Setting on the edge of a post-glacial river channel, the 18,800-seat Target Center in downtown Minneapolis is the home of the NBA Minnesota Timberwolves and is one of the top venues for sporting and music events.

The facility also features a full-service, two-level health club with a full-depth swimming pool on the lower level below the arena.

Braun Intertec was retained by the owner to be part of the design team and to conduct the geotechnical evaluation early in the design process. This evaluation showed the site was even more unique and more of a challenge than expected. The lower level slab elevations put many of the footing elevations at or below the in place limestone bedrock. Near the center of the building, the limestone had been eroded to where it was to deep to use spread footing. The evaluation also showed the northeast corner of the building was located in a glacial river channel that had eroded a channel some 120 to 135 feet deep and then refilled it with a mixture of sand, gravel, silts and clay.

Challenges

The common goal of any design team of a building project is to develop a foundation system that will be easy to design, uniform throughout, simple to build and will "react" the same when fully loaded. This goal was even more critical from a structural standpoint because this arena would have the first structurally supported, elevated, computer controlled movable playing floor in the United States. It would be in the "down position" for basketball games and the "up position" when the ice sheet was in place. Because of this, differential movement of the supporting members had to keep to "near zero."

From a geotechnical standpoint, the challenge presented was how to achieve the objectives when the layers below footing grade were so varied.

Solutions

Working with the project structural engineer, Ericksen-Roed & Associates, it was determined the use of a typical spread footing foundation system throughout was not possible because of the shallow bedrock at one end, where the system would work, and the deep channel on the opposite end, where it would not work. The use of a dual foundation system, that is spread footings for the shallow bedrock areas and deep driven piles in the channel was considered but presented challenges in the transition zone between the shallow bedrock and the channel that could not be addressed by either system. The recommended, and ultimately selected, solution was to add a third foundation system in the transition zone, drilled shafts. The final system then consisted of spread footings in areas where the limestone bedrock was near footing grade, limestone or sandstone bearing drilled shafts in the transition zone between the shallow bedrock and the channel and driven piling in the channel. The bearing pressures, load carrying capacities, settlement potential, elastic properties and structural requirements of each system were evaluated and defined so that the entire system reacted to the building loads as uniformly as possible. The first of its kind, a triple foundation system, for a major structure had been developed.

Follow Up

Working with the design team, the construction testing and evaluation program was mapped out. This program consisted of providing code required special inspections and testing for the structural components of the new building. During the installation of the spread footings and the drilled shafts, full-time observations were done to confirm the quality and competency of the limestone or sandstone bedrock using contractor drilled probe holes. As part of the pile driving operation, test piles were evaluated using the Pile Dynamic Analyzer (PDA) to establish driving criteria for the production piles. This information, along with full-time observations, allowed for the installation of the piling that met the requirements of the project specifications. With the foundation system completed, Braun Intertec provided full time inspection and testing services during the reinforcement placement, the concrete placements, the structural steel construction, the fireproofing and the structural masonry construction.

With a unique foundation system and unique structural components, this project was awarded a "Grand Award" from the Minnesota Consulting Engineering Council and a "Seven Wonders of Engineering Award" from the Minnesota Society of Professional Engineers.