Creative Secant Pile Excavation Support System-Paper

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The U.S. Air Force’s new Weapons Storage and Maintenance Facility at Malmstrom Air Force Base required a 30-by-75-foot basement excavated to a depth of 40 feet for mechanical equipment. Construction around the basement had to proceed concurrently, ruling out open excavation and conventional temporary support. The permanent secant-pile walls would ultimately span between the basement floor and ground-level slabs, but the upper slab had to remain open until the equipment was installed.<br /><br />The design team evaluated six support concepts, including grouted tiebacks, deadmen, internal bracing, and top-down construction. Tiebacks risked colliding with numerous nearby drilled-shaft foundations, while internal bracing and top-down construction obstructed equipment access. The selected hybrid system combined cantilever secant piles, shallow tie-rods connected to temporary deadman shafts, and tie-rods anchored to the adjacent permanent structure.<br /><br />One-meter reinforced concrete secant piles were designed for construction and permanent loading. A continuous reinforced-concrete wale beam at the top of the walls transferred temporary lateral loads to the tie-rods. On the east and west sides, the rods connected to 6-foot-diameter, 50-foot-deep deadman shafts. On the south side, the team used part of the permanent structure’s slab and drilled-shaft foundation system as an anchor, reducing temporary works and saving time and cost. Tie-rods were proof-loaded and locked off at 250 kips.<br /><br />Favorable stiff clay and minimal groundwater enabled efficient pile installation, including open-hole drilling. The system provided a clear excavation for equipment installation and avoided conflicts with surrounding construction. At the time of publication, the permanent upper slabs were still being completed; once they reached design strength, the temporary tie-rods would be released. The project demonstrates how coordinated evaluation of alternatives and adaptation to new constraints can produce an effective excavation-support solution.

Keywords

Malmstrom Air Force Base

Weapons Storage and Maintenance Facility

basement excavation

secant-pile walls

temporary support

tiebacks

deadman shafts

tie-rods

drilled-shaft foundations

excavation support