<p>A creative application of reinforced soil technology is for back-to-back mechanically stabilized earth (BBMSE) walls, commonly used for bridge approaches and width-restricted highway and railway embankments. Despite the widespread use of BBMSE walls, available experiments and analyses for these walls are limited, and existing design guidelines provide little information about their interaction behavior. Delineating the uncertainties associated with BBMSE walls compared to stand-alone mechanically stabilized earth (MSE) walls, this paper presents a numerical investigation of the behavior of geogrid-reinforced modular-block BBMSE walls at the end of construction. In addition, the effect of distance between two opposing walls and fill friction angle are investigated as the major determinants of the behavior of BBMSE walls. To this end, a series of detailed plane strain models are developed using a commercially available 2D finite-difference program, where the modeling process is verified against one large-scale geogrid-reinforced retaining wall. The models simulate construction stages and consider the interaction of wall components, i.e., soil-geogrid, soil-block, geogrid-block, and block-block. Finally, the results of facing displacements, maximum tensile load in reinforcements, lateral earth pressures, and bearing stresses are presented, discussed, and compared with those of a simple MSE wall. Moreover, the effect of reducing reinforcement length and their connection in the middle of the wall is investigated, and some design recommendations are offered.</p>