Operators of sports facilities should be aware that the problem can occur especially on clay soil sites. This can affect new drainage installations and possibly existing ones in severe drought conditions.
Those having drainage installed should ensure that remedial measures to cater for possible trench settlement are agreed with the drainage contractor in advance. Those with an existing drainage scheme should seek advice on how best to minimise the problem if it occurs.
A sports turf facility equipped with a drainage scheme will typically have a network of mains and lateral pipe work spaced at between 3 and 10 m centres. The pipes are installed within excavated trenches and are backfilled with aggregate. Tri-axial shrinkage of the clay soils leads to increased trench widths allowing the backfill materials to settle causing depressions on the surface.
In addition to primary piped systems, secondary slit systems are often installed, offering improved surface drainage characteristics through either excavated or injected gravel or sand slits.
Clay soil shrinkage promotes natural cracking of the soils; these cracks will take the line of least structural resistance - specifically trench or slit lines.
The clay soils of the British Isles retain soil moisture with great tenacity. Water will only be drained from a soil once field capacity has been achieved; the drainage scheme will remove surplus soil water.
This summer has been the driest on record in some areas of the UK, according to the Meteorological Office data. This could cause the drying of soil profiles to unprecedented depths with very low soil moisture content at depths exceeding 600 mm.
The resultant soil shrinkage and cracking could generate settlement of trench and slit lines on both newly installed and, more surprisingly, existing established schemes which have been installed for a number of years. Where trench or slit lines have grassed over this settlement can present an unseen hazard to players on sportsturf and to horses on equestrian facilities and racecourses.
Even in correctly installed drainage schemes some settlement can occur naturally to a greater or lesser extent in most years. Once settlement has occurred, remediation measures are limited to topping up. On intensive schemes, general sand or root zone top dressing may be an option but deep settlement will require specific application to individual trench lines by means of specialist equipment operated by specialist contractors.
Information on drainage and a list of contractors is available from the LDCA website www.ldca.org and click on Members Directory.