A soakaway is an underground structure — traditionally a stone-filled pit, now often a modular plastic crate system — that receives rainwater run-off and allows it to infiltrate into the surrounding soil. It is the standard way of disposing of surface water from a house where the ground percolates adequately.
A soakaway stores run-off temporarily in the voids of the fill or crate structure and releases it slowly into the surrounding soil. Its performance depends entirely on the infiltration rate of the ground, which is measured by a percolation test carried out in the position where the soakaway is proposed.
Soakaways are unsuitable where the soil has very low permeability, where the water table is high, or where infiltration could affect foundations, a percolation area or a neighbouring property. In those cases attenuation with a controlled discharge, or connection to a surface water sewer, is used instead.