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Are you improving drainage or just burying money?

Landscape architects and contractors usually have to plan for surface drainage and runoff. Some landscapers dig trenches and lay gravel or drainage cell over drainpipe, then cover the gravel with the soil removed from the trench. Although this idea sounds reasonable, is it usually as effective as taking a wad of cash and burying it in the ground. This is because such soils are poorly drained, on account of low permeability and poor internal structure. Placing drainage under the surface without creating a path for water to reach it is a waste of money.

On steep slopes, gravity moves water to drains, so the main concerns are erosion and soil stabilisation. On relatively flat areas, however, the water has to move along the surface until it can infiltrate downwards into the soil. If it can’t, flooding and sheet erosion become possible. To cope with high or saturated flow, trenches and contours are installed to allow drainage.

The rate at which water flows is dependent on slope and the pore size of the soil it is moving through. Soils that are heavy in texture (that is, with a high clay or silt content) usually have slow rates of water movement. Unfortunately, loams (typically 10% to 25% clay) or sandy clays (typically 35% to 45% clay content) are often used to backfill trenches, effectively blocking off the drainage.

Improving drainage

The most basic level of drainage improvement is to create enough surface slope to cause water to move (in a controlled manner) towards drainage grates or catch basins.

The next level of improvement is the installation of drainage trenches. The rationale for trenches is to decrease the distance that water travels before it is rapidly removed: the closer the trenches are built, the shorter the distance the water travels until it is removed.

The key to success is to make sure surface water can reach the gravel or cell that will conduct the water to the drainpipe. That is, it has to be able to soak in. For this to happen, the trench should be backfilled to the surface with an open, free-draining medium (such as sand), not the native soil or, worse, subsoil. Leaving a sand or gravel cap over the area will maintain infiltration and reduce the potential for adjacent topsoil to seal the trench.

Geotextile

In recent years, new materials incorporating plastic and geotextile have become available. The purpose of the geotextile is to separate the drain from the soil, thus preventing surface slumping and drain failure. However, the pores clog with fine to medium-sized particles, leading to a gradual reduction in permeability. In some cases, landscapers completely encapsulate the drainage trench, resulting in a double layer of fabric over the top. One is bad enough! Eventually, fine particles migrate downwards with the water and block the pores of the fabric, ultimately rendering it impervious to water.

Design tips

No matter what you choose to construct the drain, keep in mind a few design concepts:

• If the budget is not big enough to install proper trenches, use the money to create surface contours and install catchment pits and strip drains.
• Do not try to improve poorly drained soils by incorporating sand. It usually creates a more compacted surface and reduces drainage. It generally takes at least 80% sand (by volume) to improve drainage.
• If drainage is to be installed, make sure that surface water can permeate to the gravel or drainage cell. Use sand or fine gravel (nominal particle size between 2 and 7 mm) to backfill the trench to the surface. Mound it over the trench to prevent infill from the soil.
• Make sure drains lead to proper receptacles.
• Make sure you comply with local regulations and codes.

How not to install a subsoil drain. The slotted pipe and gravel are covered with loamy topsoil and plastic fabric, and the slotted pipe is enclosed by a geotextile sock. Best drainage is achieved by backfilling with coarse sand or fine gravel all the way to the surface. If fabric is required, place it on the bottom and sides of the trench, never over the trench.