Low Level Patio - Efficient drainage

[JonD]

There's a channel (moat!) running round my house because someone in their wisdom decided long before I moved there to install a slabbed drive and rear garden patio that went up to and above the the air bricks. The house is a three-bed semi occupying a low position on a street with a slight incline. Wanting to extend the rear patio - whilst at the same time lowering it so as to do away with the 'moat' - I removed the 3x2 flags and dug out six inches of soil to the new patio level and size then dug a nine inch trench round three edges of the new patio area. I then put four inches of concrete in the trench in preparation for a low, brick-built retaining wall. Then the heavens opened and it rained at a rate I've never known before. The trench filled with water BUT the clay soil in the patio area stayed pretty well as was. However, after the deluge, because water in the main rear section of the trench didn't drain very well, I dug a soak away just in front of the trench and inside the patio area. Result: the soakaway filled with water so I tried to put the soil I'd dug out back. Result: it turned to a sort of grey mushy mess submerged under the water so I dug out the mush and put in gravel. Result: that too became submerged. I then dug out the gravel and started bailing. Once all the water was removed and bailing ceased......the hole filled straight back up although it never overflowed on to the patio area. I continued occasional bailing over a period of days when finally the water level in the soakaway began to slow considerably.
Something I've learnt from this is that soil when left alone acts as an effective sponge. My question is, should I now fill the soakaway with concrete to prevent ingress of water to the hole after a downpour? Also, what are the flooding implication for the patio when it sits below the level of my nearest house drain?

(Edited by JonD at 12:07 am on Aug. 17, 2004)

[alan ditchfield]

In an ideal world concrete would be the best thing to refill the soakaway depending on size as it may get expensive, otherwise a very well compacted hardcore is the next best thing but this would have to be done when everything has dried thoroughly as this too will soak up the water when uncompacted. As for the patio this sounds like a likeley flood to me, if the water has nowhere to go then it will rise untill it meets the level of the lowest drain, a channel drain at the bottom of the patio piped to a drain is the best option here.
Alan.

[JonD]

Alan - Thanks for the prompt reply. What if the patio is below the level of the nearest drain? I expect by a 'channel drain' you mean a shallow trough covered by a grate of sorts? If this is necessary (which I don't know yet because the retaining wall has to be built and then the flags put in place) I'd have to pick up a low point in the existing house drainage system which likely means the inspection chamber down the drive near the front gate. But what if the inspection chamber isn't low enough? Don't tell me.....a pump?
Well, we'll see. I'll get the wall up this weekend then lay the 3x2s........wish me luck

[alan ditchfield]

There will allways be a drain low enough to take surface water the only problem is how far away it is from your project. Good luck Alan D.

[84-1093879891]

The hole you dug was NOT a soakaway: it was a hole. The key difference is that a soakaway has the ability to allow water to soakaway - your hole fails this basic test, and so could only be called a sump, at best.

Lean-mix concrete, CBM3 or DTp1 will be fine for backfilling the sump, but wil do nothing to help drain the eventual patio. It may be that a wet-well with pump is your only solution, but consider all other options first. There are linear drains (channel drains) that are particularly shallow (they're called Deck Drains and are often used on bridge structures where we don't have the luxury of depth) and may be easier to connect to the existing system at minimal fall.

Have you determined the invert level in the IC to assess just how much fall is available? For SW, fall of only 1:100 will work adequately.

[JonD]

Tony McC wrote: "The hole you dug was NOT a soakaway"
Ha....you're not wrong there!
Well (no pun intended), the retaining wall's up and the good news is there's very little surface water retention on the clay soil in the intended patio area, despite heavy rain yesterday. My bricklayer left three drain slits in the side wall. I chucked a good few bucketfulls of water - taken from my 'soakaway' - into the corner where the slits are and it soon dispersed. I'll get the flags down on a cement/sand base and keep my fingers crossed.
Tony....many thanks for your comments.

(Edited by JonD at 9:21 am on Aug. 24, 2004)

[JonD]

Tony McC wrote: "Have you determined the invert level in the IC to assess just how much fall is available? For SW, fall of only 1:100 will work adequately"
Not sure what that means but what I'm going to do is try to get an idea of how low the bottom of the inspection chamber in my drive is in relation to where a channel drain would be on the patio which is at the rear of the house. If it came to it, I'd dig a trench off the low point of the patio, onto and down the drive to the inspection chamber. If the chamber's not low enough, I'll carry on round into the front garden where the main run-off from the chamber will be lower still (recall that my street is on an incline).
That said, a plumber friend called round this morning and he thinks I should be able to doctor the existing house drain standing directly adjacent (but some way above) the intended patio area......like I've said, we'll see