When borings show uncontrolled fill, soft clay, a high water table or expansive soil beneath a building site, a conventional footing may not reach competent bearing at a practical depth, or may have to be made unreasonably large. For these sites we often turn to helical piles: steel shafts with one or more helix plates that are screwed to a depth where the soil can carry the load.
The work covers houses on poor lots, additions next to existing footings, decks, porches, boardwalks and light commercial buildings. Piles are selected from systems with a current ICC-ES evaluation report under AC358, and the drawings give the contractor the shaft size, helix configuration, minimum depth and the minimum torque that must be reached before the pile is accepted.
Helical piles under a new foundation
- New foundation wall or column above.
- Reinforced concrete grade beam or pile cap, designed to span between piles.
- Pile cap plate cast into the concrete.
- Pile shaft, with pile spacing and layout set by the foundation plan.
- Helix plates bearing below fill and soft soil.
Scope of services
- Allowable compression and tension capacity per pile from the soil profile and helix bearing
- Shaft selection, square or round, with buckling checks through soft layers
- Minimum installation torque from the torque correlation factor in the pile system's evaluation report
- New construction brackets, pile caps and grade beams spanning between piles
- Lateral resistance using battered piles or passive pressure on the grade beam
- Group spacing and helix depth so piles do not interfere with one another
- Coordination with the geotechnical engineer on parameters and site observation during installation
How we approach it
A footing on the natural soil is checked first; helical piles are used when the numbers, the schedule or the access make that the better answer. Allowable capacity is half the least of the ultimate values the building code lists: helix bearing in the soil, the torque correlation, a load test where one is run, and the strength of the shaft, the couplings and the helix plates. Drawings carry a pile plan, a capacity and torque table, bracket and cap details, and the acceptance criteria the inspector applies in the field.