Under a multiphase $2.4 billion plan, the Department of Homeland Security is consolidating 14,000 staff from 40+ locations to the west campus of the former St. Elizabeths Hospital. The plan incorporates an ambitious infrastructure program, including an off-site access road. ZGF, the prime architect under the General Services Administration for campus development, hired Schnabel as the geotechnical engineer of record.

Connecting the road to the 176-acre campus required improvements to the I-295/Malcolm X Avenue/South Capitol Street interchange, including 1.5 miles of roadway widening, six new bridges, one existing bridge widening, supplementary retaining walls, and stormwater management features. Our initial task was a comprehensive subsurface exploration to support the interchange improvements. We addressed design solutions specific to pavements, pavement subgrade stabilization, retaining wall analysis and structural design of new post and panel walls with tie-back anchors, and delivered cost estimates and construction documents for geotechnical and geostructural elements.

In addition to an aggressive schedule, there were other challenges to be surmounted. The entire project site is located along an existing slope underlain by low residual-strength Potomac clays. This required the design of a foundation stabilization system below several mechanically stabilized earth walls to offset global instability. Our geotechnical and geostructural engineering team evaluated various alternatives and designed a stabilization system, including the development of construction cost estimates and detailed specifications. The selected stabilization system consisted of over 600 auger-cast piles extending to depths of up to 95 feet below grade.