Infiltration testing guidance note
Accurate and compliant infiltration testing is critical to working out if you can satisfactorily drain rainwater into the ground and what water storage volume is needed to achieve a suitable design. The design infiltration rate, how fast water is drained through the ground, is determined by the infiltration tests. The slower the infiltration rate, the more storage volume will be needed. These site investigations go hand in hand with (and are informed by) groundwater monitoring[ii].
Groundwater levels should be fully evaluated before determining the extent that infiltration can be used on a site. This is important as it can make infiltration into the ground unviable. This is because national guidance stipulates that there must be 1m of unsaturated ground between the base of an infiltration component and the expected peak groundwater level.
It is important to recognise that if infiltration viability is not accurately demonstrated, then there can be significant delays to a proposed development if further infiltration testing is required.
This is particularly critical if a site relies on the use of infiltration into the ground as the only option to drain surface water. Examples of alternative options could include nearby watercourses or surface water sewers. In those instances, the viability of infiltration is expected to be demonstrated prior to planning permission being granted.
Infiltration tests must be strictly compliant with those described within the most current BRE publication DG 365, “Soakaway Design”. This document is commonly referred to as BRE DG 365. Alternatively testing can be completed strictly in accordance with CIRIA R156. The testing methods described within these are clear and this guidance note is not intended to replicate it.
We do highlight that the method stipulates that trial pits must be,
- located in the area where an infiltration feature is proposed,
- set at least 1m above the expected peak groundwater level[i],
- undertaken at the expected base level of the proposed infiltration feature[ii],
- tested in winter or spring[iii],
- filled with the corresponding head of water to mimic the proposed design,
- observed from full[iv] to empty, or near empty[v],
- tested three times in quick succession[vi].
We suggest that you avoid undertaking testing during rainfall as this may affect the results.
We do not generally accept infiltration testing in boreholes. If a designer believes that borehole infiltration testing is necessary, then they are advised to discuss and agree the testing methodology before making an application for planning permission.