Abstract:
The United States Air Force is constructing a new facility to consolidate weapon maintenance, storage, and training to support intercontinental ballistic missile operations at Malmstrom Air Force Base in Montana.Part of the new facility requires an approximately 30 ft by 75 ft by 40 ft depth basement to house significant mechanical equipment, with a thick ground-level structural concrete slab above. Concurrent construction of other portions of the structure on all sides precluded open cut or offset temporary support. A single pass system of reinforced concrete secant walls was selected to overcome these constraints. In the completed condition, these walls span between the bottom floor slab and ground-level structural slab, however the upper slab could not be constructed until after the major mechanical equipment was installed inside the basement, requiring alternate means of lateral support during construction. The design team considered several alternatives, including temporary internal steel wale frames and top-down construction with block-outs for equipment installation, but none provided adequate access for equipment installation. Grouted tiebacks were also evaluated, but deemed infeasible due to numerous adjacent drilled shaft foundations. Ultimately, the team devised a hybrid system utilizing three different methods of lateral support: cantilever secants, shallow tie-rods to temporary deadmen shafts, and tie-rods anchored to the adjacent permanent structure foundations. This facilitated successful construction and equipment installation with no obstructions inside the excavation, while meeting all performance objectives.