Need to drain groundwater reemerging from dirt slope under the house

[jbs1]

Hi all, I’ve got a dirt slope under the house that has groundwater coming out of it after rainfall, washing dirt and bits of rock down onto the concrete floor of the workshop. There are piers all over the place and the slope is uneven, making it hard to find an obvious path to divert the flow using ah pipe, and I don’t want to go digging because of the piers. Any thoughts or suggestions? Cheers! Jim

 

[jbs1]

Hi All, seeking advice on how to drain mainly sub-surface water from a slope of rock and soil over a concrete slab. As you can see in the pictures, I've got a (under the house) workshop floor slab which gets flooded when it rains heavily; water rises up from next to the slab, and also comes down the slope and emerges where the dirt ends and the slab starts. 95% sub-surface, a little surface water if it's deluging.

I'm thinking of a french drain system (gravel, socked slotted ag-pipe) snaking down the slope to gather and direct the water, but am not sure how best to then divert the water over the slab, as I can't direct it either side. Do I build a small retaining wall on top of the slab with a hole for non-slotted ag-pipe to then run outside? Am I going to need a permanent 'sump pump' installed in a basin next to the slab which is fed by the french drain?

Pics attached from looking up the slope and down, showing where I'm planning on putting the french drain and where I've got to get the water out to.Planned french drains above, rectangle shows where water seeps down / rises up onto the slab.From the other direction; where I need to direct the water.

Any thoughts or suggestions gratefully appreciated!

Cheers,

Jim

[MitchellMc]

Hi @jbs1,

I see you've had some work done, but are still dealing with the same drainage issue.

I think you are certainly on the right track. I'd simplify your drainage design slightly and have just one collection point for the whole slope. I believe only one deep French drain constructed correctly will be enough. It can be directed to a stormwater pit that contains a sump pump. The pump can send water out via a 32mm pipe attached to the wall with pipe saddles.

Dig a trench between the slope and the slab to be slightly deeper than the slab. Paint the side of the slab with waterproofer and lay drainage matting on the soil side to stop sediment from entering the system. The matting can be extended up the slope and pegged into it. This should assist with preventing soil erosion from the surface runoff. Personally, I'd be inclined to install a small one sleeper high retaining wall using 0 x 50 x 3mm 0.65m H Joiner Post Galvanised Steel Sleeper Uprights just to future proof the system. Optimally I'd suggest building the retaining wall right next to the slab and having the drainage behind it. Much neater and likely more function that way. 

The water will hit the edge of the slab and fall through aggregate into an ag-pipe. The ag-pipe needs to be graded towards the pit. 

The only way I can see you doing away with the pit and sump pump is if you cut a trench into the slab with an angle grinder and a diamond blade. In order to establish fall, the trench will need to be quite deep and involve a substantial amount of work.

I've spent some time creating a rendering of my proposed solution for you. I hope it assists you in your project.

There's been some great advice in this discussion already and I'm sure we are all keen to see you solve this issue.

Please let me know if you have any questions.

Mitchell

[jbs1]

Thanks Mitchell, that's great - really appreciate you taking the time to put together those images, that's really helpful. Cheers!

[MitchellMc]

No problems, @jbs1.

Now that you've made some changes let me mention @TedBear to see if he has any other thoughts.

Mitchell

[TedBear]

Thanks for the tag @MitchellMc .  I am very impressed with your design and images... I can't think of anything to add to that!

[Ben2065]

Hi @redracer01 ,

I loved your diy concrete spoon drain concept and instructions. Can I ask for my own project I am considering a 13m long run spoon drain down my soil laneway (slightly curved in its path). Is it best to install expansion joints or unnecessary? Also is there any need for mesh in the concrete as it will only be foot traffic and garbage bins (no vehicles). I understand you are suggesting the soil trench be about 3cm thick and 15cm wide? I will also be installing buffalo turf either side of the drain to assist with water slowing, sediment collection and water absorption. I really haven't seen a spoon drain construction so clearly and simply explained, Thank you!

[EricL]

Hello @Ben2065 

I suggest building it in 1-meter lengths and joining the drains with silicone. In this manner it will have a measure of flexibility in case there is any ground movement. Putting in mesh into the assembly is always a plus and will give the drain extra strength. 

The measurements you've mentioned are recommendations and if you feel that you need a larger spoon drain you can adjust it to your needs. The beauty of a D.I.Y. spoon drain is that you can adjust its size and put in modifications which you feel will improve its overall function.

For example, you can build a round soil catcher in the run. It's basically a round concave cement dish that traps the soil in the middle but still allows the water to flow away.    

If you have any other questions, we can help with, please let me know.

Eric