Rain surface water under my new back extension

4 Nov 2017
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United Kingdom
I recently had a side and back extension built and decided to pave the back garden by 50%. We have quite a bit if clay in the ground and a soakaway was installed. We have rain water from a rear garage going straight into this soakaway and we have rain water from the pitched roof of the extension also going into the same soakaway crate. We have drainage channels going right across the back of the house for the paving surface water.

What we are experiencing now is when it rains heavily there is water under the extension we built and since it has stopped raining heavily the water level under the house has reduced but still there. The lawn that we have left in the garden is water logged when it rains heavily.

We never had this problem prior to having this extension built and we had all the surface water from the roof of the old conservatory where the extension is now going straight to a public sewer.

So could our problem be caused by:

1. A lot of building material being dumped by the builders into the grounds of our garden causing blockage.

2. The clay ground we have in the back garden (although prior to the extension being built never had a problem, and also currently our next door neighbours who do not have an extension do not have the same problem as us).

3. Soakaway not installed correctly.

4. Soakaway not suitable for clay ground and will make matters worse.

5. Too much pressure put on the garden with all rain surface water going in it from all the rain and surface water. (Prior to the extension the rain water on our old conservatory was going straight into the public sewer.

I could get a Civil Engineer to look at it but I think they are very expensive so thought I'd try the forum just in case someone has experienced something similar.
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. Soakaway not suitable for clay ground and will make matters worse.
Soakaways do not work in clay.

A soakaway is a hole to contain a certain amount of water temporarily, such as when it rains, and the water will then permeate through the soil gradually.
If you have clay, it will just be a hole in the ground which will fill up with water and stay that way.
thanks flameport. what will then be the ptions to sort out my problem?
Speak with Building Control, as they can allow you to drain into the mains if the ground conditions are unsuitable. But this should have all gone through the hoops to get this done before the soakaway was ever built!
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Agreed, for our extension we had to drain into the earth combined sewer as presumably they already know that in our area the ground isn't permeable. The ground is always sopping in winter so I can see why they didn't require more.
Strangely enough we have a surface water drain at the front of the house, but half the water from the roof goes into the combined one at the back. It's not a miss connection as they are all like that.
Thanks Guys really appreciate the really useful feedback. My sacked builder should have known this :evil:.
Yes as should whoever did your Building Regs drawings (you did get Building Regs drawings didn't you?) and your BCO.
What the others said- soakaways don't work in heavy clay, if you had building control involved the local inspector would have pointed that one out.
Solution = talk to building control, dig a trench from extensions to surface water drainage (or to the foul sewer if you don't have a separate system), pipe, manholes as necessary, off you go
so i guess the soakaway can be taken out? It is curently full of water and blocked. Also do I do anything about improving the soil and getting ride of all the cement, concrete and crap that is inside my garden cos when there is heavy rain it is water logged
and shall i reduce the paving that has been laid to improve the situation?
IMHO not a lot of point digging all the rubble out if it sitting on clay but not a landscaping expert.....dunno about the logistics but if it wasn't going to mean doing too much twice I'd sort the roof drainage from the extensions first, maybe put some French drains/land drains around the edges of the paved bit (connected to surface water drainage) and see what happens. Shouldn't cost a fortune.....
If a soakaway is being installed in clay, then it needs to go down deep enough to get into permeable earth. It should have been filled with water, and you then check how long (if ever) it takes to clear; and if it takes took long, then you carry on digging further down. How do you know the soakaway is blocked and full of water. Did they fill it with rubble, or with voide crates and a permeable membrane.

If you can get the soakaway dug deep enough, then you could dig a series of land drains across the garden, and around the extension and link into the soakaway, and that would take the water away.
they definitely used voide crates but not sure if they used membrane. CCTV confirmed that the soakaway is full of water
Wasn't dug deep enough, and why didn't BCO double check it. They may have decided a membrane wasn't necessary being clay, but you should have soil on the top of the soakaway, so the membrane should be there at some point.

So why was the builder sacked.

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