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Retaining wall backfill step building questions

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kwschumm
Join Date: Feb 2003
Posts: 5764 NW Oregon
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2008-09-15          156677


So, I show up at Mutual Materials, our local hardscape supplier, to place an order for materials for a retaining wall. When asked a question about construction the guy said, "We don't build 'em, we just sell the stuff". Yeesh.

So, I come to the experts here.

When building a gravity wall without geogrid the mfr. instructions say to construct the back of the wall with a drain and compacted gravel all wrapped up in landscape fabric.

With geogrid the instructions are a little fuzzy. It says to use compacted gravel sandwiched between geogrid all wrapped up in landscape fabric. Then in another place it says with geogrid the site soil can be used as backfill, which sort of makes the landscape fabric useless except to cover the gravel area immediately surrounding the drain. If the geogrid is sandwiched exclusively with compacted gravel it will take about 12 yards of gravel since the geogrid will extend 4' behind the wall for 80' of 4' high walls.

What's the right way to do it with red clay soil?

Also, need to build steps. These blocks are expensive. I'd like to construct the steps by building a four wall box, with the side walls stepped, and then fill it with compacted gravel using the blocks only as risers. Will that work?


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Retaining wall backfill step building questions

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candoarms
Join Date: Mar 2007
Posts: 1932 North Dakota
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2008-09-16          156683


Kwschumm,

The land around me is about as flat as a pancake. Therefore, I don't see too many retaining walls in this area. However, sewer drain fields are very common in the rural areas, and they use the same system when constructing these.

The clay around here is yellow in color, as it is composed of a mix of about 50/50 clay and sand, which makes a good drain field. I don't know how well water will penetrate through your red clay. If your red clay is a water barrier, you might be better off using gravel and sand, covered with about 4 inches of top soil.

Joel ....

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Retaining wall backfill step building questions

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kwschumm
Join Date: Feb 2003
Posts: 5764 NW Oregon
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2008-09-16          156691


The red clay is porous enough that it passed a standard septic perc test. When we get a lot of rain one big depression might have a few feet of water accumulate which then soaks in at a rate of about one foot/day.
....

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Retaining wall backfill step building questions

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Murf
Join Date: Dec 1999
Posts: 7141 Toronto Area, Ontario, Canada
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2008-09-16          156696


Well, from my point of view, the biggest issue you have is the clay, luckily however you don't also have the frost problems we do, that is a wall-killer combination. I can't think of a problem more destructive than frost heave.

That being said, your three priorities should be drainage, drainage, and drainage, in that order. ;)

I wouldn't use the geotextile fabric anywhere except to protect and isolate the gravel from the native soil and around the drainage tile. Get the drainage tile that comes wrapped in fabric.

I've had good luck building a regular wall then then building a conventional set of stairs out of cedar that sit on a concrete footer at the bottom. It ends up a LOT cheaper and looks, IMHO, nicer than concrete steps do. Too much chance of built in concrete steps settling and causing uneven footing for those using the steps IMHO. Even a very slight difference in steps is very noticeable to even the average person using them. If the wall shifts wooden steps can very easily be re-leveled in short order, concrete not so easy.

Best of luck. ....

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Retaining wall backfill step building questions

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candoarms
Join Date: Mar 2007
Posts: 1932 North Dakota
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2008-09-16          156697


Kwschumm,

Being that your retaining wall is going to be about four feet high, your biggest problem is going to come from the hydraulic pressure placed against it from the water trapped in the soil behind that wall. (yikes!)

As Murf stated, drainage is the most critical issue you're facing. The more porous the fill behind that wall, the better off you'll be.

Back filling with that red clay might be a problem. However, a natural mix of rock, sand, and clay, might be considerably cheaper than buying 12 yards of clean gravel.

I just had 9 yards of clay/gravel/sand mix hauled in, to fill in some low spots in my driveway, at a cost of about 17 dollars per yard, or $150.00 for the load. Clean gravel would have cost me about 30 dollars per yard.

Joel ....

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Retaining wall backfill step building questions

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Murf
Join Date: Dec 1999
Posts: 7141 Toronto Area, Ontario, Canada
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2008-09-16          156699


Quote:
Originally Posted by kwschumm | view 156677
When building a gravity wall without geogrid the mfr. instructions say to construct the back of the wall with a drain and compacted gravel all wrapped up in landscape fabric.


As long as you have a continuous layer of gravel, protected by an also continuous sheet of geotextile fabric there cannot be any hydraulic force against the wall.

Despite what the women think, gravity is a good thing! LOL

With a proper installation any seepage behind the wall goes straight down through the aggregate and into the weeping tile, if there is more water than the weeper can handle it will just run through the gravel beneath the wall itself and back to surface in front of the wall.

It is very important BTW to make sure that the weeping tile either comes to daylight (i.e. runs downhill underground to a place where it appears) which is preferred, or at the very least has a catch basin (or several) along it's route so that you can check for silt coming out, and backwash it if necessary to keep it open and flowing.

Best of luck. ....

