Commercial Foundations Case Studies: Duplex Home Construction
A 2,600 square-foot duplex home was planned for construction on two residential lots sandwiched between properties with existing homes. The rears of the lots slope down to a heavily-wooded drainage valley. The proposed duplex was designed as attached two-story, wood-framed structures with basements. A geotechnical exploration at the site encountered approximately 10 to 16 feet of very loose to loose silty sand fill over generally medium dense to very dense sand and sandy silt (glacial till). Groundwater was observed at depths of 13 to 16 feet. The borings were completed at the preconstruction ground surface elevation before the basement excavation was made. The foundation design originally included 2.5-foot diameter drilled concrete caissons. However, anticipated limited working space within the basement excavation, environmental concerns (excessive removal of trees), and public disturbance from noise and vibration during installation, prompted the general contractor to seek an alternative deep foundation system. The relatively shallow groundwater conditions would also likely require the drilled shafts to be cased to prevent caving soils and to limit water infiltration.
Helical piles were selected as the ideal deep foundation solution for this project given the piles could be installed with smaller equipment in tight access and limited working space conditions. The product and installation equipment could also be mobilized to the site quickly. Thirty-seven (37) Model 288 (2.875-inch OD by 0.276-inch wall) round shaft helical piles with an 8”-10”-12” helix plate configuration were proposed for the design working load of 35 kips per pile. The piles were installed to torque values of at least 7,800 ft-lb for torque-correlated ultimate capacities of at least 70 kips (FOS ≥ 2). The piles were installed with a mini-excavator and a tracked skid steer to depths of seven to 16.5 feet below basement subgrade elevation. Adjustments were made to some of the pile locations after encountering tree roots or cobbles/boulders during installation. The piles were shifted several inches along the grade beams to new locations approved by the project engineer. The helical pile components were hot-dip galvanized for corrosion protection.
Structural Engineer: K.O. & Partners, Ltd.
Geotechnical Engineer: Haddad Geotechnical
General Contractor: Eco-Live Homes
Certified Pile Installer: Foundation Supportworks® of Ontario
Products Installed: (37) Foundation Supportworks® Model 288 Helical Piles, 8"-10"-12" Lead Section, Installed to Depths of 7 to 16.5 feet, Design Working Load 35 kips
Soldier beam and wood lagging wall along property line
Limited access into basement excavation
Pile locations marked
Advancing lead section
Advancing lead section