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DIY_JC 05-20-2008 03:19 PM

Retaining Wall
 
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I am in the process of purchasing a house that needs considerable work on the outside (primarily landscaping). One of the first things we have to do is build a (10-12' high) retaining wall to support the driveway.. as the house is built on a steep hill. We will be enlisting the services of an engineer, but I would like to do some research on my own to determine what my options are. What I would like to do if possible, is extend the driveway to the end of the house... to provide additional room to back out of the garage and an extra parking space. However, I do not know if this is possible due to the stepped foundation and existing brick facade. If this is the case, the only alternative that I can think of.. may be a 'L' shaped regaining wall with possibly steps between one side of the retaining wall and the house. The idea being that the retaining wall will keep soil from pushing toward the house as well as toward the back. Any ideas, thoughts, suggestions would be appreciated...


thanks in advance,

Jeff

Termite 05-20-2008 04:26 PM

You must keep the footing protected from frost, so you have to have enough earth against the house to provide 36" (more or less depending on geography and local requirements) of depth below grade to the bottom of the footing.

Your wall must be engineered, as it seems you are aware.

As for the wall, being 12' high you'll need BIG blocks. Usually a wall of that height will involve installation of geo-mesh every few courses to use the retained earth to the wall's structural advantage. A very solid footing is a must (AB3/compacted road base at a minimum, concrete would be better).

No matter what, the most important thing to remember is DRAINAGE!!! Good drainage behind the wall (clean gravel and filter fabric) and good drainage behind the footing are a must. Don't backfill the wall with soil!!!

I would also suggest cutting a swale in your yard if the grade pitches toward the wall. That will keep surface water from running behind the wall and potentially undermining it by re-directing rainwater away from the wall.

Not sure if the site allows, but terraced walls look nice and perform very well. Two 6' walls or three 4' walls would give endless landscaping options and could potentially make the wall's design/construction easier in the long run.

Termite 05-20-2008 04:27 PM

Also, the grade currently doesn't pitch away from the house, which is very important in dealing with rainwater and runoff. Your grade should drop at least 6" in the first 10' away from the house. More is better.

jogr 05-20-2008 05:03 PM

Jeff,
I agree with your idea about steps along the existing brick wall (following the current top of the step footings). I suspect that above the step footings and behind the brick is a framed wall and you do not want to backfill dirt against a brick/frame wall. Then say 4 feet away from the wall you can put your L shaped retaining wall - but I would recomend three 4 foot walls rather than one 12 foot wall for both safety and aesthetic reasons. And I definitely agree with getting the engineer.

Many commercial engineered block walls don't require or recommend a frost depth footing so you might look into those. They usually have a height limit which is also a good reason to break it up into 4 foot hign sections but you will need to find out the minimum distance the manufacturer specifies between the sections (depends on height).

DIY_JC 05-21-2008 10:44 AM

Quote:

Originally Posted by jogr (Post 124567)
I suspect that above the step footings and behind the brick is a framed wall

Yes, that is what is suspect as well. While the basement is finished.. there is a small section unfinished (toward the front of the house) that shows framing above the poured concrete portion of the wall.

Quote:

Originally Posted by jogr (Post 124567)
and you do not want to backfill dirt against a brick/frame wall.

That is what I figured. I was hoping it wouldn't be that way... but you confirmed my suspicion. Unfortunately, I am coming in after the house was completed, so it is too late to do anything about it. Would covering the brick w/ a moisture barrier (ie the black stuff) and a moisture wicking system (ie the dimpled plastic) make any difference? Or even possibly pouring a second wall right next to the brick an option. It is probably still a no-no, but I want to make sure.

Quote:

Originally Posted by jogr (Post 124567)
Then say 4 feet away from the wall you can put your L shaped retaining wall - but I would recommend three 4 foot walls rather than one 12 foot wall for both safety and aesthetic reasons.

How much distance would the 4' walls be from each other (vertically & horizontally)? I'm not sure this will leave enough room for the additional/extended driveway pad... which is what I am trying to accomplish.


thanks,

Jeff

jogr 05-21-2008 02:48 PM

I don't know of any way to backfill dirt against a brick & frame wall. Maybe there is, but I've never seen it done.

The block manufacturer usually specs how high their block can be used and the minimum distance between the walls. If they don't then your engineer should be able to. It would be somewhere around 4 feet distance between 4 feet high walls but I'm not an engineer so don't use those numbers. Use the engineer or block manufacturers numbers.


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