SOIL PREPARATION

CHAPTER 6

Topsoil

Subsoil

Plastic Gravel

Stone

Imper- meable Subsoil

FIGURE 6.5: Schematic illustration of various kinds of stone drains (redrawn after Saayman & Van Huyssteen, 1981a).

Open ditches can also be filled completely with stones from bottom to top if stones are available. A stone drain of this kind does not get blocked by soil that washes down into the openings between the stones, and vehicles can move freely across. Weeds and other vegetation do not grow between the stones. 6.4.4 Bore holes Pumping of water from a borehole can in some particular instances be considered as a method for subsurface drainage. Groundwater pumping will be more effective than normal gravitation drainage when the permeability of the deep subsoil is much higher than that of the topsoil. Drain systems in the dense topsoil will not be effective. A pre-condition for groundwater pumping is that the permeability of the deep subsoil must be approximately 10-80 m per day and the water carrying layer several meters thick. Such conditions can be found in coarse sand deposits of old river courses (Van Woerkom & Streutker, 1971). According to Webber & Jones (1992), the aquifer or sand bed should be close to the soil surface (< 15 m). In absence of experience with successful groundwater pumping in an area, the latter authors recommend a geo-hydrological investigation to determine whether a suitable aquifer exists. Such an underground investigation should also indicate whether water tables are perched or of a wider regional nature. Although the capital costs of groundwater pumping will be less than that of pipe drainage, maintenance and operating costs of a borehole are expensive. This drainage method should consequently only be considered if the water table is lowered over an economically acceptable area. The financial viability of groundwater pumping will be aided substantially if the pumped water is fresh and can be used for irrigation.

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