J. Erik Loehr
About the research
A-walls are retaining structures composed of regularly spaced deep foundation elements battered in opposing directions and connected through a grade beam to mitigate movements of a slope or embankment. Analysis of A-walls for slope stabilization applications is challenging because of complex interactions between deep foundation elements and moving soils. A previous method was successful in modeling A-walls with consideration of both lateral and axial load transfer, but interaction between upslope and downslope A-wall elements through the capping beam is neglected in the “uncoupled” analysis. To evaluate the effect of coupling, the research team analyzed slopes stabilized with A-walls using finite element models with upslope and downslope piles connected at the pile heads. Results of the analyses were compared to those of uncoupled lateral and axial analyses utilizing the p-y and t-z methods. Load transfer parameters for the analyses were calibrated to field measurements of load transfer in A-walls to demonstrate viability of the revised methodology. Results of the coupled analyses were then compared to results from uncoupled analyses to evaluate the effect of interaction between upslope and downslope piles.
Coupled analyses produced bending moment and axial force profiles in reasonable agreement with measured values. Calibration of p-y and t-z curves to achieve predictions consistent with field measurements required significant softening of ultimate lateral and axial resistance values, but the softening was less than that required for calibration of uncoupled analyses. Modeling of interaction through the capping beam resulted in more reasonable calibrated values of lateral and axial soil resistance, better agreement with measured axial force profiles, and better agreement with measured bending moments and axial forces at shallow depths. The results indicate that interaction facilitated by the capping beam has a significant effect on development of forces within the A-wall elements. For deep sliding, reasonable predictions of pile resistance can be achieved using uncoupled models. However, for shallower sliding, the capping beam is likely more consequential and a coupled analysis is prudent. Coupled analysis is also beneficial for structural design of the capping beam.
Both calibration case histories involved relatively deep sliding and relatively small values of total soil movement. Additional analyses with new datasets from A-wall applications for shallower slides and greater movement are recommended.
Deep Foundations Institute ($18,700.00)
Midwest Transportation Center
University of Missouri – Columbia ($6,100.00)
Contract Number: DTRT13-G-UTC37
J. Erik Loehr
About the research
A-walls are a slope stabilization technique involving a series of deep foundation elements installed in a failing or marginally stable slope using an A-shaped pattern of alternating inclinations. A-walls have been used to successfully stabilize several noteworthy slopes, but analysis of the walls is complicated by several complex load-transfer mechanisms. These complications have dissuaded engineers from using the promising approach.
The Deep Foundations Institute (DFI) has funded a study by the University of Missouri to improve analysis techniques for A-walls using computer code developed by the university. The MTC effort will be used to develop improved design practices for A-walls, which will be valuable for transportation agencies that face significant risks and costs associated with slope instability. The objective of this project is to develop improved design guidance for A-walls, which will be a valuable tool for departments of transportation (DOTs).