Shotcrete is concrete sprayed through a hose at high velocity onto a surface. Shotcrete is used to reinforce both temporary and permanent excavations. It may be used in connection with lagging and other forms of earth retention to stabilize an excavation for an underground parking structure or hi-rise during construction. Once the structure is completed the area between its foundation and the shotcrete is backfilled and compacted. Shotcrete is also a viable means and method for placing structural concrete. Although mostly a temporary system, some owners use the finished shotcrete facing as an exposed permanent component for its unique concrete aesthetic appearance.

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Project Details

The historic J.L. Hudson Building (Hudson’s) in Detroit, Michigan was the premier retailer in the city’s downtown district from the 1890s through the 1960s. Following years of neglect, the Hudson’s building was seized by the state and transferred into the hands of the city. The city had it demolished in 1998, to construct a new, four-level underground parking garage as well as a potential future above-grade development. The garage would remain operational until early 2017, but it was plagued with operational issues during this period. The existing concrete walls continually wept water and caused flooding on the bottom floor. The weeping water helped speed the corrosion of the concrete walls and anchors. Some attempts were made to correct spalled concrete at localized locations, only to have the weeping water migrate to the next closest corroded concrete area. By 2017 the site needed major repairs.

The property was purchased by the Bedrock Development Group in 2017. Bedrock had been planning for a new development in this location for years. Construction was slated to start on the new Hudson’s building in December of 2017. Once completed, the 890’ tall building will be the tallest building in Detroit and in the state of Michigan. It will pay homage to the prior building with multiple retail stores in the mixed-use development.

The location of the development, along with city codes, required the developer to create numerous parking spaces. Initial design had the new structure sitting on the existing parking structure and existing caissons. Due to the state of the existing structure, as well as the unknown state of the caissons, it was determined that the best plan would be to demolish the existing parking structure and install new deep foundations for a below grade parking structure. The Urban environment and proximity to surrounding streets and buildings meant earth retention would need to be utilized. The existing foundation walls were a 240’ x 430’ rectangle. These existing walls would act as the current earth retention but needed some serious repair and revitalization. The plan was to make the repairs necessary to use the existing walls as permanent earth retention for the new structure, as well as install new deep foundations. Hardman Construction was hired for this.

There were major concerns about the condition and integrity of the anchors, as well as the original 19th century concrete walls. The walls had numerous spalled areas that exposed 19th century steel. These areas constantly wept water. Calcium deposits encased the ties, making it difficult to evaluate their integrity. The plan to make the 40’ excavation safe was a four-step process. First, the perimeter walls would be stabilized to allow demolition to commence. Second, the four underground floors and top plaza level would be demolished. Third, existing anchors were to be inspected, tested, and in some cases repaired, depending on their condition. Fourth, all existing walls would be hydro-demoed, repaired, and the walls sprayed with shotcrete. Due to safety and access concerns, a robot was used for all hydro-demolition work.

The four-step process of Structural Steel Installation, Repairing Anchors, Demolition, Hydro-Demo and Shotcrete was fraught with challenges, but the construction team was able to employ new technologies using robotics to help overcome these difficulties. The implementations of this new technology on the Hudson’s site will allow for better preparation for all involved in future work with similar projects. Full excavation and stabilization of the work area was completed in October of 2018.

The Mosher Jordan Hall at the University of Michigan was renovated using Hardman Construction to reinforce the foundation of the existing building. The renovation included the addition of a new dining area which was 18’ below the elevation of the existing building footings and remodeling of the existing dorm rooms. The excavation next to the existing hall was constructed using a permanent earth retention system which was a tangential augercast wall with permanent earth anchors and a structural shotcrete facing. In addition to the permanent ERS wall, micropiles were installed in both the outside and inside areas. The micropiles that were installed inside of the existing building were placed in 8’ of headroom and were utilized to support the existing columns during the time they were removed and the new footing were placed at a lower elevation. The micropiles that were placed outside the building were installed to support the existing area ways during excavation. In these areas the permanent ERS system was not utilized as underpinning was required.