Effect of Compaction Stresses on Performance of Back-to-Back Retaining Walls

Back-to-back reinforced soil retaining walls are commonly used for approach embankments of bridges and flyovers. Existing design guidelines (FHWA/BS/IS codes) do not provide a mechanistic approach to design back-to-back reinforced soil retaining walls. Lateral pressures on the facing and at end of reinforced zone are required for stability checks (both internal and external). During stage-wise construction of back-to-back walls, compaction stresses should be incorporated to obtain realistic lateral earth pressures on the walls. The present paper describes the effect of the compaction stresses on the lateral pressures in such reinforced soil retaining walls. The variation of lateral pressures at the end of reinforced zone along the depth of the wall is obtained from numerical modeling of back-to-back reinforced soil walls. A surcharge load of 30 kPa is applied at the end of the construction of the wall. It is observed that the effect of surcharge load is not significant after certain depth of the wall for lower spacing between walls to wall height ratios. A comparison on lateral pressures with and without compaction stresses for different distances between the ends of reinforcements of two walls is presented. Effect of connection of reinforcement is also studied

Behavior of Connected and Unconnected Back-to-Back Walls for Bridge Approaches

Back-to-back mechanically stabilized earth (MSE) walls are commonly used in the construction of transportation infrastructure facilities. Federal Highway Administration (FHWA) guidelines discuss briefly the design of reinforced back-to-back walls. In this study, a numerical model was developed to study the behavior of connected and unconnected back-to-back walls under working stresses. The effect of reinforcement stiffness on tensile force profiles, the maximum tensile force developed in the reinforcement, and lateral pressures and lateral deformations for both unconnected and connected walls are discussed in detail. A well-defined critical slip surface was observed for the case of the unconnected back-to-back wall with relatively extensible reinforcement. Lateral pressures at the facing in both the cases were found to be almost equal, and the tensile forces developed in the reinforcement for the connected case were found to be uniform along the length of the reinforcement (except at higher depths)

Behavior and Design of Back-to-Back Walls Considering Compaction and Surcharge Loads

Back-to-back geosynthetic-reinforced retaining walls are commonly used as approach embankments for bridges and flyovers. Compaction and surcharge loads should be incorporated in the model to understand the realistic behavior of mechanically stabilized earth (MSE) retaining walls through numerical modeling. In this study, a finite-difference method-based numerical model, Fast Lagrangian Analysis of Continua (FLAC 2D, Version 7.0), is used to study the effects of surcharge and compaction stresses on lateral pressures and lateral displacements of back-to-back MSE walls. The ratio of the distance between walls to the height of the wall (W/H) in back-to-back walls is varied from 1.4 through 2.0, and the stiffness of reinforcement from 500 to 50,000 kN/m to cover the entire range of stiffness values of extensible to inextensible reinforcements. The coefficient of lateral pressure, Kr, at the end of the reinforcement zone for W/H = 1.4 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. Maximum tensile forces in the reinforcements along the depth of wall are also analyzed. The lateral pressures at the facing appear to be unaffected with W/H ratio. Finally, a design example showing the external stability analysis of reinforced back-to-back walls is illustrated incorporating the lateral pressures obtained from the study

Behavior of Connected and Unconnected Back-to-Back Walls for Bridge Approaches

Back-to-back mechanically stabilized earth (MSE) walls are commonly used in the construction of transportation infrastructure facilities. Federal Highway Administration (FHWA) guidelines discuss briefly the design of reinforced back-to-back walls. In this study, a numerical model was developed to study the behavior of connected and unconnected back-to-back walls under working stresses. The effect of reinforcement stiffness on tensile force profiles, the maximum tensile force developed in the reinforcement, and lateral pressures and lateral deformations for both unconnected and connected walls are discussed in detail. A well-defined critical slip surface was observed for the case of the unconnected back-to-back wall with relatively extensible reinforcement. Lateral pressures at the facing in both the cases were found to be almost equal, and the tensile forces developed in the reinforcement for the connected case were found to be uniform along the length of the reinforcement (except at higher depths)

Seismic site characterization of a few Indian coal ash deposits using multichannel analysis of surface waves

