About

Determining the foundation configuration for framed houses on difficult terrain requires precise load calculations. This tool computes the necessary quantity of screw piles (helical piers) by analyzing the total structural weight against specific soil bearing capacities. Unlike generic estimators, this calculator integrates a soil coefficient factor, acknowledging that a 108mm pile performs significantly differently in dense sand compared to wet clay. Incorrect spacing or underestimating the soil's resistance can lead to differential settlement, causing structural warping and drywall cracks.

The calculation enforces a safety factor and ensures the span between piles does not exceed structural maximums (typically 2.5 to 3 meters for standard timber framing). Use this for planning decks, extensions, or modular home foundations where ground excavation is impractical.

Formulas

The core logic determines the total bearing capacity required and divides it by the derated capacity of a single pile.

N = ceilWtotalCpile × ksoil

Where N is the number of piles, Wtotal is the building weight, Cpile is the manufacturer's stated capacity, and ksoil is the soil resistance coefficient (e.g., 0.6 for soft clay, 1.2 for dense sand).

Reference Data

Pile Diameter (mm)	Blade Diameter (mm)	Soil Type	Approx. Load Capacity (tons)	Typical Application
76mm	250mm	Clay (Plastic)	1.5 - 2.0	Decks, Fences
76mm	250mm	Sand (Dense)	2.5 - 3.5	Light Cabins
89mm	300mm	Loam (Firm)	3.0 - 4.0	Extensions, Porches
108mm	350mm	Clay (Soft)	2.5 - 3.5	Timber Frame Houses
108mm	350mm	Sand (Dense)	5.0 - 7.0	Heavy Timber, SIPs
133mm	450mm	Mixed Fill	6.0 - 8.0	Industrial, Containers
133mm	450mm	Gravel/Hardpan	9.0 - 11.0	Commercial Structures

Frequently Asked Questions

Helical piles rely on torque and friction to hold weight. Loose soils like soft clay provide less resistance (friction), effectively reducing the load capacity of the pile by up to 40%. Dense sand or gravel "locks" the blade in place, often allowing the pile to hold 20% more than its nominal rating.

For standard wood framing (2x8 or 2x10 floor joists), the maximum span is typically 2.5 to 3.0 meters. Exceeding this causes the floor beams to sag (deflect) under load, regardless of whether the piles themselves can handle the weight.

A common rule of thumb for single-story timber frame construction is 150-200 kg per square meter of floor area, plus snow loads (which vary by region). Always add a 20-30% safety margin to your calculated dead load.

The calculation assumes the pile blade is driven below the frost line. The number of piles ensures load bearing, but the depth is critical for frost protection. The length of the pile does not significantly change the bearing capacity, only the stability against heave.