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Retaining wall backfill step building questions

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kwschumm
Join Date: Feb 2003
Posts: 5764 NW Oregon
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2008-09-16          156700


Thanks, guys. So my plan is to suck it up and buy a bunch of good draining aggregate. The 4' wall will be built with 4' of compacted aggregate behind it, separated from the soil by geotextile fabric. Every few courses geogrid will be layered into the aggregate but it will not penetrate through the geotextile fabric into the soil. A fabric-wrapped drain tile will be slope down from one end of the wall to the other and drain to daylight. Sound OK?
....

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Retaining wall backfill step building questions

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Murf
Join Date: Dec 1999
Posts: 7141 Toronto Area, Ontario, Canada
TractorPoint Premium Member -- 5 Tractors = Very Frequent Poster  View my Photos  Pics

2008-09-16          156704


A little over-kill IMHO, but it shouldn't move any, that's for sure!

The other thing you should do is run 2 separate drainage tiles side by side when you build the wall. If there is a lot of water it will help a lot, if not it's a cheap insurance policy.

I've seen some really lovely landscapes ripped up for no reason other than trying to clear a blocked pipe, or replacing a broken wire. A second strand of wire, or a second run of drainage tile costs pennies to put in when the hole is open originally compared to the cost of replacing a failed one later.

Also, if it's possible, and not too unsightly, try to put some method of getting the higher end of the drainage tile to daylight also. They sell a variety of fittings for this, or you can just get a few feet of ABS pipe a little bigger than the drainage tile, and cement a threaded fitting and screw cap on one end and stuff the pipe in it bringing it then vertically to the surface. This will be invaluable later on if you ever want to flush the drainage tile out, you just open the end and stuff a hose in it and let 'er go!

Best of luck.

....

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Retaining wall backfill step building questions

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kwschumm
Join Date: Feb 2003
Posts: 5764 NW Oregon
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2008-09-16          156705


Thanks, Murf. It would be easy to bring the top end(s) of the drain lines to the surface and two drain lines would be easy. I may tie one of them into the gutter drains for the shed above the wall to drain that to daylight as well.

One last question please. The local sand and gravel company offers 3/4 minus, something called round drain rock (commonly used in drain fields and french drains), and pea gravel. It seems the pea gravel would compact best but I'm not sure how it will drain.

Which would you choose? The price is about the same.

Thanks again! ....

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Retaining wall backfill step building questions

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Murf
Join Date: Dec 1999
Posts: 7141 Toronto Area, Ontario, Canada
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2008-09-16          156709


Ken, first off, if possible try to resist using the drainage tile to get a 2 for 1 out of it, while a (presumably) small roof of a shed likely won't do much harm, you are trying to dry the area out, not introduce MORE water. Remember, anything you pour in the high end has to travel the full length of the tile to get to daylight, more than likely it will be absorbed by the soil below the wall long before it gets to the far end.

As for the stone, if the 'pea gravel' they sell is round I'd go for the crushed stuff (3/4" and less), round things don't pack as well as stuff with sharp edges, but roundish stuff does let more water pass since it leaves bigger gaps too.

The most important is to get something as 'clean' (no sand or other soil or contaminants mixed in with it) so you don't start off the process by dumping a load of stuff in there that will clog the drains from the get-go.

If you want it well packed spend a few bucks and rent a gas-powered tamper, it will save you a lot of grief down the road.

Best of luck. ....

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Retaining wall backfill step building questions

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kwschumm
Join Date: Feb 2003
Posts: 5764 NW Oregon
TractorPoint Premium Member -- 5 Tractors = Very Frequent Poster  View my Photos  Pics

2008-09-16          156711


Thanks, Murf. OK, since there will be a trench to daylight it's almost as easy to run a separate drain for the shed gutters so that will be done. I'll rent a gas plate compactor too. Picture 21 in my album shows the area where the new wall will go. You can see the need for it :) The plan was to build two walls but the cost is higher than expected and too much to do in one shot this year. ....

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Retaining wall backfill step building questions

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Murf
Join Date: Dec 1999
Posts: 7141 Toronto Area, Ontario, Canada
TractorPoint Premium Member -- 5 Tractors = Very Frequent Poster  View my Photos  Pics

2008-09-16          156713


Not that I want to screw up a perfectly planned wall by doing something silly like thinking, but here goes anyways.

Your caption on the picture "A retaining wall is being built here to keep the shed from sliding down the hill!" got me thinking.

If the main goal is to stabilize the building, why not just build a much smaller wall but deeper that runs maybe 4-6' out from the shed and around it enough that the shed is standing on flat land?

We do this all the time where we need something along side say a driveway, like a garage, and we need it to match grade to the driveway, but the ground where we want to put it falls away, so we just build a 3 sided wall out from the driveway and a few feet wider and deeper than needed, then put the building on top.

It doesn't take a lot of stable soil to keep a building secure.

Best of luck. ....

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Retaining wall backfill step building questions

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kwschumm
Join Date: Feb 2003
Posts: 5764 NW Oregon
TractorPoint Premium Member -- 5 Tractors = Very Frequent Poster  View my Photos  Pics

2008-09-16          156715


That's a good point. We decided against that because we'd like to turn that hill into flat terraces that can be used. Because the house is so tall I need to occasionally get a boom lift in there (the main reason for construction overkill) and my wife would like a small greenhouse built on one one of the terraces. ....

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