Geosynthetic Reinforced Retaining Walls with Limited Fill Space under Static Footing Loading

Abstract

Geosynthetic reinforced retaining (GRR) walls, which typically consist of reinforced soil mass, facing units, and retained fill, are extensively used for highways, bridge abutments, and service roads throughout the world. In recent decades, GRR walls have been increasingly built with limited fill space, which has posed challenges for designing such walls with satisfactory performance, especially under surface loading, such as bridge foundations. Since the GRR walls with limited fill space are retaining walls under special conditions, only limited information related to such walls is available in the literature and therefore their performance has not been well understood. The objective of this study was to evaluate the performance of limited fill space GRR walls subjected to static footing loading. To fulfill the above research objective, a comprehensive experimental study and numerical analysis were conducted. The experimental study included a series of laboratory model tests to investigate the performance of GRR walls constructed with limited fill space subjected to strip footing loading. The devised experimental program consisted of 11 model tests with different retained medium distances, geosynthetic-reinforced fill widths, footing offset distances, and reinforcement layouts. The model tests were constructed and tested in a model box under a plane strain condition in the geotechnical testing laboratory at the University of Kansas. The dimensions of the model box were 2.4 m long, 0.5 m wide, and 1.1 m high. The model walls were 1.0 m high and 0.45 m wide. In each model test, a load was applied on the top of the wall through a 200 mm wide rigid plate to simulate a strip footing. Earth pressure cells, reflection targets on the sides of the wall, and fixed benchmarks on facing units, and on the loading plate were installed to measure earth pressure distributions, wall deformations, and wall facing displacements, and footing settlements, respectively. To interpret the reduced-scale modeling results based on the scaling ratio between the model and the prototype, a scale effect analysis was performed to find the correct scale ratio between the model and the prototype wall. The wall models were designed based on the findings of the scale effect analysis. The experimental test results showed that limiting the size of the retained medium and the reinforced fill affected the internal stability of the wall, the lateral wall facing displacement, and the footing settlement. Reduction of the wall width from 0.5H to 0.3H (H is the wall height) resulted in excessive wall deformation and footing settlement, and even sudden failure of the model wall. On the other hand, the test results revealed that connecting geosynthetic reinforcement to the stable retained medium resulted in substantial reduction in the lateral displacement of the wall facing and the settlement of the footing. The test results also demonstrated that bending geosynthetic reinforcement upward along the back of the reinforced soil enhanced internal stability and resulted in considerable reduction in the lateral displacement of the wall facing and increase in the bearing capacity of the footing. The experimental test results also showed that the vertical earth pressures along the depth of the model tests increased with the increase of the depth of the model and the applied footing load. Likewise, the lateral earth pressures on the wall facing along the depth of the model tests increased with the applied footing load. In addition, the vertical earth pressures and the lateral earth pressures measured from the model tests with the reinforced fill width of 0.3H were lower than those calculated by the exiting theoretical methods (i.e., 2:1 distribution method, Rankine’s active earth pressure theory and Janssen’s equation). However, the results of Janssen’s equation were in better agreement with the experimental results as compared to the result of Rankine’s theory. The numerical analysis was performed by using the continuum mechanics-based program FLAC 2D Version 8.0 to verify the experimental results. In the numerical analysis, backfill soil was modeled as a linearly elastic perfectly plastic material with the Mohr Coulomb (MC) failure criterion. The wall facing, the stable retained medium, and the foundation were modeled as a linearly elastic material. A strip element was utilized to simulate the reinforcement and modeled as a linearly elastic perfectly plastic material. The lateral displacement of the wall facing, the footing settlement, the vertical earth pressures, lateral earth pressures, and the maximum strains in the reinforcement were computed by the numerical analysis under the applied footing loads and compared to the results from the experimental tests. The results obtained from the numerical analysis generally agreed with those measured from the experimental tests. In addition, a numerical parametric study was conducted to assess the factors influencing the performance of GRR walls with limited fill space subjected to static footing loading. The influence factors consisted of the reinforced fill width (reinforcement length), the reinforcement rear connection, the footing size, the footing offset distance, the stiffness of reinforcement, the friction angle of the backfill soil, and the wall height. The parametric study showed that the maximum lateral displacement of the wall facing, and the footing settlement increased with the reduction in the reinforced fill width, the friction angle of the backfill soil, and the footing offset distance. In contrast, the maximum lateral displacement of the wall facing, and the footing settlement decreased with an increase in the reinforcement stiffness, the footing offset distance, and the decrease in the footing size. The parametric study also showed that the connection of the reinforcement to the stable medium at rear, bending-upward the reinforcement around the back of the reinforced fill and overlapped the reinforcement from the back of the reinforced fill resulted in considerable reduction in the maximum lateral displacement of the wall facing and the footing settlement. The maximum strain in the reinforcement increased with the reduction in the reinforced fill width, the friction angle of the backfill soil, and the footing offset distance. However, the maximum strain in the reinforcement decreased with the increase in the reinforcement stiffness, the decrease in the footing size and the footing offset distance. Additionally, the vertical earth pressures computed on the wall along the wall facing of the wall models were lower than those computed along the centerline of the footing under all the applied footing loads. Also, the vertical earth pressures computed on the wall along the wall facing of wall models were generally lower than those calculated by the 2:1 distribution method. However, the vertical earth pressures computed along the centerline of the footing along were generally higher than those calculated by the 2:1 distribution method. Similarly, the lateral earth pressures computed on the wall facing of wall models were lower than those calculated by the exiting methods (i.e., Rankine’s active earth pressure theory and Janssen’s equation with the 2:1 distribution method). However, the lateral earth pressures calculated using Janssen’s equation showed better agreement with the lateral earth pressures computed by the numerical analysis as compared to those calculated using Rankine’s theory.