Land Drain - Overview, Layout & Installation
Land Drain
Land Drainage is the term given to perforated drainage pipes that are used to cure waterlogged areas in gardens or other landscaped areas such as sports fields. Essentially they are perforated pipes that allow water to enter through small perforations. The water then flows away to a more suitable disposal point such as a stream, a storm drain or a soakaway. You should always check with your local authority to confirm that your chosen point of outfall is permissible. Land Drains should never be connected to a combined or foul drain without the express permission of the Local Authority, and in such circumstances the connection must always incorporate a trap.
Types of Land Drain
Most land drains today come in the form of flexible coils made from PVC. The diameter of these pipes varies from 60mm to 150mm, although 80mm and 100mm are the most popular sizes for domestic applications. Coil lengths are generally 25m or 50m, although 100m coils are available in some of the sizes. Where there is a need for strength in the pipe, a rigid perforated Twinwall pipe should be used instead of the flexible coiled option. Examples of this would be in instances such as under a driveway or where the pipe is buried deep down and hence the backfill material above the pipe is particularly heavy.
Colour
Unlike underground ducting where colour is an indicator of what is inside the duct, colour is irrelevant when it comes to land drains. Most however are manufactured in either a black or a yellow colour.
How Land Drains Work
In waterlogged areas, closely packed soil particles hold the water in position - there is no escape route for the water. By replacing the majority of these soil particles with granular material such as shingle, a clear path for the water to travel through is created, as the gap between each piece of shingle is much greater than the gaps between the original soil particles. Gravity takes hold and the water then picks up speed, channelling downwards and into the land drain itself via the perforations in the pipe. Once inside the pipe, which is laid at a gentle gradient, the water is then free to flow away unhindered to the point of disposal.
Planning the Land Drain Layout
If you are installing land drains in a garden, you should try to ensure that any point within the area is within 2.5m of the land drain. The most common layout is a herringbone pattern, where side branches are connected into a central spine using Y junctions.
For larger areas, it would be both impractical and expensive to use the 2.5m rule - instead we would recommend that no part of the affected area is further than 10m from the drain.
What fall should the Land Drain pipe be laid to?
Only a very gradual fall is required for land drainage. 1 in 150 is ideal, and certainly no greater than 1 in 100.
Installation
It is always wise to start the installation at the lowest point and work back. The trench should be wide enough to allow for 150mm of granular material each side of the pipe, so for instance when using 100mm land drain the trench would need to be 400mm.
It should be deep enough to allow for 100mm of clean bedding (ideally fine gravel with stones no larger than 10mm) on which the pipe should be laid, then 150mm of standard 20mm shingle above the pipe. Above that there should be a minimum of 300mm of free draining material (this is also usually 20mm shingle or coarse sand), followed by 100mm of the excavated soil on which the turf can be re-laid.
For a 100mm land drain, this would give a total trench depth of 750mm.
Geotextile Membrane
Whilst it is not crucial to protect a land drain pipe with installation with geotextile membrane, it will prolong the life of the drain as it aids water flow and prevents any fine particles of soil from being washed into the drain. If you do decide to use a membrane you should opt for the non-woven type, and lay it on top of the free draining material, below the 100mm top layer of excavated soil.
Connecting Land Drains
Connectors and Y Junctions are available to fit the flexible land drains. Unlike foul drainage, these fittings do not incorporate a seal as there is no requirement for them to be watertight. To connect to standard 110mm PVC drainage pipe use either a standard pipe coupling or a flexible rubber adaptor.
Catch Pits
If you have not used a geotextile membrane to prevent soil particles from entering the land drain you may wish to consider installing a catch pit. This is essentially and empty chamber with an inlet on one side and an outlet on the other side, slightly lower down than the inlet and protected by a baffle. Any grit or sediment is collected at the bottom of the pit and this can be removed at regular intervals. A typical catchpit is constructed from brickwork laid on a concrete base, with the inlet and outlet pipes built into the opposing walls.
Outfall
The outfall to a ditch or watercourse should be made via a rigid pipe (PVC is fine) that is concreted into the bank and extends out above the bank below. The area of bank directly below the pipe should be protected from erosion from the water flow. Whilst most Local Authorities will usually provide a specification for this, the detail is usually some sort of stone facing.
If you are connecting the outfall into a soakaway, we would strongly recommend that the soakaway is formed using modular plastic water crates - these are far more efficient than the old style hardcore soakaways. The pipe connection is simple - just introduce the pipe into one of the pre-formed knockout sections on the soakaway crate.
