Case Studies Seismic Retrofit - Custom Micropile Brackets

Originally built in 1979, the warehouse was constructed for General Motors Corporation as a parts distribution center. The 634,000 ft^2 facility included dozens of truck dock openings and an interior railroad spur for the unloading of train cars inside the building. In 2015, a seismic retrofit and tenant improvement, including installation of additional truck docks, were planned, changing the structural characteristics of the tilt-up concrete building.

Situation: The installation of a moment frame on the interior of the structure required relatively high loads to be addressed in tension and compression at specific locations. VersaGrade, Inc. was contacted early in the design phase to help determine the best foundation system to address these loads.

While helical piles are ideal for seismic retrofit situations, the soils are predominately alluvium of the Truckee Meadows formation, consisting of pebble to cobble sand, silty sand, and sandy silts overlain by scattered, well-rounded volcanic and granitic cobbles. Due to the rocky nature of the site and limited soils investigations, VersaGrade, Inc. concluded that the use of micropiles would be the best system to meet the required loads. Pile groups of three to four would be necessary to achieve the 140 kip allowable loads at specific points along the perimeter foundation.

Proposed Solution: VersaGrade, Inc. was awarded the subcontract to install 40 mm Contech Systems Micro Piles along with a custom Ram Jack bracketing system. VersaGrade, Inc. contacted Ram Jack Engineering to help design the micropile bracket and casing that would carry a 35 kip allowable capacity (70 kip ultimate) in tension and compression for each pile. Each bracket was designed to work on shear strength alone and attach directly to the tilt-up panels as there were no footings to attach to in some locations.

Outcome: Six (6) 7⁄8 in. diameter epoxy doweled bolts (per bracket) were used to secure the brackets to the face of the concrete tilt-up wall panels. VersaGrade, Inc. inserted a 5 1⁄2 in. diameter x 8 ft. long steel casing sleeve through the brackets and set to elevation. After bracket installations were complete, VersaGrade, Inc. installed (14) Contech 40/16 micropiles with a 4 in. carbide cutting bit down through each casing. The micropiles were installed to an average depth of 21 ft.

Unconsolidated material and fissures created challenges, causing higher than expected grout usage during micropile installations. Some piles took in excess of 35 ft.^3 of grout to complete.

VersaGrade, Inc. used a Komatsu PC-160 track-mounted hydraulic excavator with an Excavator Mount TEI 350 Rock Drill and ChemGrout CG600 Batching Plant for the Micro Pile installations.

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