Behavior of Back-to-Back Reinforced Earth Retaining Walls

Mouli, S Sasanka and Umashankar, B (2019) Behavior of Back-to-Back Reinforced Earth Retaining Walls. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

In order to retain earth, the reinforced earth technology has been gaining tremendous popularity in the last decade among the other available techniques, mainly because they are typically more economical than traditional retaining structures. The main components of a Mechanically Stabilized Earth (MSE) wall consist of backfill, reinforcing material, and facing elements. Different wall facia are available in MSE technology. Wall facia not only determines the aesthetics of the retaining walls but also influence the behavior of MSE walls. In the initial part of the present study, the effect of different facing elements on the behavior of single MSE walls is obtained from numerical modeling using finite difference based numerical software - FLAC2D Version 7.00. Common wall facia for retaining earth -wrap¬around, segmental-panel, and full height-panel are considered in the study. The kinematics of deformation of these facia under loads is first detailed in the study. The influence of facia on the variation of lateral pressures, vertical pressures, and lateral deformations at the facing along the depth of the wall and tensile forces mobilized in the reinforcement are analyzed. The influence of the reinforcement stiffness is also studied by considering the reinforcement stiffness equal to 500 kN/m, 5,000 kN/m and 50,000 kN/m. It is observed that the lateral pressures decrease with decrease in the facing stiffness of the wall. The stiffness of reinforcement is found to have insignificant influence on the lateral pressures for wrap¬around facia, while the stiffness of reinforcement is found to have a significant effect on full height-panel walls. Back-to-back reinforced retaining walls are mostly used in approach embankments for bridges, flyovers, and many other applications. Guidelines on the design of such walls are limited. According to the Federal Highway Administration Works (FHWA) provision, the distance between back-to-back walls is an important parameter in estimating the lateral pressures on these walls. For back-to-back walls of height ‘if'with backfill angle of shearing resistance two cases are given in the code: a) when the walls are sufficiently far away with the distance between the ends of reinforcements extending from the two walls (D) is greater than i?*tan(450-^/2), the walls are designed as independent single walls, and b) when the two are very close to each other, i.e., ends of the reinforcements for the two walls overlap by a distance more than 0.3*-#; the active lateral earth pressures that need to be considered for stability analysis can be taken as zero. When the distance between the walls is intermediate between these two cases, the lateral earth pressures of the walls are linearly interpolated. However, there is no literature available to justify the above-mentioned earth pressure distribution for back-to-back reinforced walls. In the present study, the effect of distance between the facings of the walls normalized with the wall height (W/H) on the lateral pressures at the facing of the wall and at the end of reinforced zone are analyzed using FLAC2D. The numerical model accounts for complex interactions by incorporating interfaces between backfill-to-reinforcement, backfill-to-panel, and panel-to-panel. Generalized design charts are proposed showing the variation of lateral pressures, tensile forces, and facing displacements with depth for W/H varying from 1.4 to 2.0. The results are also compared with the case when the ends of reinforcements are connected at the middle (called as ‘connected walls’ hereafter). In addition, the stiffness of reinforcement is varied from 500 kN/m to 50,000 kN/m to investigate its effect on the mobilized lateral earth pressures. Stiffness of the reinforcement and W/H ratio are found to have a significant effect on lateral pressures, especially at the end of the reinforcement zone. Tensile forces in the reinforcements are also studied. The effect of facia on back-to-back walls is also analyzed. Compaction and surcharge loads should be incorporated in the numerical model to understand the realistic behavior of MSE walls. The effects of surcharge and compaction stresses on lateral pressures and lateral displacements of back-to-back MSE walls is studied. Similar parametric studies, as mentioned in the previous paragraph, are conducted. The coefficient of lateral pressure, iT, at the end of the reinforcement zone for W/H=1A is found to be 50% less than that for W/H= 2.0. Plots showing the variation of lateral earth pressure coefficients and lateral deformations versus normalized depth of wall are presented. The lateral pressures at the facing appear to be unaffected with the W/H ratio. The tensile forces in these loading cases are also analyzed. The behavior of battered back-to-back walls is also analyzed. A battered angle of 5° is considered in the study. The top width of the wall is taken as W, and different W/H ratios are modeled accordingly. It may be noted that the width at the bottom of battered walls will be higher than W. Three reinforcement stiffness values are considered for the parametric study {i.e., .7=500, 5000 and 50000 kN/m). W/H ratios of 1.4, 1.55, 1.7, and 2.0 are considered. The condition with reinforcement connected in the middle of the wall is also examined for battered walls. Various parametric studies, including W/Hn&tio, reinforcement stiffness, and loading conditions, are carried out in detail. It is found that the lateral pressures at the end of the reinforcement in battered walls are higher than those of lateral pressures in vertical walls. Finally, a design example is worked out for various W/H ratios and a given reinforcement stiffness based on the lateral pressures obtained from the study. FHWA guidelines assumption of linear interpolation is found to overestimate the factor of safety against sliding and overturning for walls with W/H ratio ranging from 1.4 to 2.0. In other words, it underestimates the lateral forces at the end of reinforcement. However, for walls with W/H =2.0, the factors of safety from the FHWA guidelines coincides with the present study.

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