SUMMARYA series of drainage trials was set up to investigate various aspects of drainage design on former opencast coal mining land. The drainage problem was characterized. Subsoiling, subsoiling direction with respect to slope and drain spacing were assessed as factors affecting drainage performance. The persistence of subsoiling effectiveness was also monitored. Drainage performances were evaluated statistically (Scullion, Mohammed & Ramshaw, 1986).Surface wetness was the predominant feature of drainage conditions on opencast land and the effectiveness of drainage treatments was limited to the control of this superficial water. Secondary drainage, in this case subsoiling, was found to be an essential feature of field drainage, reducing surface wetness and runoff. The effectiveness of subsoiling was dependent on there being sufficient gradient (4–5%) along channels for intercepted water to clear. For this reason, adoption of appropriate minimum reinstatement gradients and the arrangement of field drains to accommodate preferred directions of secondary treatments would assist drainage effectiveness.There was a gradual deterioration in drainage performance as spacing increased. Spacings in the range 20–30 m appeared to offer a reasonable compromise between cost and benefit. The persistence of subsoiling treatments, which deteriorated markedly after 2–4 years, was not affected by drain spacing. Performance at the widest spacings tested was, however, generally poor regardless of time since subsoiling.A basis for calculating design rates on individual sites and the implications for drainage theory of soil air-water interactions are briefly considered.
DYNAMIC EQUILIBRIUM FOR DETERMINING DRAIN SPACING FOR DIFFERENT SOILS