Why is fast fast percolation value bad when designing a soakaway?
I have just undertaken a percolation test on some ground to establish its suitability for a soak-a-way system.
I dug my hole and previously filled it, then later filled it with 300mm of water. the water drained at an average of 1.9mm per second. based on SEPA guidelines, anything below 15 seconds per mm drop is fast and may render the ground unsuitable for a Soakaway.
I can understand the problem if the ground drains too slowly, but I cannot for the life of me work out why a fast drainage rate would be bad!!
Does anyone know why ground that drains very quickly might be bad for a soakaway system?
Thanks in advance.
In case it helps I am located in Scotland and have been following the guidelines indicated on a site dedicated to Scottish drainage plans.
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I agree it sound counter intuitive, but a guess, mind you this is just a guess, is that if the water moves too fast it may increase the chance or reaching and contaminating ground water or near by wells. Not that wells will be near by but you know what I mean.
its an advantage not a disadvantage
Good question emailtea, I don’t know the answer. I have dug soakaways before but many years ago, when regulations weren’t so in depth. The only thing that springs to mind is….. If the ground allows liquids to pass too freely, as it rushes through constantly it is likely to drag the ground particles through with it and create a void or voids which then may lead to subsidence/sinking in unforseen places.
Hi there,
I am in the Scottish Borders and the answer I have been given by our local building standards officer is that if the water drains away too quickly it may be that the hole is close to a fault and the water would end up in the river.
regards.
water will find the easiest and least restrictive way to drain away, if the water drains to quickly it may well take soil particles with it creating voids which may undermine and cause subsidence.
their are a few variations for rainwater soakaways
1) A herringbone style soakaway, it looks like a spine and ribcage and uses perforated pipes to allow the water to drain away evenly into the soil. if a pipe breaks then just add another rib whereas you would have to replace a pit style soakaway.
2) A new method is to use what looks like milk bottle crates stuck together and wrapped in neo-preen instead of rubble etc. this allows the water to drain away steadily and will not allow tree roots to penetrate the soakaway.
3) drain the rainwater into a nearby ditch/water course (permission may be required)
4) Create a pond, a friend who is a landscape gardener built a pond with an inlet pipe from the gutters and outlet pipe to a small soakaway.
5) put water butts under each downpipe and use the water for your garden.