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A soak pit disposes of effluent through its base and side walls, so the required infiltration area is daily volume times a safety factor divided by the soil percolation rate. Percolation ranges from over 50 L/m2/day in sand to under 10 in clay, where a soak pit may not be viable at all.
Infiltration area
Area = daily effluent x safety factor / percolation rate
Pit depth
Depth = (area - base area) / circumference
Soakaway design must comply with local rules on groundwater separation, setback distances from wells and percolation testing. Poorly sited pits contaminate drinking water sources.
By a percolation test: dig a test hole, presoak it, then time the fall of water level over a measured drop. Standards specify the procedure and the conversion to a design loading rate.
Typically 1-2 metres of unsaturated soil below the pit base, and prescribed minimum distances from wells, boundaries and buildings. These are regulatory requirements, not guidelines.