Navigate Up
Sign In
Loading

My Neighbourhood

Use your postcode to find local councillors, facilities, school catchment areas and more.

Find facilities in my area
new cardiff > ENG > Resident > Planning and SuDS Approval Body > SuDS Approval Body > Frequently asked questions on infiltration

Frequently asked questions on infiltration

We have put together a selection of frequently asked questions on infiltration. 

A soakaway is an area underground that infiltrates rainwater into the ground.
Yes. Field tests are required to be undertaken in strict accordance with BRE Digest 365 in order to: 

  • Determine whether infiltration is a viable solution in accordance with Standard S1 of The Statutory Standards for Sustainable Drainage
  • Measure how quickly water can drain away
  • Allows you to size the soakaway to accommodate for storage volume
Not necessarily. Cardiff’s geology is dominated by a Glacial material that is variable in its soil characteristics. Field tests provide information on specific locations and are not always representative of the wider ground conditions. Any site investigation needs to be as extensive as possible to demonstrate that the ground conditions are representative of the wider geological unit.
We require an assessment of the wider geological soil characteristics in order to demonstrate that the proposals are not discharging into a finite aquifer. The assessment will be required to consider an area wider than the site boundary. Any application will be required to provide the following information:
 
  • Site specific ground investigation report
  • BRE365 compliant soakaway test results, including photos of the tests being undertaken
  • Adjacent site investigation information (if available)
  • Review of BGS borehole records in the area
  • BGS superficial deposits and solid geology maps
  • Observed groundwater strikes
  • Cross sections of the target strata, including: 
  • Geological map to an appropriate scale (screen grabs are not excepted)
  • OS map with north arrow and contour
  • The extent of the target aquifer through perpendicular sections to confirm its morphology and conceptual linkages to surface water and/or other aquifer bodies
  • Information from desk study to demonstrate the wider soil characteristics of the target strata. This is in order to provide a high level consideration of the potential for transmissivity
  • Annotation of relevant information, such as hydraulic gradient and borehole records
  • An appropriate scale
  • Legible key for the geological units
  • Evidence of the determination of hydraulic gradient. It should be noted that the hydraulic gradient in Cardiff does not always follow assumed principles
  • Evidence of the transmissivity of the target strata (if applicable)

Further to the consideration of the above, infiltration proposals will need to consider the following geological hazards and site constraints: 

  • Shrinking/swelling clays
  • Landslides (slope instability)
  • Soluble rocks
  • Compressible ground
  • Collapsible deposits
  • Running sands
  • Flood incidents in the area
  • Topography of the site to demonstrate there is no downstream flood risk


Only following an assessment of all relevant information will we be able to assess the potential to discharge surface water into the ground. Failure to provide any of the required information will result in delays to applications and potentially a refusal of a full application.

It is strongly recommended that applicants engage with us in pre-applications discussions as early as possible to discuss the level of detail required for a specific site.

A full SuDS Approval application requires full detailed design. If a full application is submitted without the consideration and testing for the use of infiltration, it will be refused.
 
It is appreciated that at the concept stage, applicants may not have the ability or permissions to undertake soakaway testing. In such circumstances, it is recommended that desk based research, including the provision of cross sections, is undertaken to provide an understanding of the target strata and inform the requirements of the ground investigation. We consider that the following readily available information should be considered:
 
  • Adjacent site investigation information (if available)
  • Review of BGS borehole records in the area including any information relating to groundwater
  • BGS superficial deposits and solid geology maps
  • Cross sections of the target strata, including:
  • Geological map to an appropriate scale (screen grabs are not excepted)
  • OS map with north arrow and contours
  • The extent of the target aquifer through perpendicular sections to confirm its morphology and conceptual linkages to surface water and/or other aquifer bodies
  • Information from desk study to demonstrate the wider soil characteristics of the target strata. This is in order to provide a high level consideration of the potential for transmissivity
  • Annotation of relevant information, such as hydraulic gradient and borehole records
  • An appropriate scale
  • Legible key for the geological units
  • Evidence of the determination of regional hydraulic gradient. It should be noted that the hydraulic gradient in Cardiff does not always follow assumed principles
  • Evidence of the transmissivity of the target strata (if applicable)

​Further to the consideration of the above, infiltration proposals will need to consider the following geological hazards and site constraints:

  • Shrinking/swelling clays
  • Landslides (slope instability)
  • Soluble rocks
  • Compressible ground
  • Collapsible deposits
  • Running sands
  • Flood incidents in the area
  • Topography of the site to demonstrate there is no downstream flood risk. 

