Design of soakaway
The relationship between the diameter or effective depth required for a soakaway, to suit a given collection area, e.g. roof or paved surface, and the average time (T) resulting from the test is shown in the graph below.
The diameter and effective depth below invert level are assumed to be the same dimension (D).
Test time (T) = 900 minutes
Plan area to drain = 150m2
From the graph below, the diameter and effective depth of the soakaway (D) are both 2.8m
Where the ground is of low permeability; dig separate soakaways to drain smaller but distinct parts, for example:
- one side of a roof to one soakaway
- the other side to a second soakaway
- the driveway or yard to a third soakaway.
Where the permeability of the ground increases with depth; tests in the deepened trial holes will give shorter percolation times. It may be more cost effective to build a smaller soakaway at a greater depth below the surface.