A soak pit is a hydraulic sizing problem, not a structural one — no loads, no reinforcement, no AS 3600 clauses at all. The only question is whether the pit's wetted sidewall area can absorb the design effluent flow within the available groundwater clearance.
Every check above runs automatically inside the Structyze Soak Pit module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
The Example We'll Use
- Design daily flow 1600 L/day (3-bedroom dwelling)
- Percolation time 20 min per 25 mm
- Pit diameter 1.5 m
- Depth to groundwater 4.0 m
- Separations: bore 30 m, building 6 m, boundary 3 m
Working Through It
The 20-minute percolation rate converts to an application rate ra ≈ 28 L/m²/day, giving a required sidewall area A = 1600/28 ≈ 57 m² (rounded to 60 m² with allowance). Usable depth is capped by groundwater clearance: 4.0 − 1.5 = 2.5 m, comfortably above the 1.5 m minimum clearance required.
Here's the real constraint: a single 1.5 m-diameter pit deep enough to reach 60 m² of sidewall area would need to be 12.7 m deep — far beyond the 2.5 m usable depth the groundwater clearance allows. The only way to hit the required area within that shallow depth is to split into multiple pits.
The engine solves this: 6 pits at about 2.1 m depth each, giving the same total 60 m² of absorption area within the depth constraint every single pit must individually respect.
What Governs, and Why
Every site check passes comfortably — percolation rate, groundwater clearance, and all three separation distances (bore, building, boundary). The real design decision isn't a pass/fail check at all: it's recognizing that a single pit can't physically satisfy both the area requirement and the depth constraint simultaneously, and that's why the sizing splits into six shallower pits instead.
Where Structyze Takes Over
Every check above runs automatically inside the Structyze Soak Pit module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
For Soak Pit and the rest of the full catalogue — 80+ components across RCC, Steel, Composite, Timber, Masonry and more, across 20+ design codes — buy the full Structyze desktop app or see everything it does on the Software page.
Quick FAQ
Why can't a single deep pit just be used instead?
The 12.7 m depth a single pit would need to reach the required absorption area sits far below the available 2.5 m of clearance to groundwater — drilling that deep would put the pit's lower portion below the water table, defeating the purpose of a soak pit (which relies on unsaturated soil to absorb effluent) entirely.
Is a soak pit always the right choice over a drain field?
No — the chapter itself notes that in most Australian settings, a drain field (leach field) is generally preferred, and soak pits are restricted or prohibited in many jurisdictions. A soak pit only makes sense where local rules explicitly allow it and the soil is coarse enough for effective sidewall absorption.
Final Thoughts
A soak pit's sizing logic is a clean example of a hydraulic calculation where the real engineering judgment is recognizing a geometric constraint (depth versus area) rather than checking a single formula against a limit.