SOIL PREPARATION

6.4.5 Pipe drains 6.4.5.1 DRAINAGE MATERIALS

The main components of pipe drains are various kinds of drain pipes, pipe connections and drainage filters (Table 6.1) that are placed around the pipes which discharge into access wells or into waterways. It will be economically beneficial to install the drainage pipes as deep as possible since then the length of pipe per unit surface area of land will decrease. The recommended minimum depth of drain pipes is one meter in order to prevent damage by ploughs. However, to reduce problems of root penetration, drain pipes should preferably be installed 1.5-2.0 m deep if the soil and landscape position allows it (Western Cape Department of Agriculture, 1998). High density polyethylene pipes that are well suited for both manual and mechanical installation are used for drainage systems today. These pipes are lightweight, flexible and easy to handle with good hydraulic properties. The corrugated structure also makes it more resistant to compression. Pipe made from polyvinyl chloride (PVC) is also available in South Africa. Drainage filters or envelopes are mineral, organic or synthetic material that are placed around drain pipes to both improve water flow into the drain and prevent fine soil particles from entering drain pipes and clogging them. In addition to their hydraulic and barrier function, drain envelopes are also used to improve bedding conditions, especially in unstable soils. Envelope material should have a thickness of at least 75 mm around the drain pipes. The need for a drain envelope depends on soil properties, e.g. gravelly and coarse sandy soils will seldom require drain envelopes around drain pipes, but such casing material will be needed in finely textured soils. Access wells for catching of silt and maintenance must be constructed every 200 m as well as at strategic positions, e.g. where drain pipes join at different heights, or more than one lateral drain joins the main drain, as well as in places where changes in direction of the drain pipes occur. Space of at least 15 cm deep below the lowest outlet pipe must be left at the bottom of the access well for accumulation of silt (Van Woerkom & Streutker, 1971; Western Cape Department of Agriculture, 1998).

94 | SOIL DRAINAGE