City Creek Block 75 Redevelopment: Shoring & Underpinning
City Creek Block 75 is a 20-acre urban redevelopment with five underground levels located in downtown Salt Lake City. Excavation for Block 75, the central six-acre segment of this project, required shoring and underpinning in combination with dewatering to support five adjacent high-rise commercial buildings and three heavily trafficked city streets around its perimeter. The excavation extended to 90 feet below street grade (up to 40 feet below groundwater) in soils ranging from dense cobble and gravel to fine-grained lakebed deposits. This work was complicated by existing driven pile foundations abandoned throughout the site. Shoring was performed in two phases, maintaining operating facilities on the north side until excavation was completed across the southern half of the site.
Malcolm Drilling employed a diverse range of shoring techniques on this project, taking into consideration the geometric, soil, groundwater conditions, and sensitivity of adjacent structures. Soil nails and shotcrete, combined with vertical nailing for face stabilization, were employed along the walls with street frontage. Anchored secant pile walls provided a combined groundwater cut-off system and an extremely stiff earth retention system to support the adjacent high-rise structures on the northeastern perimeter. Closely spaced micropile underpinning and A-frames were employed to support adjacent structures in limited access areas of west, south, and east walls. Where existing subgrade walls were located outside the new building footprint, they were nailed in-Situ. Around the balance of the site perimeter, cantilever, anchored, and internally braced soldier pile shoring was applied to match a range of geometric and loading conditions. The shoring was combined with multi-level wellpoint systems for construction dewatering.
