The fill material used to backfill the soakaway can also be included in the storage capacity of the soakaway. This is applicable for either soakaway pits or trench soakaways which are entirely backfilled, or for soakaway crates or soakaways with precast concrete units where the annulus between the concrete unit and the natural soils is backfilled.

Soakaway Fill Material Porosity

In order to include the backfill within the storage capacity of the soakaway, the porosity or void ratio of the fill material must be known. For typical gravel backfill this porosity can be taken as around 30%-35%. For geocellular systems the porosity can be 95% or higher. The porosity of the specific back fill material used should be acquired from the supplier or from testing. If a value higher than around 30% is assumed, this should be confirmed through material testing.

For preliminary design purposes a list of typical values is presented below. The lower bounds relate to tightly compacted poorer quality material, the upper bounds to loosely compacted higher quality materials;

Backfill Porosity	Lower Bounds (%)	Upper Bounds (%)
Geoculluar Systems	90	95
Large Clean Stone	40	50
Uniform Gravel	30	40
Graded Sand or Gravel	20	30

Stone Aggregate Fill

The stone aggregate fill should typically be single sized coarse gravel, pea gravel, Type B or Type 6C material or 4/40 aggregate in accordance with BS 7533-13.

Where the backfilled areas a subject to loadings, from traffic for example, an assessment of the load capacity must be made. The low fines content means that any load bearing capacity is provided by point to point contact between aggregate particles. To maximise the friction and interlock between particles they should be angular and rough. In this case crushed rock should be used, never rounded particles such as marine aggregates.

The aggregate should be specified in accordance with BS EN 13242. Suitable grading and specification info for 4/40 aggregate material is shown below.