Slope stability enhancements through soil arching

Earth embankments built over soft foundations require the use of ground improvement measures of the foundation such as installation of rigid inclusions or piles. The vertical piles improve the settlement and the overall stability of the embankments. The analysis of pile-supported embankments is a complex 3D problem, and is often designed using empirical methods. This dissertation was divided into two main components. The first component of this dissertation involved the load transfer mechanism for vertical support of the embankment. The second component focused on the slope stability of pile-supported embankments. The outcome of this work will help improve analyses and design of pile-supported embankments built over soft foundations. The first component included a critical review of different soil arching methodologies used to quantify vertical stress reduction effects for pile-supported embankments over soft foundations revealed that existing models show stress reductions ratio due to soil arching ranging from 15% to 45% of the overburden stress. A comprehensive parametric study was used to evaluate the existing soil arching methods and to resolve discrepancies and to develop a new robust load transfer factor that can be confidently applied in design for estimating the vertical stress reduction ratio. The second component focused on evaluating the contribution to the slope stability associated to the additional lateral load contribution through lateral arching effects between piles if these are spaced sufficiently close. Analyses of the slope stability of pile-supported embankments are commonly modeled as an equivalent 2D plane strain problem where the amount of lateral resistance contributed by the piles is typically estimated based on the available lateral load capacity of a single pile divided by the center-to-center spacing of the piles without consideration of soil arching. Experimental studies have shown that if the piles are spaced close enough additional lateral load resistance can be developed through soil arching between the piles. Neglecting soil arching effects can be exceedingly conservative for embankments stabilized with closely spaced piles. The typical design practice for pile-stabilized embankments is to perform 2D, plane strain limit equilibrium approach using an iterative decoupled procedure. First, the stability of the soil slope is evaluated without the piles to ascertain the need for pile reinforcement. Second, the required shear strength and spacing of the piles is iterated until satisfactory stability is predicted. The lateral load contribution of each pile is predicted using laterally loaded single pile p-y analyses, which often require considerable iterations to provide agreement with the decoupled analyses. Incorporation of the contribution of soil arching is challenging and problematic in this commonly used approach, as the lateral pile resistance is estimated using analyses of a single pile. This research developed a modified computational procedure that allows consideration of the contribution of soil arching effects when piles are spaced sufficiently close. The developed approach is based on finite element analyses that allow estimating the contribution to the lateral resistance due to soil arching between the piles. The proposed approach allows incorporation into the 2D limit equilibrium analysis of the contribution due to soil arching. The research presentation will include examples to illustrate the higher stability that can be realized by considering the contribution of soil arching.