Plan North Engineers delivered breakthrough structural engineering for a sophisticated modern accessory dwelling unit near the Bay Area in California, where extreme site conditions demanded unprecedented innovation. Navigating a perfect storm of site challenges—surface-level groundwater, expansive bedrock, Seismic Design Category E requirements, and near-zero-clearance property lines—our team engineered a structure that stands strong against intense natural forces. This ~3,500-square-foot ADU with subterranean living space demanded an anchored structural boat approach: capable of withstanding major seismic events, resisting groundwater infiltration without mechanical systems, and maintaining structural integrity despite aggressive soil chemistry—all while hovering above volatile bedrock. The comprehensive solution included 45 micropiles extending nearly 60 feet into the local Whiskey Hill Formation to bypass unsuitable soils and provide a stable foundation. Carefully coordinated temporary shoring systems protected neighboring properties just feet away, allowing construction to proceed safely with minimal impact.

This remarkable achievement emerged through close collaboration with SageModern (architect and contractor) and specialty contractors who embraced our unique approach at every stage. Plan North's expertise in developing innovative engineering solutions provided the foundation for success, particularly our advanced knowledge of hydrostatic design, complex foundation systems, and seismic performance in highly corrosive soil environments. This project establishes a new benchmark for residential engineering in challenging environments, demonstrating pathways to resilient construction in high-groundwater regions and showcasing how thoughtful engineering can enhance both safety and livability even in the most demanding geological conditions.

Engineering Challenges 

• Surface level groundwater table

• Expansive bedrock in the upper 20 feet

• Seismic Design Category E

• Near zero-clearance property lines

• Highly corrosive soils

• Deep foundation micropile design
  

Construction images courtesy of American Drilling, Inc.