Bridge Design Soil-Structure Interaction Series Post#3: Abutment Piles in Lateral Spreading Areas

When designing bridges in seismically active regions with liquefiable soils, understanding how lateral spreading affects bridge abutments and piers is crucial. This article examines the complex interaction between bridge superstructures and pile foundations in lateral spreading zones, with a focus on practical approaches for single- and two-span bridges (Figure 1).

Figure 1. Bridge deck resistance and pile pinning forces affecting abutment and pile response

Practical Approaches for Analysis

For single- and two-span bridges, design budgets typically don't accommodate extensive soil-structure interaction analyses. Consequently, engineers often employ decoupled approaches using specialized software like LPILE to assess pile foundations. In North America, the prevalent practice follows guidelines developed by Caltrans, which extend the methodology initially outlined by the Pacific Earthquake Engineering Research Center (PEER).

Caltrans/PEER Methodology

The Caltrans guidelines, detailed in their 2017 Technical Memo for Designers and its 2020 update, represent a significant advancement in this field. These guidelines benefit from contributions by prominent scholars and practitioners and are widely adopted by transportation agencies due to their ability to model complex phenomena using simplified techniques and user-friendly software.

A critical advantage of this approach is its consideration of the bridge superstructure's influence on pile response. Case histories have consistently demonstrated the significant impact of the superstructure on bridge abutment and pile behavior in liquefaction-induced lateral spreading zones. Compared to free-field movements, superstructure resistance and pile pinning effects can substantially reduce lateral spreading displacement near bridge abutments.

Limitations in Current Practice

Despite these advancements, current methodologies present significant limitations in their treatment of superstructure resistance (deck resistance). The deformation patterns and pile demand predicted by these methods can deviate substantially from those observed in actual earthquakes and those derived from more sophisticated numerical analyses.

For instance, lateral spreading typically induces pile displacement towards the river, while the bridge deck simultaneously restrains pile head movement. This interaction results in a characteristic pile deformation pattern commonly referred to as "back-rotation" of abutment piles (Figure 2), a phenomenon frequently observed in post-earthquake assessments.

Figure 2. Schematic showing the back-rotation of piles

 Back-Rotation Phenomenon

The back-rotation mechanism subjects the bridge deck to significant compressive forces, potentially leading to buckling failure. In contrast, current practice often assumes unrestrained pile head movement towards the river, mimicking fixed or free pile head conditions. This scenario, requiring the entire bridge to displace against the opposite abutment, is inconsistent with observed earthquake behavior.

The back-rotation phenomenon alters both the magnitude and location of maximum bending moments in piles, deviating from predictions based on conventional pile head fixity assumptions. For example, back-rotation induces tension cracking on the river side, contrasting with the land-side tension predicted under free pile head conditions. Appropriate structural analysis must capture this behavior for realistic damage and service quantification.

Key Considerations for Engineers

When analyzing abutment piles in lateral spreading areas, engineers should:

• Consider superstructure effects: Account for the restraining influence of the bridge deck on pile head movement.

• Recognize back-rotation patterns: Understand that the characteristic back-rotation of piles differs significantly from conventional assumptions.

• Assess actual bending moment distribution: Evaluate how back-rotation alters the location and magnitude of maximum bending moments in piles.

• Validate with observed behavior: Compare analytical results with documented post-earthquake observations to ensure realistic predictions.

  
  

Looking Ahead

Subsequent discussions will explore methodologies to mitigate these limitations by refining analysis techniques to incorporate soil-pile-deck interaction. The next article will present observed pile deformation patterns from past earthquakes, highlighting the prevalence of back-rotation and its implications for bridge design.

References:

• Ashford, S. A., Boulanger, R. W., & Brandenberg, S. J. (2011). Recommended design practice for pile foundations in laterally spreading ground (PEER Report 2011/04). Pacific Earthquake Engineering Research Center, University of California, Berkeley.

• Caltrans (2017). Memo to Designers 20-15: Lateral Spreading Analysis for New and Existing Bridges, dated May 2017.

Authors:

Saqib Khan, P.Eng., SE, M.A.Sc., is Principal Engineer at Spannovation

Lalinda Weerasekara, Ph.D., P.Eng. is Principal Geotechnical Engineer at ECORA