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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).

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.

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.

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.