The following guidance has been put together to improve the quality of submitted infiltration reports to North Yorkshire Council by summarising the submission requirements for infiltration testing. This guidance should be read in conjunction with the North Yorkshire Council sustainable drainage systems guidance – 2022 update.
To adhere to the drainage hierarchy, set out in Building Regulations Part H, developments should seek to dispose of surface water via infiltration, before exploring connections to water courses. The applicant is to demonstrate to the Lead Local Flood Authority that infiltration is or is not viable by providing sufficient evidence. To do this, infiltration testing should be carried out in accordance with BRE365, CIRIA C753 SuDS Manual, North Yorkshire Council sustainable drainage systems guidance – 2022 update (SuDS guidance) and the guidance below.
To ensure a valid application, the applicant should check they have met the submitted information required, outlined below
Standard information for all submissions:
- the Lead Local Flood Authority should be invited to witness all infiltration testing
- soakaway testing is a form of ground investigation and thus ground descriptions should be provided
- made ground is an unsuitable stratum to develop a soakaway on, and ground remediation will be required for soakaways in made ground
- soakaways are unsuitable in areas of karst topography, please see local development plans for guidance
- soakaways proposed within a coal mining reporting area, the Coal Authority must be consulted, and a coal mining report prepared
- as per section 8 of the North Yorkshire Council SuDS guidance, adequate infiltration tests should be provided for all development sites, including outline and full applications
Infiltration testing for standard soakaways
The potential for infiltration can vary at a county level and at a local level, due to soil types, historical site use, groundwater levels and topography. Intrusive investigation is required to evidence the suitability per site for infiltration. Standard soakaways are generally square or circular excavations, filled with aggregate or a precast storage structure backfilled with engineered backfill.
The following should be submitted as acceptable evidence for infiltration tests:
- risk assessment of the site, to avoid contamination
- log of the infiltration pit, which includes a description of the strata within the infiltration pit, grid references, AOD and groundwater as per BS5930
- a ground investigation report, or desk study if a ground investigation is not available
- mapped plan of all trial pit locations
- a table of the raw recorded dip levels and times throughout the infiltration testing, per test, per trial pit
- the calculations and graphs used to calculate the infiltration rate (reminder, the lowest value infiltration rate per trial pit should be used, it should not be averaged)
- structural AIP for soakaways over 900mm in diameter
- images of the trial pit, including date and time stamps, images of the arisings and images before the water was added, during the test and at the end of the infiltration test. As per best practice, a scale should be included in all images
Deep bore soakaways
Deep bore soakaways may be appropriate where infiltration can take place at a greater depth, such as where near-surface soils are unsuitable. However, deep bore soakaways come with greater risks, maintenance difficulties and require approval from the Environment Agency. The use of deep bore soakaways should be assessed for suitability alongside the drainage hierarchy, however, the Lead Local Flood Authority does not consider deep bore soakaways equal to traditional soakaways due to the risks mentioned above.
Investigation parameters:
- falling head tests should be carried out in accordance with ISO 22282-2:2012
- the tests should be completed a minimum of three times, should be completed within five metres and to the assumed depth of the proposed deep bore soakaway
- acceptable infiltration rate 1*10-5, this is to reduce the risk of siltation
- groundwater monitoring over wet months should be undertaken to ensure groundwater variability has been included within the design and results provided to the Lead Local Flood Authority
- groundwater monitoring should ideally take place over a 12 month period and should include at least one seasonally high groundwater event (peaks in the spring months). Should the applicant require monitoring over a reduced time, this should be agreed with the Lead Local Flood Authority before monitoring
- there shall be no discharge into groundwater and a 10 metre unsaturated zone should be present below the bottom of the chamber
- minimum catchment size of 1000m2, larger sites will require dividing into sub-catchments
Design parameters:
- acceptable in Brownfield sites, however, a chamber must not sit in the made ground. Engineered fill is acceptable (type 2 or similar)
- for infiltration at depths at 10 metre or greater, a 10 metre buffer between soakaway chamber edges must be present
- the storage volume of the soakaway should be half empty within 24 hours
- the soakage area of the liner is calculated using the borehole diameter, not the chamber diameter
- the borehole should be a plastic or steel liner between 100mm and 150mm in diameter
- the first two metres of the pipe/liner should be a non-perforated pipe and sealed with bentonite
- the siphon head is to be one metre above the base of the chamber to reduce the risk of siltation
Figure 1 Sketch of deep bore soakaway
The following should be submitted as acceptable evidence for deep bore infiltration tests:
- risk assessment of the site to avoid contamination
- Environment Agency response and/or approval for deep bore soakaway, further guidance is available in section G9: The Environment Agency’s approach to groundwater protection document on the government website
- log of the borehole, which includes a description of the strata within the borehole, grid references, AOD and groundwater as per BS5930
- a ground investigation report, or desk study if a ground investigation is not available
- mapped plan of all exploratory ground investigation locations
- a table of the recorded raw data
- the calculations and graphs used to calculate the infiltration rate (reminder, the lowest value infiltration rate should be used, it should not be averaged)
- site-specific maintenance schedule, specifically demonstrating the management of silt and contamination control
- structural AIP for soakaways over 900mm in diameter
- any images that will be of relevance when demonstrating the above information
Consultation should be taken with North Yorkshire Council should any deviation from the above be requested, or if the applicant requires any clarification regarding the above. To contact the Lead Local Flood Authority, please contact us.