Effects Of Geosynthetic Reinforcement Spacing On The Behavior Of Mechanically Stabilized Earth Walls
Chapter 7. Conclusions And Recommendations
The results of numerical analysis of MSEWs with modular block facing and geosynthetic reinforcement using program FLAC have been presented. The emphasis was to identify the effects of reinforcement spacing on wall behavior, considering the effects of soil strength, reinforcement stiffness, connection strength, secondary reinforcement layers, foundation stiffness, and reinforcement length. Parametric studies were conducted on numerical models constructed, layer by layer, up to failure under gravity loading with reinforcement spacing in the range of 0.2–1.0 m, 3 soil types with different soil strength, 2 different foundation conditions with respect to soil stiffness, 3 different connection conditions, and 2 reinforcement stiffness values. The material properties were based on literature data representing typical values used in design practice. The reinforcement length was kept constant, equal to 1.5 m in models investigating the wall response with respect to failure. It was increased to correspond to length-to-height ratio in the range of 0.3–0.5 in models investigating the effects of reinforcement length on wall stability. The most important conclusions are summarized below:
- Four MSEW failure modes were identified numerically: external, deep-seated, compound, and connection mode. They correspond to the following 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).
- The reinforcement spacing was a major factor in controlling the behavior of MSEWs. Either increasing reinforcement spacing from 0.2 to 1.0 m or decreasing soil strength decreased the wall stability and changed the failure mode from external or deep-seated to compound and connection mode. Two types of spacing were considered: small (less than or equal to 0.4 m) and large (larger than 0.4 m). The failure of walls with large reinforcement spacing was always accompanied by some degree of instability within the reinforced mass. The critical wall height (defined as a general characteristic of wall stability) always increased when the reinforcement spacing decreased. The only exception was observed when the failure was controlled by the strength of foundation soil and the wall failed as a result of deep-seated sliding. In this case, the critical wall height remained the same for models, with reinforcement spacing equal to 0.2 m and 0.4 m (case 10, figure 4.1–b).
- The behavior of walls with small reinforcement spacing was similar to the behavior of a conventional retaining wall. The identified modes of failure were external or deep-seated mode. The analysis of displacement fields, failure zone distributions, grid distortion, and horizontal displacement distributions confirmed that the reinforced soil was internally stable and moved as a block.
- All walls with large reinforcement spacing experienced internal instability to some degree. The predominant mode of failure was the connection mode. The reinforced mass did not move as a block. Movie analysis of failure zone distributions showed that failure zones evolved first in the reinforced soil, initiating large deformations that led to connection breakage. At the critical state, the predominant part of the reinforced soil was at yield in shear or volume, while the backfill was affected minimally. At failure states, the failure zones propagated within the backfill because of the large deformations at the facing. The analysis of displacement fields, grid distortions, and horizontal displacement distributions showed significant deformations in the reinforced soil.
- The change of critical wall height with respect to soil strength demonstrated the effect of soil strength on wall stability. The critical wall height decreased as soil strength decreased. An important observation was that, with smaller reinforcement spacing, the same or higher critical wall height can be achieved with lower strength soil. The effects of soil strength on failure mechanisms were different for the cases with very stiff foundations and the cases with baseline foundations. For the cases with very stiff foundations, deep-seated failure mode was not identified. The decrease of soil strength changed the mode of failure from external to compound mode, or from compound to connection mode.
- Most of the models experienced connection mode of failure when the reinforcement spacing was equal to 0.6 m or larger. The only exception was observed for the cases with reinforcement spacing of 0.6 m, high strength reinforced and backfill soil, and very stiff foundation, which experienced compound mode of failure.
- The decrease of reinforcement stiffness affected the failure mode and stability of walls with small reinforcement spacing by increasing the wall displacements and the possibility of internal instability. For all cases with DR, the critical wall height was less than the critical wall height of the corresponding cases with BR, and the maximum forces in reinforcement were identified either at the connections or close to them. The maximum horizontal displacements for the cases with DR were approximately 2.5–3 times larger than the maximum horizontal displacements of the corresponding cases with BR.
