Innovative Secant Pile Excavation Support: Weapons Storage and Maintainance Facility

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At Malmstrom Air Force Base in Montana, a 30-by-75-foot basement, approximately 40 feet deep, was required for mechanical equipment in a new weapons storage and maintenance facility. Construction around the basement had to proceed concurrently, ruling out open-cut excavation and conventional formed basement walls. The secant-pile walls would ultimately span between the basement floor and a thick ground-level slab, but the slab could not be built until after the equipment was installed. The excavation therefore needed temporary lateral support that left its interior clear.<br /><br />The team evaluated six approaches, including grouted tiebacks, deadmen, internal wale frames, and top-down construction. Nearby drilled-shaft foundations made tiebacks difficult, while the mechanical equipment’s installation requirements ruled out top-down construction and interior bracing.<br /><br />The selected design combined three support methods: cantilever secant piles where feasible, shallow tie-rods connected to temporary deadman shafts on the east and west sides, and tie-rods anchored to part of the permanent structure on the south side. A reinforced concrete wale beam at the top of the secants distributed the tie-rod forces. Using the permanent structure as an anchor avoided some temporary shafts and reduced cost and schedule impacts. The secants were increased to one metre in diameter to provide capacity and reduce reinforcement congestion.<br /><br />Construction proceeded without obstructions inside the excavation, enabling installation of the mechanical equipment. Tie-rods were proof-loaded and locked off at 250 kips; testing showed negligible loss of load. Once the permanent top slab reached design strength, the rods were detensioned as planned. The project demonstrates how evaluating multiple concepts and coordinating closely across the design and construction teams can produce an effective excavation-support system under tight sequencing, space, and access constraints.

Keywords

Malmstrom Air Force Base

Montana

secant pile walls

temporary lateral support

basement excavation

tie-rods

deadman shafts

reinforced concrete wale beam

mechanical equipment installation

excavation support system