Challenge

A new maintenance building was planned at an existing water treatment plant. The maintenance building would be a steel-framed structure with an approximate building footprint of 2,400 square feet. A geotechnical investigation was not performedspecifically for the proposed structure; however, based on previous construction at the site, the subsurface profile was assumed to consist of up to 16 feet of “beach sand” backfill underlain by weathered shale. Deep foundations were required to support the structure and minimize the risk of settlement. Drilled caissons were originally proposed with specified ultimate pile capacities from 450 to 767 kN (≈102 to 173 kips).

Solution

Helical piles were considered to be the most viable deep foundation solution as they can be installed quickly with relatively small equipment. Helical piles were chosen over drilled caissons due to the limited working area and to reduce the risk of disturbing/ damaging existing structures, which included neighboring buildings and adjacent water basins. Drilled shafts would also require casing within the beach sand backfill and if groundwater was encountered within the excavations, which would add significant cost to the project. A full-scale, compression load test was performed on a sacrificial helical pile to verify torque-correlated capacity and the deflection-to-load response.

The foundation design included thirty six (36) Model 288 (2.875-inch OD by 0.276-inch wall) and eighteen (18) Model 350 (3.50-inch OD by 0.313-inch wall) round shaft helical piles with 8”-10” double-helix lead sections. A pair of battered helical piles was installed at each proposed caisson location to reduce the load per pile. Standard extensionsadvanced the piles to depths ranging from 12 to 18 feet below grade. The helical piles were installed to torque-correlated ultimate capacities exceeding the revised ultimate “per pile” design loads. The installed piles were fitted with standard new construction brackets to be cast into concrete grade beams. The helical pile components were hotdipped galvanized for corrosion protection.

Print (PDF)

Project Summary

Architect: Hatch Mott MacDonald

Structural Engineer: City of Toronto Engineering & Construction Services

General Contractor: CIR General Contracting

Certified Pile Installer: Foundation Supportworks® of Ontario

Products Installed: (36) Foundation Supportworks® HP288 and (18) HP350 Helical Piles, 8"-10" Double-Helix Lead Sections; Installed Depths from 12 to 18 feet, Design Ultimate Capacities from 51 to 86.5 kips