Soakaways
Soakaways are one of the greatest causes of complaints and disputes that arise regarding
specifiers – garden designers or design and build landscaper firms. Those who recognise
the need to dispose of water from a given area of a garden build project, but fail to carry
out due diligence tests to establish the potential amount of water that will need to be
dealt with.
So many schemes become subject to financial and insurance claims and disputes that may
go well beyond the immediate region of the garden being worked on due to the siting or
specifying of soakaways and their installation because the designer did not conduct sufficient
research into the history and water issues of the site and neighbouring properties.
Simply recognising that a project plan will require water disposal, and nominating various
drainage channels, with levels and falls included on drawings may not be sufficient diligence
to prevent surface or sub-surface flooding from occurring – sometimes even exacerbating
existing site problems by allowing water to enter a site by creating space for inflows.
As with all site surveys, a designer should seek information regarding the land taking into
consideration the topographical siting of the plot. Is the garden in a dip, or at the end of a
sideways sloping series of houses on a new build housing estate? Is there a bank behind the
site, where rainwater could run off into the garden? On new build sites – have you checked
aerial photographs showing the plot in previous years? Was there heavy tree cover, the loss of
which could herald problems with ground heave and a higher water table due to reduced leaf
cover and ground water take-up.
Is the ground on heavy clay? If so, problems with soluble sulphates will need to be taken into
consideration as they may destroy ordinary Portland cement used in brick chambers, or clog
drains. Or is the site over sedimentary sandstone? If present, the likelihood of silt build-up in
any system will need to be catered for.
Have you checked with the customer reference ground water tables? Do certain areas of the
garden flood during winter months? Are there are ditches or culverts in the area – not only in
the client’s garden. New builds should have geological and hydrologic survey plans available
for inspection. Request these during designer/client interviews and record any relevant points
in your survey notes.
All of these factors are crucial to the designer when specifying and siting drainage and soakways
in particular. The siting, dimensions and pipework runs of the system must be recorded
in the Construction (Design Management) Regulations 2015 Plan of the project for future
reference and to comply with Planning Permission. On some sites, the inclusion of a soakaway
may require such permission to be sought.
In many cases too, it may be advisable to obtain professional advice from a drainage specialist
to ensure due compliance, much as a structural engineer may be engaged for construction
elements of a plan that require mathematical equations to decide strengths and stress
testing. If Planning permission is required, this professional aspect will be required by the
Planning Authority
What is a Soakaway?
A soakaway may be described as a dry well, formed as an underground structure to dispose
of unwanted water, either surface water or run-off from other garden features such as a pond
overflow or drainage from areas of hard standing. It may be a simple hole in the ground
(minimum one cubic metre in size) filled with rubble, topped with a geotextile membrane
to prevent soil from filling the gaps between the stones and lined with shingle at the sides to
secure the membrane. Essentially, it is a covered, porous walled chamber that allows water to
soak into the ground thereby dissipating the water.
Water must be able to dissipate freely. Simply digging a hole in non-porous ground such as
clay will create an underground water holding tank, which will quickly fill with water directed
into the pit by means of drainage pipes!
Another system is called a Crate Soakaway, comprising of (usually) recycled polypropylene
crates which are purchased as flat packs and constructed on site. These crates do not require
any fill material, and although more expensive to buy than rubble, labour costs may be lower.
These crates need to be protected with geotextile membranes to the outer walls and floor, and
are usually held into position with shingle.
Pipe works of various types bring water from the extremities into the well. These may be
flexible perforated pipes, rigid pipes with holes drilled into the sides or a combination of both.
All will require protection with membranes and shingle to prevent silting. These pipe runs
should be covered in topsoil to a depth of around 300mm.
Size and Siting
Whilst there is no exact science involved in deciding the size of the well, it is generally held that there
should be sufficient storage capacity which is an open-ended specification! However, they must be
considered sufficiently large in the informed opinion of the specifier/designer, otherwise they may
be deemed not fit for purpose. Which is of course, where problems lay for the unwary designer… As
long as you can show that you took all available information into consideration, and can show that
your chosen capacity was reasonable, you should be covered in the event of a claim.
There are a number of rules and regulations that apply to soakaways. These come under
Building Regulations (Soakaways) Part H (2015) Soakaways must not be built closer than
five metres from a building or road. They must not be closer than 2.5 metres to a boundary.
They must not be constructed in unstable ground or lower than the water table at any time of
the year. These are very strict rules, which can often prevent soakaways from being installed
in small rear gardens, or those front gardens that are close to a road or driveway.
To summarise – a designer may wish to nominate a soakaway as being the chosen method of
disposing of surface water. If so, the designer should either instruct a specifier – a specialist in
drainage systems – or carry the responsibility for deciding the capacity and siting of a water
dissipating disposal underground chamber and connecting pipe-works that is fit for purpose.
Simply indicating a soakaway on a plan then expecting a contractor to take responsibility may be
considered complacent unless clearly stated in the drawing notes and contract pre-tender instructions.
