Deep Foundation Investigation and Design

City of Woodbury

Princeton Hydro was contracted by the City of Woodbury to assess the sub-soils of a building whose renovation included a new elevator; the elevator shaft created a loading condition of over 242,000 pounds, including the transfer of loads from an immediately adjacent column. During construction, it was found that the existing basement slab was supported by timber piles, raising concerns about the sub-soils below the building.

Princeton Hydro designed an investigation that included the use of a tri-pod-mounted boring apparatus to test the underlying soil strata for bearing capacities and adequacy of structural support. The soil samples were collected and delivered to Princeton Hydro’s AMRL (AASHTO Materials Reference Laboratory) and US Army Corps of Engineers-accredited laboratory for geotechnical analysis and characterization.

The investigation included the progression of one boring at the location of the proposed elevator shaft. Due to the site constraints, the soil boring tri-pod was mounted on the first floor and the rods and sampling spoons lowered to the exposed subsoil at the basement level. Princeton Hydro’s field engineers directed and logged the boring to 30 feet below the basement floor elevation.

Due to limited site access, Princeton Hydro recommended the use of steel helical piles because of their relative mobility due to five-foot sections that are joined as they are installed. Using helical pile design software, the site specific soil conditions and various diameter helices were input into the model in an iterative process until the appropriate configuration and depths were established. The final design included eight helical piles and a 12 inch-thick pile cap that was configured to allow the entire foundation to fit within the originally designed shallow spread foundation.