Influence of Backfill Soil Permeability on the Hydraulic–Mechanical Performance of Mechanically Stabilized Earth Walls

Recent updates to the National Concrete Masonry Association (NCMA) best-practice guidance limit reinforced backfill to a maximum of 15% fines for mechanically stabilized earth (MSE) walls, whereas earlier recommendations permitted up to 35% fines, provided plasticity criteria were met. The revised guidance reflects concerns regarding reduced drainage capacity and the potential for pore-water pressure development within reinforced-soil structures. This study develops a coupled hydraulic–mechanical evaluation framework to investigate how variations in backfill soil permeability influence the hydraulic and structural performance of tall MSE walls.

A two-dimensional finite element modeling framework is developed in GeoStudio to simulate staged construction, steady-state groundwater seepage, transient rainfall infiltration, stress–deformation behavior, and wall stability. A parametric numerical study systematically varies backfill permeability to represent materials associated with a range of fines contents, including those permitted under previous and current NCMA recommendations. Selected geometric and design parameters, including wall height and drainage configuration, are also varied to investigate their interaction with backfill permeability. Chimney and base drainage systems are explicitly modeled under regional groundwater and rainfall-induced infiltration conditions.

The framework integrates hydraulic and mechanical responses to evaluate relationships among backfill permeability, drainage and geometric characteristics, pore-water pressure, reinforcement demand, wall deformation, and overall stability. Performance measures include phreatic surface progression, pore-water pressure distribution, drainage discharge, reinforcement tensile demand, lateral wall deformation, and global stability. Results from the cases analyzed to date indicate that variations in backfill permeability have a limited influence on wall response under steady-state seepage conditions, with no substantial increase in pore-water pressure observed as permeability decreases. In contrast, chimney and base drainage reduce wall deformation and reinforcement demand, highlighting the importance of drainage provisions in controlling wall response.

The study provides a mechanistic basis for evaluating the influence of backfill permeability, geometry, and drainage on MSE wall performance. The ongoing parametric analysis will further identify conditions under which reduced permeability becomes critical and quantify the effectiveness of drainage provisions in mitigating adverse hydraulic–mechanical responses. The resulting performance-based framework is intended to support future refinement of MSE wall design guidance where hydraulic performance governs long-term behavior.