- Connection strength appeared to have insignificant effects on the behavior of walls with small reinforcement spacing. However, it affected the behavior of walls with large reinforcement spacing, i.e., the increase of connection strength decreased wall displacements, improved wall stability, and changed the failure mode. The analysis of cases with high strength soils, very stiff foundations, and different connection strengths and reinforcement spacing showed that the connection strength effects on wall behavior were significant for the cases with large reinforcement spacing. The increase of connection strength of walls with large reinforcement spacing improved global wall stability (i.e., critical wall height increased), decreased wall displacements, and, as a result of the improved local stability at the wall facing, the mode of failure changed from connection to compound. The wall with structural connection experienced compound mode of failure, while the wall with baseline frictional connection experienced connection mode of failure. The change of connection strength did not affect the behavior of walls with small reinforcement spacing significantly. The model with the low strength frictional connection developed larger reinforcement forces and displacements than both the model with structural connection and the model with high strength frictional connection, but the mode of failure and the critical wall height remained the same.
- Foundation stiffness had significant effects on wall response. A very stiff foundation was investigated and compared to foundations with varying strengths. Decreasing the foundation stiffness or strength decreased the wall stability, changed the mode of failure, and increased the displacements and reinforcement forces. For all cases with very stiff foundations, the bottom reinforcement layers were less stressed. Effects of foundation stiffness on failure mechanisms were investigated by changing the stiffness and strength of foundation soil. Foundation properties had significant effects on wall response. Decreasing foundation stiffness or strength decreased the critical wall height, changed the mode of failure, and increased the displacements and reinforcement forces. The artificially high foundation soil stiffness and strength prevented the development of deep-seated mode of failure and increased wall stability. When the foundation soil was changed from baseline to very stiff soil, the cases with small reinforcement spacing that experienced deep-seated mode of failure changed their mode of failure to compound mode. The walls with large reinforcement spacing were more sensitive to the change of foundation properties.
- Increasing the reinforcement length improved wall stability and decreased wall displacements and reinforcement forces. Analysis of stable states of models representing external, compound, and deep-seated failure modes identified that increasing reinforcement length increased wall stability, and decreased wall displacements and reinforcement forces. Increasing the reinforcement length of models that experience connection mode of failure does not affect wall stability.
- Introducing secondary reinforcement layers in a model with large reinforcement spacing changed the mode of failure from connection to compound, improved global wall stability and local stability at the facing, and decreased the displacements and reinforcement forces. Introducing of secondary reinforcement layers in a model with large reinforcement spacing changed the wall behavior significantly. The following effects were identified: mode of failure changed, global wall stability and local stability at the wall facing increased, and displacements and reinforcement forces decreased. The secondary reinforcement layers improved the wall's global stability by improving the local stability at the facing.
- Analysis of models with different soil dilatancies showed that the mode of failure did not change. The model with zero dilation was less stable and experienced larger deformations and larger reinforcement forces. The failure zones were located in narrower bands in the model with zero dilatancy, compared to cases with baseline dilation.
- A comparison between FLAC predictions and MSEW 1.1 calculations according to AASHTO design method showed good agreement between the results. It indicated that the existing design method was capable to distinguish the modes of failure identified by FLAC analysis especially these due to external instability.
- The slope of slip surfaces that developed at the critical and failure states of all models were measured from FLAC plots of failure zone distributions. The slip surfaces were approximated as planes starting from the top of the backfill. The planar approximation was accurate for models that experienced external or connection failure mode. The slip surfaces of models that experienced compound or deep-seated failure modes were nonplanar, and can be approximated more accurately by circular arcs, as in the slope stability analysis. In most cases, the slip surfaces became nonplanar in the zone close to or within the reinforced soil. The slip surface slopes measured from FLAC plots were lower than the values given by Rankine's and Coulomb's earth pressure theories at both critical and failure states. For all cases, FLAC slip surface slope was closer to the value given by Coulomb's theory, and decreased with the progression of failure.
The results of the parametric study clearly show the influence of reinforcement spacing, connection strength, reinforcement stiffness, and soil properties on the behavior of MSEWs with modular block facing and geosynthetic reinforcement. In general, the MSEWs can sustain higher loads with less deformation when reinforcement spacing is smaller and connection load is higher. Since the emphasis of the current study is on the effects of reinforcement spacing on wall behavior, the study was designed to investigate and quantify these effects with respect to failure. The effects of connection strength and reinforcement stiffness were investigated, but only qualitative evaluation of their effects are possible.
Further parametric studies that implement experimental data from laboratory and large-scale tests must be conducted to quantify the effects of connection strength, reinforcement stiffness, and soil properties such as soil stiffness and dilatancy on the behavior of MSEWs.
The reported numerical simulations supported by laboratory and large- scale tests and further numerical analysis may be used to verify or modify the existing methods of analysis and MSEW design with modular block and close reinforcement spacing.