Date of Award

Spring 5-31-2025

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Civil Engineering

First Committee Advisor

Aaron Gallant

Second Committee Member

Luis Zambrano-Cruzatty

Third Committee Member

Eric Landis

Additional Committee Members

Bill Davids

Michael McGuire

Abstract

Adopting unreinforced rigid columns to support earthwork systems has become a common approach to accelerate construction under challenging subsoil conditions. However, evaluating global stability for these systems using conventional limit equilibrium (LE) methods remains elusive, often violating the complex changes of soil stresses at depth due to the soil and column interaction. Additionally, conventional LE methods typically neglect column failure modes and rely on column strength to provide stability.

This dissertation proposes a rational methodology for evaluating the global stability of reinforced column-supported earthwork systems, integrating the assessment of factor of safety and lateral deformation at the toe. A new limit equilibrium model called LE-RISE (Limit Equilibrium model for Rigid Inclusions Supported Earthwork) is developed, which explicitly capturing the fundamental physical mechanisms governing these systems. LE-RISE is founded on three fundamental assumptions: 1.) columns do not resist lateral load, but modulate vertical stresses imposed on foundation soils; 2.) The slip surface geometry is simplified into three distinct wedges: passive, shear, and active; and 3.) the global stability is evaluated using the strength reduction method, whereby horizontal equilibrium is achieved by reducing the available shear strength of the soil, consistent with conventional global stability analyses.

To address the complex changes of soil stresses, the Load Displacement Compatibility Extension (LDCE) is introduced. LDCE integrates stress-displacement compatibility associated with soil arching and subsurface load transfer by employing t-z and q-z curves to characterize interface column friction and end-bearing resistance, respectively. The resulting changes in vertical stress are subsequently utilized in the LE-RISE model to primarily compute lateral driving stresses.

LE-RISE is verified through comparisons with existing simplified LE models and validated with a comprehensive 3D finite element (FE) parametric study of hypothetical embankments. Complex changes of vertical stresses at depth in the reinforced zone predicted by the LDCE are verified through comparisons with existing analytical models, and validated with field observations of the CBIS field case study, and the FE parametric study. Comparative predictions of the vertical stress changes, factors of safety, lateral deformations at the toe, tensile stress in the geosynthetic reinforcement, and critical slip surface locations demonstrate strong agreement among the proposed models, field observations, and parametric FE results. LE-RISE provides engineers with a tractable approach for making physically meaningful and computationally efficient predictions. These methodologies are expected to enhance communication and decision-making among engineers, clients, and contractors.

Comments

To aid with the designing reinforced column-supported earthwork systems, a standalone software has been developed. The software computes the changes of vertical stresses using the LDCE, and then compute the factor of safety using the LE-RISE. Reinforcement tensile can also be predictive using the framework in this study. For more information about the software contact dboterol24@gmail.com, danilo.botero@maine.edu, or aaron.gallant@maine.edu.

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