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Land Drainage

Garden Land Drainage.

Poor drainage in the garden?

If you have a high seasonal water table, you will find that the water sits on or close to the surface during periods of high rainfall. If your soil structure is heavy, this too can reduce soil permeability and cause poor drainage. Low points in the garden, can allow water to collect and drain poorly. Walking on your poorly drained lawn when it’s wet can further add to the problem because the wet soil compacts and loses its structure.

Does the problem exist in only a small low point?

It might be worthwhile considering turning a problem into a design feature! If you dig out a border and then by filling it with plants that love their feet to be wet. But if you want a lawn you can actually walk on after the rain without it turning into a mud bath, then you should install an underground drainage system of land drains, and that’s where J&F Drainage Can Help!!

First you need to take a good look at your site and decide where you want to drain the water to. The thing to remember here is that water will not travel uphill so your trenches must finish downhill from where they start. It may be possible to ‘daylight’ your pipe into an area beyond your garden but there are certain restrictions attached to this. Even waste ground belongs to somebody so you have to check that the owner of the ground you plan to drain onto doesn’t object. You can’t drain onto a road either because water running off your property onto the road surface may cause cars to skid, nor should you drain into a river or stream as nutrients from your garden may upset the ecosystem and you definitely cannot drain into your neighbour’s property thus transferring the problem onto them. You can daylight you drain if it is possible to slope the trench downhill until it comes up above ground and it’s certainly the easiest way of dealing with the excess water.

Types of Pipes

Most land drainage systems consist of lengths of perforated or slotted plastic or clay ware pipe laid in a trench with a porous surround. There is a wide range of sizes:

  • 80mm Flexible Plastic
  • 100mm Flexible Plastic

Up to 1000mm or more for large agricultural or commercial schemes, and they come in a wide variety of materials, including uPVC, clay ware, fibre-cement, concrete and ductile iron.

80mm diameter perforated flexible/corrugated plastic pipe is ideal for garden drainage.

How They Work!

image1Land 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 side fill to each side, so, for a pipe with an OD of 115mm, the trench width would need to be…. 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 side fill 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!

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