This study presents the outcome of an extensive field experiment undertaken to characterize coal ash deposits in terms of dynamic properties. In-situ shear wave velocity (Vs) was measured over three hydraulically deposited coal ash deposits (two active ponds and one abandoned pond) situated in the central and southern parts of India. Multichannel analysis of surface waves was performed at twenty-one different locations on the ash deposits and the dyke top to measure the Vs profiles along the depth (z). Vs profiles measured for the abandoned coal ash deposits were greater than those of the active deposits. Moreover, the empirical association of Vs profiles from active and abandoned coal ash deposits in India, based on the present study and reported literature, yields a piecewise linear relationship between shear wave velocity and depth. © 202

Analysis of Single and Back-to-Back Reinforced Retaining Walls with Full-Length Panel Facia

Back-to-back mechanically stabilized earth (MSE) walls have several applications in infrastructure development. Present study firstly discusses on the kinematics of deformation of MSE wall with full-length panel facia. Secondly, single and back-to-back MSE walls with full-length panel facing are modelled using finite difference based software (Fast Lagrangian Analysis of Continua). The reinforcements in the back-to-back walls extend through and through from one wall facing to the other wall facing. Lateral pressures, vertical stresses, and lateral deformations at the facing for various reinforcement stiffness values are evaluated for both single and back-to-back walls under working stresses. Reinforcement stiffness values of 500 kN/m, 5000 kN/m and 50,000 kN/m are considered. For single MSE wall with stiff reinforcement, lateral pressures at the facing are higher than those for active earth pressure. Lateral deformations of facing with reinforcement of low stiffness are higher than those with reinforcement of high stiffness. In back-to-back walls, the lateral deformation of wall facing with reinforcement of high stiffness is slightly inwards near the top of the wall. Reinforcement stiffness is found to have significant effect on the vertical stress in both single and back-to-back walls. Lateral pressures of both single and back-to-back connected walls are compared. Lateral pressures on single MSE wall are found to be lesser than those for back-to-back wall at the top of the wall. However, lateral pressures near the bottom of single wall are higher than those of back-to-back wall

Site Characterization of Existing and Abandoned Coal Ash Ponds Using Shear-Wave Velocity from Multichannel Analysis of Surface Waves

Coal ash ponds or lagoons are surface impoundments constructed to store the unused coal ash generated from thermal power plants. In India, by the end of 2020 nearly 82,200 ha of land will be under ash ponds. Once the capacity of these ponds is reached, they are closed in compliance with the statutory regulations. Recent trends suggest that these lands could be reclaimed to accept light- to heavy-weight structures over them, which makes in situ characterization essential to ensure the safety of structures built over them. This paper presents the site characterization in terms of shear-wave velocity (Vs) of four hydraulically deposited coal ash lagoon sites (two existing and two abandoned) at two ash pond sites in India using the multichannel analysis of surface waves (MASW) technique. The ash is deposited using the wet slurry technique, and the age of deposition is found to have a significant effect on the shear-wave velocity of these deposits. An empirical correlation is presented between the mean Vs and depth (z) for all the sites considered. The measured Vs values were extrapolated using three different methods to obtain Vs,30 values, and the sites were classified as per the recommendations of the International Building Code. Standard penetration tests (SPTs) were conducted near the MASW testing locations at one of the abandoned ash ponds, and the SPT values (N values) had significant scatter. An empirical correlation is proposed between the mean Vs and N values obtained

Reinforcement Tensile Forces in Back-to-Back Retaining Walls

Back-to-back reinforced retaining walls are mostly used in approach embankments of bridges and flyovers. In the internal stability check for mechanically stabilized earth (MSE) walls, earth pressure theory is used to obtain the tensile forces in the reinforcement. However, tensile forces calculated from this method are found to be very conservative (by a factor of two) in comparison to the actual field measurements. The objective of this study is to examine the mobilization of reinforcement tensile forces at different levels within back-to-back MSE wall at working stress condition. Tensile forces in the reinforcements with reinforcement connected in the middle (i.e., reinforcement extending from one wall to the other) are also obtained. Parametric study is carried out with the stiffness of reinforcement varying from 500 to 50,000 kN/m and the ratio of spacing between two walls to wall height (W/H) varying from 1.4 to 2.0 to investigate their effect on tensile forces at every level of the reinforcement. Charts are also proposed showing the variation of the maximum tensile forces along the height of the wall