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ground water coming up through paving

I'm quoting to replace a patio. With all the winter wet there hs appeared a 'spring' at the foot of some steps, the area is cut into quite a long slope. Is the answer to lay some sort of permeable pipe along the foot of the retaining wall either below the paving or in a narow gravel ditch? if so what's the right sort of pipe to use and is it best to lay and point with mortar or lay on sand (butt joined).

All advice gratefully recieved!

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Replies

  • PRO

    ooh, I would be running away, or having some really crunchy terms re groundwater damage in your T&Cs on the quote.

    ...Of course if the client was wiling to embrace the problem and have a spring (rill) coming through the area where the groundwater comes out that would cast a different light on things

  • PRO

    Simple stuff first. Pipe is called perforated pipe. Comes generally in 60mm, 80mm or 100mm diameter. I always use 100mm as it gives you increased capacity in eventof flash flood.

    For paving i always lay and point with mortar. I've seen it done on sand but i personally wouldn't do it

    You can run a gulley like this at the edge of the paving where it meets the wall.

    http://www.pavingexpert.com/drain14.htm

    We have used this quite a bit over the years depending on the size of the patio. We don't always run this to an outlet. It is just a fancy French drain.Please note though that this will do nothing to help with the water coming through the steps, see below.

    With all the rain we have had it sounds like your water problem could be water seeping through from the slope. You mention a reataining wall. Has this got drainage weep holes in it. A picture of the situation would be helpful. 

    It sounds like the water is finding a way out through the steps. A possible quick fix could be to drill a series of drainage weep holes along the wall length aprox every metre or so. Any water would now seep through here and onto your new patio.If you have a gulley installed on the patio against the wall this could be a good solution.

    Ideally i would prefer to have drainage behind the wall running to an outlet as a spring indicates an awful lot of water. This would consist of perforated pipe set on the slope side at the bottom of the wall on a bed of peashingle and covered almost to the top of the wall or soil with more peashigle and all set in a sheet of terram to stop the soil fines getting into the gravel. Then one end of the pipe needs to run to an outlet or into the drainage system. As you are doing the patio now is a good time to deal with running in any pipe work you may need. Are you fitting a gulley along the wall to collect paving water runoff. If you are you could connect perf pipe into this and then gulley obviously to the drainage point. This is good for you but not your client as it is all extra work.

    Obviouly not having any images i am only basing the above on the situation as you desribe it.

    Pavingexpert - SUDS - Drive and Patio Infiltration Drain
    A simple SUDS installation for private patios and driveways
  • PRO Supplier

    Hi, would agree with perforated pipe within gravel filled trench, aka French Drain, the pipe can be shrouded in it's own sock to prevent fines getting in and yes the gravel should be surrounded on all four sides by Terram membrane. All as Ed say.

    However, you would need to check out where you are draining the water to, generally springs occur as ground water levels rise and so the French Drain may not be able to function if these are too high.  In my experience French Drains are designed and used to deal with surface water leaching down through soil layers, not rising ground water. 

     

    • Thanks all, I've attached a very rough sketch. yes the french drain with perforated pipe looks ideal, except if I'm to send the water to the existing drainage system the pipe will be nearer the top of it rather than 2ft down as shown. I don't suppose thats a problem3966947348?profile=RESIZE_710x

      • PRO Supplier

        Don't forget the French Drain will only be functioning when the ground is saturated and so not necessarily everytime it rains, in fact this is probably umlikely.  I cant make out the 2 foot you refer to on your drawing, but all I would say is as long as the pipe has got a reasonable fall on it just get it as low as you can.

         

  • PRO

    Without some where for the drain to discharge to you are completely wasting your time and the clients money.

    Discharging the water into a foul sewer, even if physically possible, is likely to result in a big argument with the water board if they find out and they will win the argument.

    A soakaway in ground that gets saturated is not viable.

    So just where do you plan to discharge the pipe to?

     Andy 

    • PRO

      Agreed 110% Andy

  • PRO

    If there is the possibility that there is a water main or sewer leaking tell the client to get a sample of the water tested, testing will show if it is mains drinking water as it is treated and if it is sewage it will have  fecal waste and detergents in it.

    I think the water board may do the testing for them as it saves water fixing leaks.

    Andy

    • Its definitely water coming off the hill and its finding its way into the drain anyway quite happily; there isnt a huge volume of it. The 2 foot I referred to was the depth of the drain detail kindly supplied by Ed fox.

      All good caveats though! thanks again

      • PRO

        If you have some where to discharge to i'd actually always favour a linear drainage gully over the french drain option. Thats really just a workaround in certiain situations. Its generally easy to install gullies and less digging out of trenches than a french drain. What drainage is there on site? Where i am there are a lot of new builds and the gutters nearly always run into purpose built storm drains. That is your ideal option to tie in to where you have a gutter downpipe. If the gutters go to an existing soakaway instead be warned you will likely have a problem as you will be adding extra capacity that the soakaway likely won't be able to handle. Don't ever run drainage into the main sewer system as has been highlighted as that is a big no no. Read up on drainage on the website i gave you a link to. Its pretty comprehensive regarding the subject.

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