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Federal Highway Administration Research and Technology
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Publication Number: FHWA-RD-03-048
Date: September 2003

Effects of Geosynthetic Reinforcement Spacing on The Behavior of Mechanically Stabilized Earth Walls

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Chapter 1. Introduction

Mechanically stabilized earth walls (MSEWs) are retaining structures constructed of a facing and selected fill material that is stabilized with embedded reinforcement elements. The interaction of the backfill material and the reinforcement form a flexible, coherent block that can sustain significant loads and movements. In principle, the reinforced soil is analogous to the reinforced concrete, and it is logical to assume that the behavior of reinforced soil will depend on the "soil-reinforcement ratio," expressed in terms of reinforcement spacing.

The existing American Association of State Highway and Transportation Officials (AASHTO) design methodology of MSEWs is based on internal and external stability analysis using limit equilibrium methods. Internal stability calculations are based on the assumption that the most critical slip surface will develop through the reinforced soil. Therefore, in many cases, the internal stability analysis controls the wall design, because of the unnecessarily large reinforcement length specified in the preliminary sizing of the wall. This sizing corresponds to a length-to-height ratio of at least 0.7. However, internal failure can occur only when the reinforcement spacing is relatively large. The relation between the reinforcement spacing and the failure mode is not considered in current design, although there is ample evidence that closely spaced reinforcement may lead to a composite material. That is, no soil plasticity (or failure) develops within the reinforced zone. This may lead to an overly conservative design of MSEWs with continuous facing. Conversely, it may lead to a nonconservative design of MSEWs with modular block facing. The effects of reinforcement stiffness, connection strength, secondary reinforcement layers, and foundations stiffness on failure mechanisms are not involved directly in the current design of MSEWs.

Presented are the results of numerical analysis on the behavior of MSEWs with modular block facing and geosynthetic reinforcement, considering the effects of reinforcement spacing, soil strength, reinforcement stiffness, connection strength, reinforcement length, secondary reinforcement layers, and foundation stiffness. The two-dimensional finite difference program Fast Langrangian Analysis of Continua (FLAC) (Version 3.40, Itasca 1998) was used to conduct the numerical analysis. The material properties are based on data reported in the literature, which represent typical values used in design practice. A set of computer runs was conducted to identify failure mechanisms of MSEWs as a function of geosynthetic spacing. The effects of soil strength, reinforcement stiffness, connection strength, secondary reinforcement layers, and foundation stiffness on failure mechanisms were identified with respect to geosynthetic spacing. Numerical analysis also was conducted to investigate the effects of reinforcement length on reinforcement stresses and wall stability. FLAC predictions were compared with AASHTO design method. Additional numerical experiments were conducted to investigate the effects of some modeling parameters on the wall response.

The wall behavior was investigated for all relevant reinforcement spacings in the range of 0.2–1.0 m. Soil properties were changed with respect to soil strength (three types: high, medium, and low strength soil) and soil stiffness (two types: baseline and very stiff soil). Three types of connection strength were modeled: frictional connection with low strength, frictional connection with baseline strength, and structural connection. Reinforcement stiffness was modeled to represent baseline reinforcement (BR) and ductile reinforcement (DR).

The results of this work should be viewed only qualitatively. They represent a comprehensive numerical analysis of various idealized cases. However, this work provides a perspective on the effects of reinforcement spacing with clear design implications. Before applying the results to design, the numerical observation must be determined. Therefore, results can be viewed as important guidance toward more focused research.


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