French drain next to foundation?

[westcoastr]

as a chemical eng, the only thing i know about soil is not to spill any toxic chemicals on it. i was hoping for some cross discipline help on my latest project.

I am putting in a gravity french drain around my house to dry out my crawl space (6"-1' of standing water during big rains). they are very popular in my neighborhood due to heavy water runoff from the surrounding hills. my house has a perimeter foundation (likely with a t-footing) with about 2-3' above the ground (believe 2-3' below grade also) and has concrete piers inside.

some drainge contractors locate their trenches 2-3' on outside of foundation and 12" below, while others put it right against the foundation. if you put it next to foundation, wouldn't you undermine the structural intergrity? especially if you dig 12" below it-are there not some type of lateral forces on the soil that could cause foundation shifts since you back fill with 3/4" rock? my pad and back yard are flat with about a 3' slope from front door to curb.

thanks

steven

 

[sam74]

The line of influence should be at a 45 degree angle from the bottom of the bearing surface, I believe. As long as don't disturb the area below that line I don't think you would have any problems.

 

[eric1037]

I wouldn't put the drain below the elevation of the bottom of the footing. I have seen drain pipe compromised and soil sucked into the pipe, thereby undermining the footing. I would place it adjacent to the footing at or above the bottom of footing elevation.

 

[LHA]

Since you are just trying to minimize standing water above grade, why not just dig a trench 1 foot deep around the perimeter. Place geotextile, then 4" clean stone AASHTO #3, 4" perforated pipe, then fill to grade w/the #3s? You don't gain anything by digging to the footing...and eric is right, why disturb the earth around your footing?

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[westcoastr]

thanks all for the replies, i will be incorporating all of these responses for my drain system. my neighborhood has both surface and subsurface water issues. from sam's response and some more investigations, it looks like if I stay out of a 45 deg angle down from the foundation I will be OK. this will gather most of the subsurface water (although hydrostatic water forces could still move water under it and back up into my crawlspace). then i will be adding a second piping system similar to steve's idea , 12" from the surface to gather my gutter water and a number of surface drains to pick up the surface water.

thanks all,

westcoastr

 

[Pondmaster]

Have you tried talking to a pipe supplier? I know ADS (Advanced Drainage Systems) offers a product called AdvanEDGE. This is basically an elongated pipe that can be placed right against your foundation wall just above the footing. These work well for draining subsurface water. If you would like you can find out more on their web site

http://www.ads-pipe.com/us/en/index.shtml.

 

[mas]

The question I have is this: is the water in your crawl space due to surface runoff, OR! do you have a high water table in your area (perhaps this is the case since you do not have a full basement).

If it's surface runoff (from nearby highpoints), the you really only need to follow lha's advice above (simple perimeter drain just below surface, smack adjacent to your crawl space wall). You would really only need to do this on the sides of the house that are in the path of the surface runoff.

If it's a high water table, then I probably would go with a full perimeter drain. As others have stated, don't undermine your footing by putting the drain below the very bottom of it - that's overkill anyway as you just want the water to stop ponding whereas you really don't need to drain every bit of water from 0" to 36" below the surface.

The trench you dig can be put right adjacent to the foundation wall and then sit and rest atop the footing (the T as you call it). Again follow lha's advice on how to construct. This will keep you dry and safe.

Aside from putting in a drain, does the ground surrounding your house pitch down, so that rainwater runs AWAY from your house, and NOT towards it? Are your downspouts properly disbursing water AWAY from the house? Consider these simple-fix items too!

Do overthink this one! & good luck!

 

[buffaloman]

One item that has not been mentioned is the fact the water from your french drain needs to leave the site. If you can't drain the pipe to daylight ie doownstream and away from your house, you will need to install a sump pump then pump it to your roof gutter drain. You do have roof gutters don't you and they are connectted to a drain aren't they? If not install a drain for the roof gutters separate from the foundation drain. The roof gutter drain pipe should be solid pipe. Like once you have it the pipe, don't let it infiltrate out. Good luck always remember one thing WATER RUNS DOWN HILL. HA HA many people forget this simple law of physics