SOIL PREPARATION

CHAPTER 6

SUMMARY INSTALLATION AND MAINTENANCE OF DRAINAGE SYSTEMS

• Installation starts with a good lay- out plan that must afterwards be filed for future reference. Exca- vations and installation always begin at the discharge end of the system and progress backwards. This can be done either manual- ly or mechanically, but the latter method is more accurate, faster and cheaper. • Drain spacing is determined by the permeability of the soil, depth to the impenetrable layer, and the design water table depth. Soil per- meability can be determined, but there are also guidelines available that are based on soil texture. If permeability becomes too low, drain spacing will have to be too narrow to make the system eco- nomically viable. Guidelines are also available for spacing of drain pipes depending on depth and texture. • Pipe sizes can be calculated or de- termined using existing standard

diagrams. According to experts, plastic piping of 70-90 mm diame- ter is the most common for later- als while 110-200 mm is the most common diameter for collectors. Drain pipes must be installed at a certain slope to prevent silting up of the pipes. Large pipe diameters require a smaller slope. • Subsurface pipe drains have a long life, and properly-installed systems will have minimum maintenance after the first two years. The first year after installation is the most critical. Ponding of water, holes in the backfill, and blowouts on slopes are all tell-tale signs that certain reparations are necessary. • Certain practices such as planting of trees close to drain lines and vehicle traffic close to drain lines before the soil has settled are ob- vious practices to avoid. Much can also be learnt about the function- ing of the system by inspecting its outflow.

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