Technical
with pipe drainage installed in the invert of the swale. Water does not move quickly laterally within the soil surface. On the surface, compaction, and the presence of organic matter and thatch, reduces infiltration significantly and, with persistent rain, saturation soon develops. Run-off is inevitable provided there is a suitable gradient over which to run. The degree of run-off after short sharp showers is underrated - yet it is always evident in depressions in a pitch, and has been significant in the swales down the sides of three cambered pitches without installed drainage over the last eighteen months. What are the gradients necessary for satisfactory surface drainage of a sports pitch? Adams and Gibbs, 1994 are not specific, suggesting a diagonal fall between 1:67 and 1:100, but McIntyre, 1998 contends that a cross-gradient should not be flatter than 1:70. In cut-to- fill construction, gradients of 1:40 to 1:50 are well accepted and have been very effective. On level ground, the creation of a camber with side slopes of 1:70 is hardly noticeable and has also proved very successful. In fact, both Sport England and the Football Foundation do not state
preferred gradients other than to say the maximum gradient across the line of play should not exceed 1:40 to 1:50. If a gradient is essential to move water laterally over the surface, and this water is to be removed as quickly as possible to retain firm topsoil conditions, then it goes without saying that close-spaced slit drains, intended to bypass the heavy relatively impermeable topsoil, should be as close as possible. Spacing of one metre appears to be most practical and suitable to retain firm conditions. What we do know, is that this method of bypassing the heavy clay soils does work, and soft muddy conditions can be prevented if this surface water is removed quickly in this manner.
Contrary to desired normal summer
procedures of aerating with the vertidrain and earthquake, any loosening and opening up of the firm clay loam topsoil in the winter months can lead to disaster - surplus water enters and is collected in the upper layers, making them wetter and softer. At this time, firm surface conditions are vital to sustain play, and surplus water should be despatched quickly into the bypass system of slit drains or grooves.
Water moving below the soil surface
Infiltration rate is critical and so dependent on the condition of the soil matrix, the homogeneity of the particle size distribution and the organic matter content. At this point, it is worth mentioning the folly of ameliorating the upper rootzone by incorporating relatively small quantities of sand into heavy clay loam soils. Since the objective is to improve resistance to compaction and increase porosity, the particle size distribution of the sand is vital and there must be a dominance of sand in the resultant mixture (Waddington et al, 1974). Where an improved rootzone is imported, it should be fully evaluated in laboratory tests. The depth is determined on assessment of the critical tension. There has been extensive research into the criterion in rootzone design. Awareness of the capillary fringe above a drainage carpet or slow draining base is essential in drainage design - particularly the fact that water is held in the fringe to almost saturation before being released into the lower layers or adjacent drainage (Adams, and Gibbs, 1994, McIntyre,
“Any loosening and opening up of the firm clay loam topsoil in the winter months can lead to disaster - surplus water enters and is collected in the upper layers, making them wetter and softer”
FEBRUARY/MARCH 2013 PC 121
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