Concrete slab 2m down advice

Discussion in 'Builders' Talk' started by paulsx, Oct 11, 2019.

  1. paulsx

    paulsx New Member

    Hi all I am currently building a new garage with an inspection pit at my place. The building will be timber frame and clad etc, but I need some advice about the its construction which is like a very deep footing.

    The area for the base is now dug and the pit is dug to a size of 4.5m x 2.3 m and 2m deep. I now need to lay a slab in the bottom containing steel mesh and the frame for reinforcing the sides. I know how to construct the steel work etc but over the last couple of weeks the hole has filled with about 30cm of water. I have read some good advice on here, so I am now digging a sump for a pump to empty it. The bit I am not sure about is the base once pumped out is very sloppy and you sink into it a good 6”. I am going to dig it a bit deeper for more sub base material but I am concerned once the concrete is poured its going to be covered by water within a few hours.

    Do I need to lay it ontop of a water proof membrane or do I need to use a fast-drying concrete and then just keep pumping it out until I am ready to pour the sides?

    Any advice gratefully received
     
  2. KIAB

    KIAB Super Member

    Inspection pit!:eek:

    Not many years ago I was filling them in.
     
  3. paulsx

    paulsx New Member

    lol not enough height for a ramp!!
     
  4. stevie22

    stevie22 Screwfix Select

    Water won't hurt concrete indeed it will cure better underwater but flow across the wet concrete is a no no.

    That's a heck of a hole: how big is the finished pit? I assume you are leaving room to shutter both sides of the walls in which case dig a grip around the outside to collect the water and keep pumping it out for 24 hours.

    If there's a lot of ground water then you should tank the outside. I would blind the formation first to form a good surface to lay waterproofing membrane on extending out past he base so you can lap to it.

    Oh and if it was mine I'd cast a little sump inside the pit so you can deal with any water that does get in.

    Good practice is always to dig to depth and then get concrete into place asap especially with water in the ground
     
  5. paulsx

    paulsx New Member

    Finished pit will be 3.6m x 1.6m. Walls will be 10" thick steel reinforced as will the slab. I have left a lot of room to shutter and have dug half the depth wider at the top to stop any cave ins which has worked well given the amount of rain. I will have a permenent sump and pump as part of the base so not worried about water ingress once its finished. I do realise I should have laid the slab earlier but wasnt ready and then its just been to wet here. I will try the trough around the outside and see if it drys out at all so i can get the sub base down
     
  6. techie

    techie Screwfix Select

    A nice two post lift only needs about 10 feet headroom. Have to be careful using it though...
    It’s too easy for all kinds of heavy harmful gases to build up in a pit,that’s one of the reasons a good many have been filled in.
     

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