This paper presents the performance assessment of Rammed Aggregate Pier® (RAP) system used as a ground improvement solution under a railway embankment founded on soft to medium stiff clays. The railway embankment heights vary from 10 to 15 m with a crest width of 14.5 m. The slopes of the embankments were constructed with 1V:2H gradients. Prior to the placement of the embankment, RAP elements were installed to improve bearing capacity and settlement responses and accelerate the time rate of settlement (i.e.: consolidation) at the project site. Site investigations revealed that the underlying soil profile is composed of, scattered relatively thin sandy layers, low to high plasticity, soft to stiff clay layers with a total thickness of 15 m. Under this layer, there exists a very stiff silty clay layer extending to the depth of 40 m. Prior to the placement of embankment fill, in-situ tests and full-scale Geopier Impact TM load tests were performed to optimize the ground improvement method and pattern. Further, hydrostatic profillers were installed to monitor the settlement response of the embankment and foundation soils. Monitored performance of Rammed Aggregate Pier® system is compared with the performance predictions estimated by analytical and numerical assessment studies. The foundation settlement was estimated by using Rocscience-Settle 3D software tool, which incorporates 3D finite difference-based solutions of elastic media. Settlement predictions are compared with actual measurements, which revealed that the RAP improved foundation soils have an increased bearing capacity, and decreased compressibility compared to the unimproved site. After the application of RAP elements, bearing capacity problems were eliminated along with a reduction of settlements from a value of exceeding 100 cm to more reasonable 40 to 50 cm range. Additionally, it was also noted that, 3D settlement calculations based on elastic theory produced reasonable estimates of the actual settlement performance.