Foundation Supportworks, Inc. Case Studies: Enger Lofts

A $6 million renovation was planned for a historic building constructed in 1897 in the Lincoln Park Craft District of Duluth, MN. The three-story building was being used as a furniture store prior to the renovation and would be transformed into the Enger Lofts building, which would have commercial and retail space on the lower level and apartments on the upper levels. As part of the renovation, spread footings had been specified to support two new staircases and an elevator shaft in the basement. The only access to the basement was through a narrow staircase from the main level, which limited access for construction equipment. During excavation for the new footings, very soft organic soils were encountered that required removal and replacement with structural fill. The excavation extended to a depth of 15 feet in one area which created concern for undermining of the existing building footings. At that point, the excavation was backfilled and a deep foundation was considered rather than shallow footings. The access restriction to the basement would limit the equipment size that could be used for the deep foundation installation. Furthermore, geotechnical information below the depth of excavation was not available and a pile load test was not feasible; therefore, a deep foundation system that could verify capacity during installation was preferred.

Given the access limitations and lack of geotechnical information, helical piles were chosen as the best deep foundation alternative. Helical piles can be installed with compact equipment such as mini excavators or skid steers and the final installation torque of a helical pile can be correlated to an ultimate capacity. A deep foundation design was developed that included 12 helical piles with new construction caps incorporated into a 24-foot long, 12-foot wide by 2-foot thick concrete pad. The helical piles would have service compression loads of 15 kips. With the lack of deeper geotechnical information, a factor of safety of three was specified for a required ultimate capacity of 45 kips per pile. The helical pile design included the Model HP288 (2.875-inch O.D. by 0.276- inch wall thickness) shaft with a 10”-12”-14” helix plate configuration. A compact excavator, capable of access through the narrow stairwell, was fitted with an 8,000 ft-lb drive head and used for the helical pile installation. The installed pile lengths ranged from 20 to 59 feet with final installation torques of 5,500 ft-lb or more for torque correlated capacities greater than 45 kips. A two-man crew was able to install the helical piles over a period of two days. The concrete for the new pad was poured the day after the helical pile installation was completed, encasing the pile caps and allowing the project to continue with only a minor delay in the schedule. 

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