Garden Drainage: The soakaway system
This is the simplest drainage method. The soakaway, which is simply a large hole into which the drainage water is channelled, is best sited at the lowest point in the garden, preferably not too close to any buildings or other structures, to avoid damaging foundations.
The soakaway should be 90 cm-2.1 m (3-6’) in diameter and at least 2.1 m (6’) deep. It is most effective if it can be dug right down to a permeable layer below subsoil, although this may mean digging to a depth of 3 m (10’) or deeper. Carefully dig out the topsoil, keeping this separate—you need it to cover the completed pit. Next, remove the subsoil in 30 cm (1’) layers until the required depth is reached, taking care not to damage any pipes or cables while digging.
Then fill the pit with builders’ rubble, broken bricks and stones, to within 90 cm (3’) of the surface. Cover these large stones with a layer about 45 cm (1-1/2’) deep of small stones, and then return and spread the topsoil, adding moreto raise the level as required. Water from the surrounding garden area can be drained into the soakaway by means of drainage trenches, thus extending the drained area. Consider your drainage system carefully, especially in a new garden, so that your scheme is not seriously affected by any new building, and you can dispose of any roof water.
Trenches to trap surface water need not be as deep as underground drains: about 45-60 cm (1-1/2 —2′) is a good depth. Surface drains should also have a fall of 1:100 to a suitable outlet.
The simplest form of surface, or ‘catchwater’, drains are 30 cm (1’) wide, of the depth and gradient indicated above, and filled with rubble or reject gravel to within 5 cm (2”) of the surface and topped off with pea gravel or a similar material.
Most drainage schemes involve digging a network of trenches to drain into a sump or ditch at the lowest point of the ground. A word of caution is called for here; before embarking on the disposal of water onto land other than your own, consult the owners and your local authority. This can often avoid difficulties later.
On short runs of drain, trenches can be lined with cither brushwood or rubble; the latter is preferable in that it lasts longer.
Rising from the outlet or lowest point of pegged out before you excavate the The drains should have a gradient not less than 1:100—a rise of 10 cm (4”) in 10 m (11 yd). Use a spirit level to check this gradient. The path of the drains should be cm (1’) wide trenches. As with the soakaway, keep the topsoil separate from the subsoil. Place a 30 cm (1’) layer of brushwood or rubble in the bottom of the trench, then return some of the subsoil and finally the topsoil. Remove the surplus subsoil from the site. Be prepared for the soil to sink after a year or two, especially if you use brushwood to line the trenches. Hazel is one of the best kinds of wood to use. You can also lay turves on top of the brushwood, grass side downwards, before returning the soil to the trench.