Deep Foundation Advancements in Fargo-Paper

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The Block 9 Redevelopment in downtown Fargo replaced a parking lot with a 19-story mixed-use tower, podium and parking ramp. The site’s upper 100 feet consist mainly of highly compressible Glacial Lake Agassiz clay, underlain by strong glacial till. Driven piles were ruled out because of vibration concerns for neighboring buildings, while drilled shafts and large helical piles were considered costly.<br /><br />The project team selected augered cast-in-place (ACIP) piles, a relatively uncommon system in the Fargo area. Advances in drilling equipment, automated construction monitoring, load testing and integrity assessment made the system more practical. To manage the risks of adopting it locally, the team used design-build contracting and required an instrumented static load test and substantially more integrity testing than usual.<br /><br />An 18-inch-diameter, 108.8-foot test pile was embedded about seven feet into the till. A bi-directional load test demonstrated a net ultimate resistance of 939 kips; at an equivalent top load of 400 kips, movement was under half an inch. The results supported the design and allowed the contractor to reduce production-pile embedment in the till from six to five feet.<br /><br />Thermal integrity profiling was performed on 125 production piles. It identified a potential anomaly in one pile, which pulse-echo testing also indicated; that pile was abandoned and replaced. A second unusual profile was traced to a grouting delay and cold-weather heat loss, rather than a pile defect, so the pile was accepted.<br /><br />The project showed that ACIP piles can be a viable, low-vibration deep-foundation option in the Red River Valley. Design-build risk allocation, careful monitoring, instrumented load testing and enhanced integrity testing helped address uncertainty and build confidence in the system’s performance.

Keywords

Block 9 Redevelopment

Fargo downtown

mixed-use tower

augered cast-in-place piles

deep foundation

Glacial Lake Agassiz clay

design-build contracting

bi-directional load test

thermal integrity profiling

vibration concerns