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Effects Of Geosynthetic Reinforcement Spacing On The Behavior Of Mechanically Stabilized Earth Walls
Chapter 6. Implications To Design
The existing design methodology of MSEWs is based on internal and external stability analysis using limit state methods. Internal stability calculations are based on the assumption that the most critical slip surface will develop through the reinforced soil. This can happen only when the reinforcement spacing is relatively large, and the reinforced soil does not behave as a solid coherent block. However, the relation between the reinforcement spacing and the failure mode is not considered in current design.
With respect to existing design methods for MSEW with modular block facing and geosynthetic reinforcement, the numerical results imply the following:
- MSEW design should use the design methodology of reinforced steep slopes. The identified MSEW failure modes numerically corresponded to the failure mechanisms considered in stability analysis of reinforced steep slopes using limit equilibrium methods: two-part wedge mechanism considered in direct sliding analysis; rotational mechanism considered in deep-seated stability analysis and in compound stability analysis; and log-spiral failure mechanism considered in internal stability analysis (tieback analysis). A comparison between FLAC predictions and MSEW 1.1 calculations according to AASHTO design method showed good agreement between the results. The existing design method was capable to distinguish the modes of failure identified by FLAC analysis especially these due to external instability.
- The critical slip surface always develops behind the reinforced soil when the reinforcement spacing is less than 0.4 m, and the reinforcement stiffness and soil strength are not very low. For commonly used in design values of the reinforcement stiffness and soil strength, no slip surface developed entirely in the reinforced soil, and the reinforced soil behaved as a composite block.
- The length-to-height ratio is lower than 0.7 (the value used in current static design) for walls with small reinforcement spacing on competent foundation. In the analysis, it was identified in the range of 0.23 –0.34 at the critical state for cases with competent foundation and small reinforcement spacing, The critical length-to-height ratio was larger than 0.4 only for cases with reinforcement spacing larger than 0.6 m and medium or low soil strength. Values close to or larger than 0.7 were identified only for cases that experienced connection mode of failure. However, in these cases, the wall stability is controlled by other factors and not by the reinforcement length (or length-to- height ratio, respectively).
- The performance of walls with large reinforcement spacing can be improved significantly by increasing the strength of connections (e.g., by using structural connections instead of frictional connections).
- The performance of walls with large reinforcement spacing can be improved significantly by using secondary reinforcement layers at each connection between the primary reinforcement layers. The secondary reinforcement layers have the same effect on wall behavior as the increase of connection strength.
- When the foundation is not competent, the wall stability can be improved by either increasing reinforcement length or decreasing reinforcement spacing.
- The slope of the critical slip surface identified numerically was less than the values calculated by Rankine's and Coulomb's earth pressure theories. For all cases, slip surface slope was closer to the value given by Coulomb's theory, and decreased with failure progression.
- Small reinforcement spacing in MSEW with modular block facing should be considered in cases of incompetent foundation, space constraints, low strength, or frictional connection between modular blocks, and lateral movement limitations.
It must be noted that the design implications mentioned above are based on the numerical results of current study. Further parametric studies that implement experimental data from laboratory and large-scale testing must be conducted to quantify the effects of connection strength, reinforcement stiffness and soil properties on the behavior of MSEWs.
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