​ 
It is also recommend that alternative proposals for the discharge of surface water are also considered in accordance with Standard S1. This will ensure if infiltration is not possible, the site can still be effectively drained.

Field tests should always be undertaken when possible.

 

Sites that are unable to drain into the ground quickly are not suitable for soakaways. These sites are usually dominated by clays.

Sites that have an infiltration rates of 10-5m/s or greater will be required to consider alternative proposals for the discharge of surface water, irrespective of the wider assessment criteria. Sites with poor rates may still be able to use infiltration to account for interception and may be able to reduce required storage volumes, potentially saving money and enhancing the design.

There are a number of potential geological constraints that may prevent the use of infiltration, even is site infiltration values are considered acceptable. The consideration of Desk Based research is essential and it is strongly recommended that developers engage in pre-application discussions with us.
No. All development proposals must leave sufficient space for SuDS, infiltration or otherwise. The type of SuDS to be used is driven by site characterisation not the development layout proposals.
For example if you wish to use infiltration to transmit runoff specifically to a stream 0.5km from your site.​

In principle yes, however you will be required to evidence transmissivity for the entire length of the infiltration pathway to the final outlet. The outlet should also be demonstrated to be capable of accommodating the runoff.

In practice it will prove very difficult to adequately prove the viability of this option. We consider this type of SuDS solution is only likely to be considered in large greenfield development sites where large scale investigations will take place and masterplanning is implemented. Such an option should not be a first choice solution and should be investigated thoroughly and well in advance.​
Yes, all schemes need to be building regulations compliant, for example soakaways should not be located less than 5 metres from buildings, new or existing. Irresolvable conflicts between SuDS standards compliance and building regulations compliance will require the development proposals to be reconsidered.
We will only accept geocellular crate soakaways as they improve the ability to maintain the system, reduce the risk of siltation and are less likely to fail. These systems also take up less room due to a larger void space.

More traditional rubble filled soakaways, including stone filled perforated ring soakaways, are not permitted as these are prone to silting and are difficult to maintain.
No. We do not permit this type of soakaway. It is difficult to ensure they are installed and functioning as intended and impossible to repair should they fail. They are also cost prohibitive to replace.
No. The use of distribution boxes prevents the treatment of surface water through the permeable surface of the paving. We do not believe a below ground distribution box delivers sufficient additional benefit compared to free surface discharge across the surface of permeable paving. Discharge to the top surface of permeable is cheaper to install and maintain.

Permeable tarmac is not permitted and will be refused under Standard S6 of the Statutory Standards for Sustainable Drainage due to maintenance issues.
We do not support the use of individual plot soakaways, as they are often inaccessible for maintenance or replacement. The ability to maintain or replace the soakaway is vital and assessed in accordance with Standard S6 of the Statutory Standards for Sustainable Drainage.
 
When considering the potential use of an individual plot soakaway, it is strongly recommended that you engage in the pre-application process with us to discuss whether it is viable at the concept stage. We will consider the following questions when assessing the use individual plot soakaways:
  • Can the soakaway be maintained (access and egress can be problematical in rear gardens)? 
  • Can the soakaway be removed and replaced (access and egress is problematical in rear gardens)? 
  • What is the proposed construction of the soakaway? We do not permit the use of perforated ring soakaways and support the use of geocellular products
  • What is the potential for property extensions to prevent access in the future?
  • Whether the soakaway is: 
  • 5m away from any building
  • 2.5m away from any boundary
  • 5m away from any adopted highway
  • Has a site specific maintenance plan been written for the development incorporating full access requirements, future site development restrictions and will this be provided to the home owner?
  • Has soakaway testing been undertaken in accordance with BRE 365 at the locations of the proposed soakaways?
  • Has the wider soil characteristics of the target strata been considered to evidence that the soakaway is not discharging into a finite aquifer?
  • Has the cumulative impact of multiple soakaways in the area been considered?
  • Has a reasonable factor of safety been incorporated into calculations? A minimum factor of safety of 5 of greater will need to be provided, dependant on the site specific environment. 
  • Has exceedance flow routing been considered to ensure that failure of the soakaway does not flood receptors? 


Infiltration solutions are encouraged to be designed in areas easily accessible from the highway, such as communal areas to the front of properties. We will refuse soakaways where they are proposed to the rear of a property with no consideration of the above.



​​​​​​​​
Cymraeg