The Site C Clean Energy Project, located in North-Eastern BC, is operated by BC Hydro, and includes a large earthfill dam with an integrated Powerhouse and spillway. The dam and associated structures are founded on Shaftesbury Shale, which contains bedding plane shears that govern the performance of the foundation. A system of shear piles consisting of steel pipes filled with concrete was designed to reinforce the foundation and enhance the long-term serviceability of the overlying Powerhouse structure. This paper presents the 3D deformation modelling undertaken to evaluate and optimize the pile design and configuration. As part of this work, a series of 3D numerical model were developed to assess each configuration considered by simulating the interaction between the piles, concrete structures, and foundation clay shale. The piles were modelled using 3D volume elements, while the soil-structure interaction was captured through interface elements to simulate load transfer and potential slip at the pile-rock boundary. Multiple pile configurations were assessed, including variations in pile length, diameter, spacing, and layout, with comparisons between staggered and aligned arrangements. The performance of each configuration was evaluated based on several criteria, including total displacement, loading within the piles, and tensile stress development in the shale. These results were used to evaluate the performance of the different pile configurations. Through an iterative design process, an optimized pile configuration was selected that met the displacement design criteria while minimizing the number and size of piles. The study highlights the value of detailed 3D deformation modelling for performance-based foundation design in complex geological settings.