Soakaway Design and Construction Advice

A soakaway is a bespoke designed system to discharge water into the ground. Practically every new development from the smallest house extension to multimillion pound developments now have soakaway systems in the UK. In the past, especially on small developments, not much thought was given to the design of soakaways. It is perhaps because getting water into the ground appears like a natural process and therefore "should be easy". However, a building or road development – often located in an urban environment, is not a natural arrangement. Nature does not normally produce large areas of completely impermeable ground. So a key is to engineer a solution which mimics the natural process to support the development in the remaining land and provide mitigation.

An Architect would often specify "1x1x1m hole filled with broken rubble to Building Control officer's agreement", or it would just be left to a contractor or builder who would "fill a hole with whatever broken rubble they needed to get rid of, similar to the last job". Unfortunately, it is nothing like as simple and such approaches commonly result in flooding and flood damage. Indeed most flood related claims in the UK relate to failure of soakaways and drainage that is part of a site, not from rivers. 

You may find the following equation a useful starting point:

Runoff or UKWIR

 

First came the Pitt Review then the Flood Water Management Act and soon any soakaway shared by more than one household will need to be designed and constructed to adoptable standards.

Why do I need a soakaway designed? Can the builder not design the soakaway?

Apart from the legal requirements, you naturally want a design which is appropriate – not " just what was done on the last job". The question to ask yourself is – what would happen if in a heavy storm the soakaway filled up and caused flooding. Such a situation could be expensive and disruptive. Is the builder really qualified to undertake engineering design, does he have the design liability insurance? Some soakaways even correctly designed do flood, but the decision about when should be an informed one, not a guess. As a client you need to decide what risk you are willing to take. Perhaps spending a small amount more to protect against a 1:100 year event, rather than 1:10 year could be the correct decision.

Can I just DIY a soakaway design?

Do you know the various standards? Do you have the computer software? Do you have any professional design liability insurance for such work? If you want electrics done, is it not less money and trouble just to pay an electrician rather than study and pass the electrical exams yourself? Some jobs can be easily done and others are best left to those qualified to do them.

What is involved in the design of a soakaway?

The design of a soakaway is actually complex. It all depends on many factors including all of the following:

  • The permeability of the ground
  • The groundwater level
  • The type of ground
  • The contamination risk
  • The space available
  • The type of foundations
  • The risk of ground instability, soluble rocks, mining hazards or made ground.

We have been designing soakaways for decades and have never had a project with the same arrangement.

Soakaway / permeability testing

Obtaining the permeability of the ground is critical. Ideally, the test mimics the performance which is likely to be achieved in practice. The earlier the test is undertaken in the design process the more cost effective the solution is likely to be. Soakaway design is based on evidence, so a lack of evidence can result in variations and delays.

There are also lots of different types of soakaways, including:

  • French drains
  • Concrete ring soakaways
  • Stormcrate systems
  • Swales
  • Permeable block paving
  • Porous asphalt
  • Borehole soakaways

Often these arrangements work together on single projects to form complete sustainable drainage systems.

Soakaway failures and flooding

As a designer of soakaways, what is critical, is practical experience of their construction and failure. We have been called in to many schools and private house developments where soakaways have failed causing tens of thousands of pounds worth of damage. Just like you should always have a structural engineer design your structure, you should always have a civil engineer experienced in soakaways design your drainage.

Soakaway system design also needs to consider the structural loading and risk and implications of settlement.

Common reasons for hydraulic or structural failure include:

  • Failure to obtain permeability rates applicable to the location/discharge soils
  • Failure to undertake the correct permeability test (there are numerous – Local Council/BRE/EA/BS EN/NHBC etc)
  • Collapse of soakaway, due to failure to consider structural design and loading
  • Failure to consider discharge mechanism / strategy early enough in a development.

The most accurate and widely applied test is that provided by the Building Research Establishment in digest BRE365.However, elements of this are often not cost effective for small sites, so again practical experience in soakaway design critical.

Soakaway design – lets go back to basics…

Avoid making absolute observations about the ability of soakaways – your site is highly unlikely to be completely impermeable. Permeability is on a scale. There can be considerable variation across a site in permeability. Every site should have at least 2 tests. This will show if there is consistency in rate. It is not uncommon to find a site where one area is permeable and another not. Clearly this is a critical factor, which would effect where best to locate the soakaway, which in turn is critical to the design of the surface water drainage discharging to it.

It is not uncommon for CLAY to have bands of sand running through it, which results in good permeability, even with predominantly impermeable clay ground.

In soakaway design one needs to consider the following:

  • Attenuation (in relation to the peak rainfall events) – that is a storage void for water
  • Discharge (permeability) – that is discharge of water into the ground
  • Water quality – how dirty the water was and how clean it needs to be for discharge
  • Cost of construction/materials

So for example:

  • if the water is clean (off a roof only) discharge is to gravels with a high permeability it is likely attenuation is the critical design element, such as with a concrete ring or stormcrate system.
  • if the water is clean (off a roof only) discharge is to clay with a low permeability it is likely discharge is the critical design element. This could result in maximising the wetted perimeter, such as with a long French drain.

​So it's far more complex than – "how much storage?"

Can you provide a hydraulic strategy and soakaway calculations for my job? 

Yes. We can assist project managers, architects, clients, builders or others who have a need for soakaway / drainage design on a cost effective basis. Often it is required to support a planning condition or to show compliance with the building regulations.

We are typically very busy and booked up months in advance. It is rare that any new project proposal will commence in under a month from instruction. A lead in time of 2 months should be  programmed for.

Outline client costs

1) A charge of £500+VAT which covers the financial, administrative, legal and insurance elements of setting up all new clients for their proposed project.

2) Professional advice (travel, meetings, design etc) associated with a specific project is charged at £80+VAT per hour.

It may be possible to provide a fixed fee, where the scope is reasonably certain and an accurate understanding of time can be conservatively evaluated. Given the considerable time necessary, the smallest single house arrangement will have fees exceeding £850 +VAT.

We understand many clients would just like "some free advice". We are not able to provide any free advice.

Due to strict legal liability, professional responsibility and commercial viability – professional advice can only be given with a written agreement in place.

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