Pile Stability Analysis in Soft or Loose Soils: Guidance on Foundation Design Assumptions with Respect to Loose or Soft Soil Effects on Pile Lateral Capacity and Stability
Wang, Y., Lim, J., Salgado, R., Prezzi, M., & Hunter, J. (2022). Pile stability analysis in soft or loose soils: Guidance on foundation design assumptions with respect to loose or soft soil effects on pile lateral capacity and stability (Joint Transportation Research Program Publication No. FHWA/IN/JTRP-2022/24). West Lafayette, IN: Purdue University. https://doi.org/10.5703/1288284317387
The design of laterally loaded piles is often done in practice using the p-y method with API p-y curves representing the behavior of soil at discretized points along the pile length. To account for pile-soil-pile interaction in pile groups, AASHTO (2020) proposes the use of p-multipliers to modify the p-y curves. In this research, we explored, in depth, the design of lateral loaded piles and pile groups using both the Finite Element (FE) method and the p-y method to determine under what conditions pile stability problems were likely to occur. The analyses considered a wide range of design scenarios, including pile diameters ranging from 0.36 m (14.17 inches) to 1.0 m (39.37 inches), pile lengths ranging from 10 m (32.81 ft) to 20 m (65.62 ft), uniform and multilayered soil profiles containing weak soil layers of loose sand or normally consolidated (NC) clay, lateral load eccentricity ranging from 0 m to 10 m (32.81 ft), combined axial and lateral loads, three different pile group configurations (1×5, 2×5, and 3×5), pile spacings ranging from 3 to 5 times the pile diameter, two different load directions (“strong” direction and “weak” direction), and two different pile cap types (free-standing and soil-supported pile caps). Based on the FEA results, we proposed new p-y curve equations for clay and sand. We also examined the behavior of the individual piles in the pile groups and found that the moment applied to the pile cap is partly transferred to the individual piles as moments, which is contrary to the assumption often made that moments are fully absorbed by axial loads on the group piles. This weakens the response of the piles to lateral loading because a smaller lateral pressure is required to produce a given deflection when moments are transferred to the head of the piles as moments. When the p-y method is used without consideration of the transferred moments, unconservative designs result. Based on the FEA results, we proposed both a new set of p-multipliers and a new method to use when moment distribution between piles is not known, using pile efficiency instead to calculate the total capacity of pile groups.
lateral loading, pile buckling, pile stability, p-y method, p-y curves API, laterally loaded piles, pile groups, p-multipliers, pile efficiency
Joint Transportation Research Program
Indiana Department of Transportation
West Lafayette, IN
Date of this Version