Technical
“Planners can be very demanding and often expect the design to cope with the most severe storm that might occur in a 100 year period, and with an additional 30% or so on top to allow for climate change”
Dr Tim Lodge, Agrostis Sports Surface Consulting
identification of how much rain you actually want the surface to cope with. In this respect, planners can be very demanding and often expect the design to cope with the most severe storm that might occur in a 100 year period, and with an additional 30% or so on top to allow for climate change. That’s a lot of rain, the actual amount varying in relation to the geographical location of the site.
Obviously, there are very few subsoils that will have rates of infiltration fast enough to cope with this lot. However, even that very severe storm won’t last forever. So, during the storm, a given quantity of water will land on the surface over a particular time. Some of this needs to be located somewhere whilst it soaks into the subsoil, if it is not to flow away down a ditch and flood the houses downstream.
The purpose of a soakaway is to
provide somewhere for that water to go while it soaks away. So, the volume of water that the soakaway needs to retain is the difference between the input (rainfall) and the output (infiltration). Simples!
BRE soakaway preparation
The soakaway will be more effective if it has a large surface area over which water may actually soak into the subsoil. The models that are used to help you design soakaways assume that all of the water moves out of them through the sides only and not through the bottom. So, long trenches tend to be more efficient than large single holes in the ground. If you fill the soakaway with stone, as you often have to do, you’re taking away potential water storage space and so necessarily increasing the overall
volume required. These are all design features to be attributed to the soakaway. Similarly, the models assume no water is absorbed vertically into the subsoil surface immediately below the sports facility; these horizontal surfaces are assumed to clog up with finer particles. You can argue about this point but, in practice, it doesn’t make that much difference. And arguing with planners is not something you want to be doing too much of anyway.
So, assuming the infiltration rate of the subsoil is sufficiently rapid for a soakaway system to be possible (sometimes they are not), that system might consist of a series of trenches beneath or close to the facility. Those trenches need to be sufficiently deep and long to provide the necessary side surface area to give the output (infiltration) rate to cope with the storm. They also need to provide sufficient storage volume to cope with the backlog that develops during the storm. Clean stone backfill, typically, has a storage volume of around 30%, which immediately demands a three-fold increase in the volume of material that needs to be dug out. You might use a cellular system, perhaps in a single excavation, that would greatly reduce this, but these are very expensive. An analysis of the combined costs of
excavation and of stone associated with any particular soakaway design, against that of cellular systems, needs to be made to give best value for the client. But, artificial surfaces have layers of material beneath them which can themselves hold water. So, whilst the capacity for the vertical absorption of
Soil structure sampling kit
Cellular soakaway system JUNE/JULY 2013 PC 115
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