Ways to improve water-carrying capacity of soil when infiltration is compromised
These soils that have no obvious permeable layer and very low hydraulic conductivity are more difficult to drain. The aim of these drainage techniques is to fracture and crack the soil and to form a network of closely spaced channels.
Mole drainage is suited to soils with a high clay content, which form stable channels. Mole drains are formed with a plough comprised of a torpedo-like cylindrical foot attached to a narrow leg, followed by a slightly larger diameter cylindrical expander.
Their success depends on the formation of cracks in the soil that radiate from the tip of the mole plough at shallow depths as the soil is displaced forwards, sideways and upwards. Therefore, the ideal time for mole drainage is during dry summer conditions, for maximum cracking in the upper soil layers, and adequate traction preventing wheel-spin on the surface.
Gravel-filled moles are required where an ordinary mole will not remain open for long enough — as in unstable soils with lower clay content. The mole channel is filled with gravel which supports the channel walls.
Gravel moles require washed aggregate of 10-20 mm size, to function properly.
Collector drains across the slope at 0.75m are also required for mole drains, at spacing from 10m to 60m, depending on site. Stone backfill for collectors should be filled to within 250mm of the surface.
The mole drains are at a right angle to the collectors at spacings of 1m to 1.5m and depth of about 0.4m to 0.5m.
