Omohundro Water Treatment Plant is a 90-mgd facility operating 365 days per year with a history dating to the 1880’s. The current Process Advancements Project will reshape the plant with an all-new process train while preserving its historic character. The filter building at the center of the site is a striking unreinforced masonry structure over concrete dual-media filter boxes. Clearwell chambers at the lowest level provide disinfection downstream of filtration. The structure was built in four phases from the 1920’s to the 1960’s on variable subsurface conditions, partially founded on hard limestone and partially on native soils. Working in and around it creates some of the project’s most complex challenges. The facility is critical to Metro Water Service’s ability to deliver water to customers and is treasured for its architecture and well-preserved historic nature. Over the course of the current project Omohundro’s filter building will first be surrounded by large-scale rock-removal activities while in service as excavation for new adjacent facilities proceeds. In a later phase, it will undergo extensive structural modifications to convert the existing boxes to deep-bed granular activated carbon (GAC) contactors, including under-pinning portions of the foundation not founded on rock, excavating rock around and within the footprint, and removing numerous concrete walls and slabs. In each phase the project team (design engineer, CMAR, and owner) will seek to mitigate risks to the structure and its architectural features through conservative construction methods and work restrictions, preventative shoring, and careful real-time monitoring and evaluation. This presentation will discuss the characterization of site conditions, the challenges associated with analyzing this type of structure, and the rigorous modeling techniques that were justified by the stakes involved. The project team used three-dimensional Applied Element Method (AEM) analyses to evaluate vibration response of the structure under various construction conditions including blast-induced ground motions. This approach allows explicit simulation of nonlinear behavior, progressive cracking, and load redistribution in unreinforced masonry and mass concrete elements—failure mechanisms that directly affect the structural integrity, long-term serviceability, and architectural and historic elements of the nearly 100-year-old facility. The results of the numerical modeling will guide general risk evaluation, design of monitoring instrumentation, establishment of blasting control procedures, and work restrictions associated with the GAC retrofit of the structure.