The installation depth for helical piers is not a fixed number; instead, it is determined by the specific subsurface profile and the need to reach stable, load-bearing strata. According to the provided guide, depth requirements can vary significantly based on soil type:
- Soft Clay: Approximately 5–15 feet.
- Silty Clay or Clay Loam: Approximately 8–20 feet.
- Loose to Dense Sand: Approximately 10–25 feet.
- Gravel or Sandy Gravel: Approximately 12–30 feet.
### Determining the Correct Depth To ensure the pier has reached competent soil, contractors use two primary methods:
- Geotechnical Investigation: Soil borings and test pits identify the depth of the load-bearing strata before installation begins.
- Real-Time Torque Monitoring: As the pier is screwed into the ground, a hydraulic torque motor measures resistance. This torque reading is correlated to soil capacity using ICC-ES AC358 formulas. The installation continues—often using extensions—until the required torque and design depth are achieved.
At a minimum, piers are typically installed beyond the frost line to prevent movement caused by freeze-thaw cycles.
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