Land drainage is principally used to alleviate waterlogging in fields and gardens. Land drainage is NOT the same as a soakaway, although they may both be used to dispose of surface and/or ground water.

 

If you are looking for information on waterlogged gardens, you may want to check your soil type, before opting for drainage excavations rate.

Other uses for land drainage are as dispersal drains for septic systems and as collector drains where more formal drainage fittings would be inappropriate.

Dispersal Drain -

Returns water to the ground


Collector Drain -

Removes water from the ground

Land drainage works by providing an open conduit for groundwater to follow to a disposal point, or, in the case of dispersal drains, to a dispersal area. There are a few variations on this theme, involving the type of backfill material, and the type of pre-formed conduit that is used, but all rely on two simple principles; that the land drain provides a 'path of least resistance' for groundwater to follow, and that, left to its own devices, water flows down even the gentlest of slopes.


Groundwater is held by soils within pores, which are the small voids or interstices between the grains that make up the soil. The bedding material surrounding a land drain has significantly larger pores between each stone, providing more space for the groundwater to occupy. The centre of the pipe itself can be thought of as one big pore. Therefore, in a collector drain, there is a 'gradient' of hydrostatic pressure from the soil, via the bedding into the pipe, along which the groundwater will migrate to the point of lowest hydrostatic pressure, ie, the inside of the pipe.     Hydrostatic gradient
The flow of groundwater in a typical land drain
In a dispersal drain, the gradient is reversed because water is being continually added to the pipe from the septic tank or other source, and so the hydrostatic pressure is greatest inside the pipe, causing the flow to be from pipe to soil, rather than from soil to pipe.

Soil mechanics and hydro-dynamics is a major field within the discipline of civil engineering and is far beyond the remit of this site, but, hopefully, this will have given some insight into the basic principles involved in groundwater management.

The disposal point of a land drain system, whether it's a manhole, a soakaway, a leach field or an outfall, is always at the lowest point of the system. Try as we might, we haven't yet managed to get water to flow uphill! wink
land drain markings    

During the Summer, land drainage can sometimes be identified by parched growth of vegetation over the line of drainage. However, this phenomenon may be reversed in situations where land drainage has been used to disperse the processed effluent of a septic tank, where surface vegetation, trickle fed by a near constant stream of nutrient enriched moisture, is that bit more lush.



Types of Pipes

Most land drainage systems consists of lengths of perforated or slotted plastic or clayware pipe laid in a trench with a porous surround. There is a wide range of sizes, from 80mm flexible plastic, up to 1000mm or more for large agricultural or commercial schemes, and they come in a wide variety of materials, including uPVC, clayware, fibre-cement, concrete and ductile iron.

80mm diameter perforated flexible/corrugated plastic pipe is ideal for garden drainage. Larger diameters are also available and they are typically used on larger schemes, such as carriageway edge interceptor drains. The smaller flexible plastic type is readily available in 25m coils from your local builders' merchant at around £1-2 per metre. It is very simple to install, well within the capabilities of a DIY enthusiast. This type of land drain has holes or perforations around the entire circumference, so there is no 'top' or 'bottom'.     perforated plastic

Perforated Plastic


Rigid land drain pipes may be fabricated from clayware, plastic or other some other solid material. Some types will be perforated or slotted around the entire circumference while others will be 'half-perforated', ie, have holes on only one face of the pipe. As with their non-perforated cousins, rigid land drain pipes are normally joined together by means of a coupling, although some systems use 'sockets and spigots'. See Drainage Installation page for more details on couplings.     clayware land drain

Perforated Clayware

Land drain being installed at the foot of a stone wall     As mentioned above, there are a variety of different types of land drain available. Clayware and rigid plastic perforated pipes are generally more expensive than the flexible type and they should always be used where a land-drain passes beneath a trafficked area, such as a driveway, or where the depth of the drainage is such that the weight of the backfill material would cause a flexible plastic pipe to collapse. As a general rule of thumb, we do not use flexible plastic pipe at depths greater than 1.2m.

And don't be confused by colour. Some pipes may be yellow, white, blue, black or faux-terracotta. It means nothing: once buried, they are all just pipes!

Theoretically, a rigid perforated clayware land drain could be used at depths up to 6m, but this would require machine excavation and shoring, and is best left to professionals.


 
Historic land drains

Clayware pipes

Land drains or field drains or field tiles as they are variously known have been used for hundreds of years and, historically, they didn't rely on perforated pipes.
The most commonly encountered 'historic' version is the field tile - so historic it was still being used in the 1960s! These consisted of nothing more than short lengths (12"-18" or 300-450mm) of fairly crudely formed plain-ended clay pipes with a diameter of anywhere from 2½" - 5" (65mm to 125mm) which were butted up against their neighbours in a basic trench, all too often with no gravel surround, and then covered over with the excavated material. They were, as the name suggests, commonly used to aid drainage in soggy fields and many of them have carried on that function for decades without a problem.     land tiles

Occasionally, they can be found silted-up, which is often a result of the outfall being blocked or obstructed in some way rather than any real problem with the pipes themselves, which were nearly always laid to a respectable gradient of 1:60 or thereabouts.

They are sometimes encountered when landscaping works are undertaken on a property. Sometimes, there are remnants of an old field drain system that was left in place when the land was originally developed, but in some cases, particularly with pre-war houses, they were deliberately installed as an aid to drainage for lawns or gardens.    

