Provided the Stage 1 report is positive for ECO-90™ system, we move to test-drilling to a depth of 12 metres or to the ground water level if higher. We conduct a minimum of four clusters of four drilled holes across a site. The clusters are drilled holes to a range of depths: 2.5m, 3m, 6m and 12m. For this we use a geo-hydraulics failing head test to a range of depths from which we extract infiltration rates.
The test-drilling results are then provided to a Consulting Drainage Engineer who uses micro-drainage software to perform calculations regarding required storage capacity (attenuation), ECO-90™ outflow rate and half drain-down time. From this the Engineer produces a Drainage Design Statement (summarised in the example table below) showing the required size of:
- The infiltration trench (soakaway)-this is needed to buffer storm water from major rainfall events
- The ECO-90™ array(s)-these are located at the base of the infiltration trench.
To err on the side of caution, the Engineer applies a safety factor against infiltration rates and also assumes no later infiltration as this takes times to establish and cannot be recorded at test-drilling stage. The maximum storage volume is based on required parameters, typically in a 1 in 100 year event plus 40% climate change allowance.