| Scenario | Soil | Working Load | FS | Shaft / Helix | Min Embed | Estimated Depth | Notes |
|---|---|---|---|---|---|---|---|
| Raised bed frame | Clay | 20 kN | 2.0 | 76 mm / 300 mm | 1.20 m | ≈ 1.82 m | α=0.70, su=50 kPa, Nc=9, reduction=0.85. |
| Small pergola | Sand | 15 kN | 2.5 | 60 mm / 250 mm | 1.00 m | ≈ 3.55 m | φ=32°, γ′=10 kN/m³, K=0.70, δ/φ=0.75, reduction=0.85. |
| Light deck corner | Clay | 12 kN | 2.0 | 60 mm / 250 mm | 0.90 m | ≈ 1.24 m | Higher su or extra helix can reduce depth. |
qb ≈ Nq × σv
Qult(depth) ≈ a·depth + b·depth²
- Pick units and enter a project name for exports.
- Enter working load per pile and a safety factor.
- Set pile geometry (shaft diameter, helix size, helices).
- Choose soil model and provide matching soil parameters.
- Set minimum embedment using frost depth or site minimum.
- Calculate and review recommended depth and utilization.
- Download a CSV/PDF, or save results to build a log.
Design Loads for Garden Structures
Working load is the service load one pile carries in use. For raised beds, pergolas, light decks, and trellis frames, total dead load from timber, soil, and finishes, then add live load from people and wind. Enter the load per pile, not the project total. If four piles share 12 kN, start at 3 kN each, then add margin for uneven load sharing.
Choosing the Clay or Sand Model
The clay model uses undrained shear strength (su) with bearing factor Nc and adhesion factor α, fitting cohesive soils that stay intact when wet. The sand model uses friction angle φ and effective unit weight γ′, so capacity grows with depth as overburden increases. Choose sand when soil is granular and drains well. If uncertain, run both and keep the deeper depth.
Pile Geometry and Helix Effects
Helix diameter drives end bearing because helix area scales with D². Adding helices multiplies total helix area and can reduce depth when shaft friction is not controlling. Shaft diameter mainly affects perimeter, so it influences friction along the shaft. Keep helix diameter larger than the shaft, and select helix counts that match available hardware and installer limits.
Safety Factors and Reduction Controls
The calculator converts working load to a required ultimate load using the safety factor, then applies a reduction factor for installation variability, disturbance, and limited soil data. For typical garden builds, safety factors of 2.0–3.0 are common. Reduction factors of 0.75–0.90 are typical. Increasing either value raises depth and adds reliability for seasonal changes.
Interpreting Outputs and Planning Installation
Required depth is the embedment that satisfies the chosen soil model. Recommended depth is the larger of required depth and your minimum embedment, which should consider frost, landscaping changes, future maintenance access, and overall lateral stability. Utilization compares working load to allowable capacity; keep it at or below 1.0. Use the optional torque check as a field sanity test, then export reports for records.
1) What depth should I use if soil layers vary?
Use conservative values for the weakest layer, or run several cases and select the greatest recommended depth. If a strong layer occurs deeper, the sand model may benefit more than the clay model.
2) Is the working load the total structure load?
No. Enter the load that a single pile carries in service. Divide the structure load by the number of piles, then increase for uneven load sharing, corner loads, or uplift.
3) Why does the sand model often give deeper results?
In granular soil, end bearing and shaft friction depend on overburden stress, which grows with depth. Lower γ′ or φ quickly reduces capacity, so more embedment is needed to reach the same demand.
4) How do I choose safety factor and reduction factor?
Use higher values when soil data is limited, loads are uncertain, or consequences are higher. Many garden projects use FS 2.0–3.0 and reduction 0.75–0.90 as a practical starting range.
5) What does utilization mean?
Utilization equals working load divided by allowable capacity at the recommended depth. Values at or below 1.0 indicate the design meets the target checks; values above 1.0 suggest deeper or larger piles.
6) Can I rely on the torque check alone?
Torque correlation is empirical and varies by pile type, soil, and installation method. Use it as a field sanity check and keep the model-based depth and safety settings as the primary planning basis.