Projects Section:
Project 1: Land drains, French Drains and Soakaways.
Laying land drains is the easiest method of dealing with surplus water in
your garden. Perforated plastic drainage pipe (80mm) can now be bought from many
garden centres and all builders merchants in
25rolls. This type of pipe is easily laid in small
trenches and surrounded by shingle (small stones) to allow water through but filter out
silt. With the introduction of what is commonly called Weed Fabric into the market
place, it is now possible to wrap the pipe in weed fabric which should stop any silt
getting into the pipe. Many builders, ourselves included, actually line the trench with
Weed Fabric or Geo Fabric to give it its proper name, to stop any silt getting into the
trench. This gives the land drain a much longer life as silt can build up in a matter of
weeks and clog all of the voids which are vital to proper drainage. The Geo Fabric
will also stop, or at least dissuade, any local roots seeking out a greater water source.
A root system can clog or even break a land drain system quite easily.
Building a Soakaway.
Land drains in gardens are usually drained to a central point and this is
usually a soakaway. A soakaway is simply a hole in the ground filled with rubble and
coarse stone with a drainage pipe laid to it removing surface (rain) water from other
areas. The soil in which the soakaway is placed must be granular with good drainage
properties. It is pointless sinking a soakaway in clay unless there is a more porous
layer underneath. The diagram below is not to scale.
A Soakaway must be at least 5 metres from any
habitable building by local authority regulations.
The pipe flowing to them should be of at least
75mm diameter which is the minimum pipe size for
any surface water drainage. 80mm is the recommended size. This pipe should be laid
to a fall of 1 in 100 which means for every 1 m of pipe length the slope should be
about 10mm or 1cm. However gradual the slope, water will find its way down it.
The size of the soakaway should be a minimum of 1m x 1m square x
1m deep below the bottom of the incoming pipe. The stone infill should surround the
pipe and finish approximately 100mm above it. An impervious layer should then be
placed on the stone such as thick polythene, tarpaulin, or even a bed of concrete (A1).
Topsoil can then be placed on top of this layer to restore the garden level (A). If you
can afford it we also suggest lining the soakaway with weed fabric. As the soakaway
needs to be 1m deep below the bottom of the incoming land drain it makes sense to
measure the distance of the land drain you intend to lay and work out, using the fall of
1:100 how deep the bottom of the pipe will be when it gets to the soakaway. For
example, if your soakaway is to be placed at the bottom of the garden and your garden
is 30m long, the pipe will be 300mm deeper when it gets to the soakaway. If the pipe
starts in a trench 300mm deep the bottom of the pipe will be 600mm below the
ground when it gets to the soakaway. This means, in total, your soakaway is 1600mm
or 1.6m deep.
Care should be taken when digging a hole this deep and we suggest
digging the sides back at a fairly shallow angle to avoid any of the sides caving in.
Wet soil weighs over 1 tonne per cubic meter and it is extremely dangerous to
carry out any excavation over 600mm deep without either shoring up the sides of
the trench or raking (digging at an angle) them back. There are no rules to say
that a soakaway has to be a perfect cube and your safety is more important than
the few extra barrow loads of rubble required to fill a bigger hole.
Laying the Land Drains:
To ensure that all applicable parts of your garden are drained, the most
effective method of laying land drains is to lay them in a herring bone pattern. The
pipes should be l;aid so that no point in the garden is any more than 2m away from a
pipe. The maximum distance between
pipes is therefore, 4m. The centre line,
or spine, of the drain takes the water to
the soakaway and all of the other lines,
or ribs, feed into the spine. To set this
formation out it is as well to buy a bag
of sand and, after measuring the
distances involved, sprinkle sand down
each line of pipe you want to insert.
Excavation of the trenches should ensure that any turf is cut carefully and stored to be
re-laid later. Dig the trenches three times the width of the pipe and, after lining the
trench with weed fabric, lay the pipe in the centre. Surround the pipe with small stone
which is available from the builder’s merchants and garden centres. Clean stone with
n average diameter of 20mm is fine. When the pipe is surrounded by an equal
thickness of stone, i.e. there should be a top covering of stone equal to, or greater than
the side covering, the weed fabric should be folded over the top. At least 150mm of
topsoil is then replaced to lay your turf on. This area can then be rolled to assist the
turf with regrowth.
One further word of warning is required here. The land drais you have
put in are there to remove water from areas of your garden. When laid properly they
will still be doing this in drier spells and can cause parts of the garden, especially in
prolonged dry spells, to scorch.