SOIL PREPARATION

CHAPTER 6

6.4.5.3 DRAIN SPACING Drain spacing is determined, amongst other factors, by permeability of the soil and depth to the impermeable layer in the soil and by design water table depth. Soil hydraulic conductivity ( K s ) is the main factor that determines the feasibility of a subsurface drainage system. Low K s values, such as found in silty loam and heavier textured soils, put the economics of subsurface drainage in question. If the K s values become too low, drain spacing will be too narrow to be economical. The permeability of a soil can be estimated from its textural class (Table 6.2). Although not accurate, such values can be used in preliminary planning. TABLE 6.2: Permeability values according to soil texture (Western Cape Department of Agriculture, 1998). Soil Texture Permeability ( K s ) (m/day) Clay (0.0005-0.002 mm) 0.01 Silty clay 0.15 Sandy clay 0.40 Clay loam 0.50 Loam 0.80 Light loam (silt loam) 1.20 Fine sandy loam 1.50 Sandy loam 1.80 Fine loamy sand (fine sand fraction = 0.074-0.2 mm) 2.00 Medium sand (0.2-0.6 mm) 3.00 Sand (combination of fine, medium and coarse sands) 5.00 Coarse sand (0.6-2.5 mm) 20.0 Gravel (12.7-76.2 mm; no soil between gravel) 35.0 Stones/pebbles (no soil between stones/pebbles) 45.0 Soil hydraulic permeability can also be determined in situ using the borehole test developed by Ernst in 1950 (Reinders et al ., 2016). In short, the method entails drilling a hole with a soil auger to 0.6 m below the water table, emptying the borehole a few times and subsequently allowing the water to rise to the original water table depth. The water level in the borehole must then be lowered quickly by approximately 0.3 m through pumping or bailing and finally measuring of the rise in the water level with time. Criteria for successfully conducting the borehole test are provided in Reinders et al . (2016). In South Africa, the Hooghoudt formula (Reinders et al ., 2016) has been used for many years to determine drain spacing. A guideline for drain spacing in relation to soil depth and texture is provided in Table 6.3.

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