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Old 04-03-2012, 05:03 PM   #1
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Sub-surface water; french drain


Hi,
My backyard slopes gradually towards the back of my home.
I have corrected downspouts and grade, etc.
I have a swale 10 ft. from the home which leads to a sewer grate.

Would a subsurface french drain help with the hydrostatic pressure on my full basement?

My basement is dry, except (before regrading, re-directing downspouts, etc.) for two heavy rains where I had minor leaks on two wall ties in that wall facing the yard.
Now that I mention this....is water directional towards the basement wall...in other words, is there greater hydrostatic pressure on the wall facing the yard...or is the pressure equal on all sides/basement walls?

I was planning on digging a trench where the swale currently is, about 3 feet deep. I would install a french drain and connect underground into the sewer (the county will do the connection for me actually).

My question is, will a 3-4ft deep french drain help take pressure off of my drain tile down at the footers? Would it be a waste of time/expense?

I am concerned about the sub-surface water mostly...the swale takes care of the surface pretty well.

Thanks!

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Old 04-03-2012, 05:38 PM   #2
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Sub-surface water; french drain


Water pressure is non-directional, meaning that the pressure in any direction is identical. The pressure is easily computed, it is the unit weight of water (62.4 lbs/cubic foot) multiplied by the difference in water elevation across the wall. So for example, if the water is three feet higher on the outside of the wall than the inside, the pressure at the base of the wall is 3*62.4 = 182.2 lbs/sq ft.

All that said, why do you care what the pressure is? Are you concerned that your wall is going to fall over? If the problem you are addressing is leakage through the wall, the solution is to maintain the groundwater level below the slab elevation. But from your post, you already have a perimeter drain, which if it is functioning correctly, should be designed to keep the groundwater level below the slab. If your perimeter drain is not functioning correctly, that is a different matter.

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Old 04-04-2012, 08:14 AM   #3
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Sub-surface water; french drain


You do not need to take the pressure off of the perimeter drain tiles at the footings. Those drain tiles are the pressure relieving means.

All you need down there is to have the sump pump get rid of the water faster than the drain tiles collect the water.

Holes in the foundation wall, such as where concrete form ties used to be, can intercept water percolating down through the earth before that water reaches the drain tiles further on down. Filling these holes and cracks should eliminate overt leakage and water flowing down the inside wall.

Grading at least the first 5 feet of land to slope away from the house should reduce the overall amount of water percolating through the earth up against the foundation so as to improve things further.
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Last edited by AllanJ; 04-04-2012 at 08:18 AM.
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Old 04-04-2012, 01:16 PM   #4
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Sub-surface water; french drain


ok, no not afraid of the wall falling down. The one wall tie was near the top
of the wall, just a fyi.
I assume the footing draintile is working.

It sounds that a new french drain 3' below the surface will have very limited effect. I was just wondering if I could intercept the water from the sloping backyard...but it sounds like thats not quite how subsurface water works.

Thanks
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Old 04-04-2012, 03:13 PM   #5
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Sub-surface water; french drain


you may benefit from reading the thread I have going:

Please Help Me With This French Drain System!
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