Geosynthetic-reinforced soil (GRS) walls in a tiered configuration are acceptable alternatives to conventional retaining wall systems because of several benefits such as cost, stability and construction constraints, and aesthetics. Current design methods for analyzing GRS multitier walls are based on the lateral earth pressure method, an extension of the design method for analyzing single-tier reinforced walls. The design approaches in these guidelines are considered empirical and are geometrically derived based on the offset distance between upper and lower tiers. However, some studies have questioned using this empirical approach. Very few studies have confirmed that the earth pressure method is effective for designing multitier reinforced walls, and few have investigated the behavior and performance of multitier geosynthetic-reinforced soil (GRS) walls with various offset distances.
Consequently, this dissertation presents the procedure and results of the limit equilibrium (LE) and finite element (FE) analyses of a series of centrifuge tests on two-tier geosynthetic-reinforced soil (GRS) wall models with various offset distances. Both methods were considered useful tools for engineering application. The objectives were: 1) to evaluate the applicability of LE and FE methods for analyzing and designing two-tier GRS walls; 2) to examine the modeling assumption of reinforcement tensile loads in LE analysis and the design methods in current design guidelines; 3) to investigate the performance and behavior of two-tier GRS walls in various stress states; and 4) to investigate the interactive mechanism between two tiers. The variables considered in the centrifuge testing program were offset distance and reinforcement length.
In the first part of this research work, the evaluation of limit equilibrium method studies undertaken to assess the validity of LE as a basis for design of two-tier GRS walls. The LE analyses are the most common approach for slope stability in current practice. Parametric studies were first performed to evaluate the effects of modeling assumptions of reinforcement force on LE results, including reinforcement force orientation, reinforcement overlaps layers, and reinforcement tensile load distribution with depth. The suitability of LE for the analysis of two-tier GRS walls and design implications were then discussed. According to LE results, good agreement existed between LE and centrifuge models in locating failure surfaces. The LE results also indicate that offset distance correlated negatively with the effective overburden pressure on the reinforcement and the resulting confined Tult of the reinforcement. The critical offset distance of 0.7 times the lower tier height was determined when the decrease in confined Tult value as D increases reached a constant value. Last, the effect of offset distance on the normalized reinforcement tension summation coefficient, KT, indicates that single and independent wall models yielded a single consistent KT value. For compound wall models, the KT value decreases as offset distance D increases.
In the second part, the finite element study investigated the performance and behavior of two-tier GRS walls in various stress states. The FE simulations were first verified according to the centrifuge test results by comparing the locations of failure surfaces. The FE results were then used to investigate the effective overburden pressure, mobilization and distribution of reinforcement tensile loads, and horizontal deformation at the wall faces. The interaction between two tiers was investigated based on the FE results, which were also used to examine the modeling assumption of reinforcement tensile loads in LE analysis and to evaluate the design methods in current design guidelines. This study demonstrated favorable agreement between FE and the centrifuge model in locating the failure surface. The FE results indicated that as the offset distance increased, the reinforcement tensile load and wall deformation decreased in both the upper and lower tiers, suggesting the attenuation of interaction between the two tiers. The maximum tensile loads of all reinforcement layers at the wall failure predicted using FE and LE analyses, assuming uniform distribution of reinforced tensile loads, were comparable. Compared with the FE results, the FHWA design guidelines are conservative in determining the effect of overburden pressure, required tensile strength, location of maximum tension line (for designing the reinforcement length), and the critical offset distance.

Geosynthetic-reinforced soil (GRS) walls in a tiered configuration are acceptable alternatives to conventional retaining wall systems because of several benefits such as cost, stability and construction constraints, and aesthetics. Current design methods for analyzing GRS multitier walls are based on the lateral earth pressure method, an extension of the design method for analyzing single-tier reinforced walls. The design approaches in these guidelines are considered empirical and are geometrically derived based on the offset distance between upper and lower tiers. However, some studies have questioned using this empirical approach. Very few studies have confirmed that the earth pressure method is effective for designing multitier reinforced walls, and few have investigated the behavior and performance of multitier geosynthetic-reinforced soil (GRS) walls with various offset distances.
Consequently, this dissertation presents the procedure and results of the limit equilibrium (LE) and finite element (FE) analyses of a series of centrifuge tests on two-tier geosynthetic-reinforced soil (GRS) wall models with various offset distances. Both methods were considered useful tools for engineering application. The objectives were: 1) to evaluate the applicability of LE and FE methods for analyzing and designing two-tier GRS walls; 2) to examine the modeling assumption of reinforcement tensile loads in LE analysis and the design methods in current design guidelines; 3) to investigate the performance and behavior of two-tier GRS walls in various stress states; and 4) to investigate the interactive mechanism between two tiers. The variables considered in the centrifuge testing program were offset distance and reinforcement length.
In the first part of this research work, the evaluation of limit equilibrium method studies undertaken to assess the validity of LE as a basis for design of two-tier GRS walls. The LE analyses are the most common approach for slope stability in current practice. Parametric studies were first performed to evaluate the effects of modeling assumptions of reinforcement force on LE results, including reinforcement force orientation, reinforcement overlaps layers, and reinforcement tensile load distribution with depth. The suitability of LE for the analysis of two-tier GRS walls and design implications were then discussed. According to LE results, good agreement existed between LE and centrifuge models in locating failure surfaces. The LE results also indicate that offset distance correlated negatively with the effective overburden pressure on the reinforcement and the resulting confined Tult of the reinforcement. The critical offset distance of 0.7 times the lower tier height was determined when the decrease in confined Tult value as D increases reached a constant value. Last, the effect of offset distance on the normalized reinforcement tension summation coefficient, KT, indicates that single and independent wall models yielded a single consistent KT value. For compound wall models, the KT value decreases as offset distance D increases.
In the second part, the finite element study investigated the performance and behavior of two-tier GRS walls in various stress states. The FE simulations were first verified according to the centrifuge test results by comparing the locations of failure surfaces. The FE results were then used to investigate the effective overburden pressure, mobilization and distribution of reinforcement tensile loads, and horizontal deformation at the wall faces. The interaction between two tiers was investigated based on the FE results, which were also used to examine the modeling assumption of reinforcement tensile loads in LE analysis and to evaluate the design methods in current design guidelines. This study demonstrated favorable agreement between FE and the centrifuge model in locating the failure surface. The FE results indicated that as the offset distance increased, the reinforcement tensile load and wall deformation decreased in both the upper and lower tiers, suggesting the attenuation of interaction between the two tiers. The maximum tensile loads of all reinforcement layers at the wall failure predicted using FE and LE analyses, assuming uniform distribution of reinforced tensile loads, were comparable. Compared with the FE results, the FHWA design guidelines are conservative in determining the effect of overburden pressure, required tensile strength, location of maximum tension line (for designing the reinforcement length), and the critical offset distance.

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