Solution

Helical piles were selected as the best option for providing the tension and compression capacity needed for underpinning the existing building and at the south wall of the addition. Helical piles are often preferred over other deep foundation alternatives given the ability to work around tight access areas, quick installation, and lower costs relative to other systems. After comparing the cost of helical piles as a deep foundation alternative to the cost of the shallow foundation system recommended at the other area of the project, helical piles were selected for the foundation system at the exterior walls of the entire building addition. The final design included 43 helical retrofit pile locations along the north wall of the existing building and 189 new construction helical pile locations for the building addition. A custom uplift bracket attachment was designed to provide a service tension capacity of 6 kips at the retrofit pile locations. The service compression loads for the helical piles ranged from 22 to 41.2 kips. Given the lack of quality soil strength information below a depth of 15 feet, a factor of safety of 2.5 on the service load was used for the pile design and a minimum pile termination depth of 15 feet was required. The Model HP350 (3.5-inch O.D by 0.340-inch wall hollow round shaft helical pile system with an 8”-10” helical plate configuration was used for the retrofit piles and the new construction piles at the south wall of the addition. The Model HP288 (2.875-inch OD by 0.276-inch wall) hollow round shaft helical pile system with an 8”-10” helix plate configuration with new construction brackets was selected for the remaining building addition piles. The helical piles were installed to an average depth of 19 feet after achieving a torque-correlated ultimate capacity of at least 2.5 times the service load. The 232 helical piles were installed over a period of 18 days.