 


French Drains

 


Another commonly encountered form of land drainage from the days of yore is the so-called French Drain, which is essentially nothing more than a gravel-filled trench. Nowadays, the industry prefers to use Fin Drains which are much more reliable.
A big part of the problem with French Drains was that the vast majority of them were poorly constructed. As already stated they often comprise nothing more than a trench backfilled with any old gravel that happened to be lying around, but a true French Drain should include an unjointed pipe and the better ones would be surrounded by a graded filter of selected gravels and sands.     french drain cross section


 

Roman Drains


There is archaeological evidence to show this type of drain in use over 2,000 years ago when it was probably part of a combined system, carrying surface water and foul, and they can occasionally be found, still functioning, on properties dating to the 17th - 19th centuries. However, in some parts of the country where flagstone was more readily available than gravel, they were sometimes used as a crude land drain.
roman drain     The structure is pretty basic. There would usually be a flagstone base (invert) with parallel stone edges and then a flagstone cap, all of which served to create a rectangular-section 'hole' through which water and worse could flow.


Before discarding any historic land drain or blocking them off, it's best to ensure they are actually redundant. In extended rain-free spells, they can sometimes seem dry and unused, but are actually working, and working well, once the rain (inevitably) returns. They should be tracked, if at all possible, using drain rods or cctv, to determine whether or not they can be disregarded or whether, in fact, they'll need to be repaired.

A drain which is demonstrably capped-off or breached at both ends of a run can be disregarded or removed. However, if there is even the slightest suspicion that the drain is functioning, then it's best to replace it with a modern equivalent, either a perforated pipe or a filter drain as appropriate.



Land drainage systems rely on a combination of gravity and hydrostatic pressure to create a flow within the pipes. Systems are typically installed with a gentle slope in the region of 1:100-1:200 towards the outfall or disposal point. For this reason, it is best to lay land drainage 'uphill' i.e. start at the outfall, which should be the lowest point on the land drainage system.
The trench width is usually calculated as external pipe diameter (abbreviated to OD, as in Outside Diameter) plus 150mm sidefill to each side, so, for a pipe with an OD of 115mm, the trench width would need to be....

115 + (2 × 150) = 415mm
    Backfilling

Trench sizing

The trench should be excavated in advance and lined with the geo-textile if one is to be used. The bedding material is placed in the base of the trench, then the pipe itself laid and checked for alignment and gradient before the sidefill is added and finally the backfill.

Ensure a clean gravel is used for the surrounding material, and do not use limestone chippings unless nothing else is available - Limestone can precipitate calcium carbonate in wet conditions, depositing minerals that can clog up the drain. Make sure there is at least 75mm of gravel around and above the perforated pipe. The geo-textile filter is not essential, but will prolong the life of the drain by filtering out smaller clay and soil particles, and deter thirsty roots from seeking refreshment in the land drain. Small rootlets in a land drain can quickly expanded to a pipe-blocking mass!


Catch Pits

Also known as an Interceptor or Interceptor Pit

On filter/collector land drainage systems that have not used a geo-textile to minimise soil particle or sediment migration, a catch-pit should be considered preceding any outfall to a natural watercourse or sewer system, to remove grits, silts and excess sediment. A catch-pit is, essentially, an empty chamber with an inlet pipe and an outlet pipe set at a level above the floor of the pit. Any sediment carried by the system settles out whilst in the catch pit, from where it can be periodically pumped out or removed.

 

Outfalls

Land drainage, whether it has been installed in order to drain an area of soggy ground, or to disperse water collected from hard surfaces or the effluent from a septic tank, must have an outfall, ie, a point where it can 'empty' itself.

Filter and collector systems (see below) often discharge, or "outfall", into a ditch or stream, but they can be connected to the surface water system of a property if there is absolutely no possible alternative, or if your local water board do not permit outfalls to natural watercourses. Only in the most exceptional of circumstances should land drainage be outfalled to a combined or a foul water system, and, if it is, the connection MUST be via a trap to prevent smells, effluent and/or raw sewage entering the land drainage system.

For outfalling to a ditch or other watercourse (assuming there is no problem with consent from the land-owner or water authority), the outfall pipe, preferably clay or rigid plastic, needs to be anchored into the bank with a large block of concrete. It's essential that the section of bank directly beneath the outfall pipe is adequately protected from erosion by means of a stone or concrete apron. A typical cross-section is shown opposite.

 

Most local authorities and/or water boards have their own specification for an outfall detail, but the illustration opposite shows the the type of scheme favoured by many authorities for small outfalls. Larger outfalls, such as those serving an extensive land-drainage or surface water system, those discharging into a fast-flowing watercourse or those with pipes of diameter greater than 225mm, may involve cast in-situ concrete structures and possibly steel piling driven into the river-bed. In all cases, consult your local authority before commencing work.

 

Where land drainage is used to drain a larger garden, it should be installed in the classic herringbone pattern to ensure no point within the area is more than 2.5m from a drain. Some pre-planning is essential to ensure the best use of the drain and to allow for unavoidable features such as trees, walls, etc., and to ensure that the drainage runs to a convenient outfall at an acceptable gradient.

 

When used to drain fields or other agricultural land, the distance between individual drain runs can be 10 metres or more, depending on local conditions. Such systems are almost always installed by specialist contractors, often on behalf of the local water authority, and a full survey is usually undertaken before the ground is opened.

 

Another use for land drainage in the garden is as a collector drain, installed as a single line, 300-450mm from the edge of a pavement to carry away the surface run-off and prevent the garden from becoming water-logged. In this sort of scenario, a decorative gravel can be used to dress the surface of the drain, making it a feature of the hard landscaping.

 

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