means of increasing reliability.

Strength and serviceability checks should be treated separately. Conventional limit-equilibrium analysis does not predict slope deformation; deformation-sensitive projects may require carefully calibrated continuum numerical modeling. Overall, the guidance emphasizes selecting the right analysis method, clearly identifying the controlling mechanism, and coordinating geotechnical and structural design.

Keywords

deep foundations

limit-equilibrium slope stability

passive piles

drilled shafts

lateral pile-soil interaction

p-y method

structural resistance factor

global factor of safety

Method A

landslide stabilization