System design brief
The site had previously been used for vehicle maintenance and refuelling, and contaminated soil had been treated on site by bioremediation. Existing soakaways had been removed as part of the remediation process.
The site had planning for 11 homes dependent on a storm water attenuation and drainage system with all storm water generated by the sites roofs and hard sanding areas being dealt with on site, without the need connect to water storm drains or combined sewers. The system was designed to deal with a 1-in-200 years storm event plus 30% for climate change.
The Hydrorock Natural Aquifer SuDS System
The solution at the time (before Hydrorock was on the market in the UK) that was chosen was one of 400 micro boreholes, to a depth of 3.3M lined with EGRP plastic tubing, in a trench 25m long, 4m wide and 1m deep, lined with Inbitex Geotextile and backfilled using single sized crushed angular stone to 1m and the top 500mm being back filled/capped with local clay arisings.
As the permeable strata of sandy silty clay was between 1.2m and 4m, using Hydrorock Blocks and excavating into this layer, no drilling would be required. However due to the very low infiltration rates, the Hydrorock Natural Aquifer SuDS System would require capacity of 61,700l to enable half of the required buffering of 23,000l to infiltrate in 24 hours. This volume would be spread across three modules each consisting of 42 blocks, placed in three trenches each 20m long, 1.5m wide and 1.4m deep. The floor of each would be covered with sand to a depth of 100mm. The blocks would be back filled/capped with local clay arisings to a depth of 300mm.