Technical
The types of aggregates used have a bearing on the speed with which the excess soil water reaches the pipes
using the smaller diameter land drainage pipes fitting snugly in trenches, filled to the surface with free draining aggregates.
It should be remembered that, when excess soil water enters the land drainage pipe, very small particles of soil enter also. When there is a good flow of water through the pipe, these ‘fines’ are carried along and eventually exit the system. However, if too big a pipe is used, water flow through it is sluggish and the ‘fines’ settle out; they slowly build-up, constricting the pipe size and, consequently, its effectiveness. Hence, using unduly large pipe is a waste of money. Not only does the pipe cost more, but it also means digging a wide trench which, in turn, leads to substantially more free-draining back filling aggregates.
For example:
100m of 60mm diameter land drainage pipe in a trench dug 75mm wide and 500mm deep:
Excavated soil in the loose = 5.6 cu metres
Backfilling aggregates = 3.75 cu metres
100m of 100mm diameter land drainage pipe in a trench dug 115mm wide and 540mm deep
Excavated soil in the loose = 9.3 cu metres
Backfilling aggregates = 6.21 cu metres
Notes: Trenches need to be dug a little wider than the diameter of the pipe, for they become narrower soon after being dug due to the latent forces in the ground.
The excavated soil has to be carted away. In the loose, it is approximately 50% more bulky.
Normally, two different backfilling aggregates will be specified, so two passes with machinery will be required.
Very considerable saving can be made by selecting the right pipe size
Let us return to the football or rugby pitch occupying an
area 100m x 50m.
Assume 25mm of rain falls on it in an hour. Assume the soil is close to field capacity and can only absorb 25,000 litres. The drainage scheme has to cope with 100,000 litres of water. The use to which the pitch is put will normally determine the spacings of the drains - a first division pitch will need speedier drainage than that used by an infants school. In either case, 60mm pipes are to be used running across the pitch. At 10m intervals there will be eleven runs, each run will have to evacuate 9,090 litres of water. At 5m intervals, there will be 21 runs. Consequently, each run will have to evacuate 4,700 litres of water. The types of aggregate used have a bearing on the speed with which the excess soil water reaches the pipes. Round aggregates drain more quickly than crushed aggregates, for there is considerably more pore space. The depth of the land drains wants to be such that 400mm of the topsoil is not saturated. Installing drains too deep can lower the water table to such an extent irrigation becomes necessary sooner. We now have advanced specialist machinery for draining sportsfields. The optimum width of trench can be dug precisely. The elevators on the trenchers convey the excavated soil into tractors or dumpers; the sward is left clean. Reinstatement of the narrow
trenches can be by seeding or turfing. It is now possible to drain today and play tomorrow. Best of all, this greater efficiency has resulted in lower costs. Hats off to the engineers and management consultants!
David Shelton will continue this theme of technical talk on land drainage in a future issue, looking at secondary land drainage systems.
David heads Shelton Sportsturf Drainage Solutions LLP in the UK. The company designs and manufactures a range of equipment for draining sportsturf.
APRIL/MAY 2012 PC 125
Multi-use Trailer Three way discharge:
Rear: To a spinner for topdressing Front: Trench filling in view of the driver
Side: Filling of trenches, filling other machines, placing sand into bunkers, etc.
TRENCHERS
01787 311811
info@trenchers.co.uk www.trenchers.co.uk
AFT45 for compact
tractors from 20hp With chain or slitting wheel, augers or conveyor to quickly install drainage systems, pipes or cables as and when needed. Ideal for sports turf, agricultural, domestic and industrial markets.
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