A customer asked me to pour two 6 foot tall retaining walls to form a stairwell leading down into her pump house. I’ve seen many older retaining walls tip from the pressure behind them and I’d like to avoid that.
What have you done? Drainage rock behind the wall with weep pipes? Perpendicular deadmen poured behind the wall? Extra wide footing on the backfilled side of the wall?
This ground drains well, glacial till, but still, I’m looking for advice. Any opinions welcome. Thanks – Jim
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Replies
been a long time since I looked at stuff before I built my much shorter retaining wall.
is it actually 2 retaining walls or a U shaped unit attached to the pump house.
but what I remember was that the footing extended back into the soil. don't remember the ratio, but the L formed the soil on top helps keep it from tipping.
alternate since this is similar to a basment bulkhead, would a preformed unit be possible?
bobl Volo, non valeo
Baloney detecter
Ask an engineer. He should be able to tell you how big the footing needs to be and if you need to tie it back.
Jim, I just posted 58836.5 .
That's from a standard retaining wall book. 6' is a little smaller. Not sugesting that one-size-fits-all, but I'll dig out a shorter wall if you'd like. The footing can be almost entirely under the fill if that's appropriate. I don't use deadmen, just large footings with substantial rebar.
We generally use an underground plastic umbrella to shed ground water away from the wall. No drainage necessary. Your groundwater situation may work differently. The drawing above shows what's typical.
PAHS Designer/Builder- Bury it!
Thanks, Tom. I was thinking the horizontal leg of the "L" would be under the backfilled side of the wall, not the open side. I wonder if that right angle is stronger one way or the other. I'm thinking it's either in compression or whatever the word is for the opposite force.
Yup, Jim. According to my engineer, putting the footing under the fill works a little better. Also means I have no need for that stub below the footing, making the footing excavation much simpler.
As I normally use a 2x4 nailed (or screwed) flat to the footing to keep the form bottom from kicking out, I plan for 3 1/2" of footing on the exposed side, the rest under the fill. Also means I have a lot of room to work back there. Over 8', for my fill height.
Won't make any difference to the footing/wall connection whether the footing is buried or exposed. That's where reinforcement size and placing is critical (you know, tension).
I got a great education when I hired my guy for my buried wall, with 15' of dirt. Had, and took, the opportunity to ask a lot of questions. As a Realtor at the time, I'd seen a lot of foundation problems. I now know where they came from. Usually either too little footing, or weak connection between wall and footing, either allows the wall to lean. Amazing how many houses around here, usually with walkout basements, have that happening. Generally the side walls show the stress.
Getting a drawing is fine. Understanding the "why" is even better.
Have fun. I always do.PAHS Designer/Builder- Bury it!
Around here the plans would require an engineer's stamp. Anything over about 3 feet high requires an engineer. For good reason.
Jim,
It sounds like you're getting some good advice. My guess is that you will need an engineer's stamp for a 6 ft. wall, but you know the codes in your neighborhood better than I do. And you will want drainage: either a perforated pipe in gravel backfill that drains to daylight or weep holes.
Good luck,
BP
I'm looking for advice
Having had retaining wall design inflicted upon me in school, I'd have to say I always get an engineer's advice. Which may sound like using a sledge on a carpet tack--but much simpler (cheaper) than having almost any size wall tip over.
Do you need and engineer? I don't have the foggiest. Wish I could offer more.
Senor Jim
I've done bulkheads on sloping terrain to lakes
taking into account what has been said before re: footings, rebar & concrete - I do that inverted umbrella thing ( have used ice & water leaving the peel on ) then the backfilled gravel then landscape fabric to keep the gravel from exiting the abs weepholes
tough to picture the "landscape" you'll be dealing w/ but I see a descent to the pumphouse - could you walk down through a rebarred arch?
This is a really natural setting in the woods, John, on the banks of the Deshutes river. Lots of hazel nut shrubs, alder, some fir, pretty much wild looking. Nice place.
I can pin one end of each wall to the concrete walls of the pump house, then the stairs ascend up from there. I'm thinking the stairs themselves and the backfill under them will provide plenty of resistance to tipping at the end away from the building, plus I plan to pour a right triangle perpendicular to each wall, about 3 feet from pumphouse. The walls and these triangles will be a continuous pour atop a large footing.
These things ain't tipping.
Based on that description, if you make the top edges of the walls extra strong (extra rebar, maybe a little extra width), and have enough rebar in the rest of the wall to prevent it from crumbling, you should be good. Be sure to backfill with sand/gravel for a foot or so, to minimize the pressure from expansive soils and frost.Also make sure that, where the wall is pinned to the pumphouse, there's enough rebar in the pumphouse to resist the pressure. The pins should be fairly substantial.
are the steps themselves going to offer strength in preventing the walls from pushing in ?
we have always done them with large rebar connections from footer to wall. #5 ell bars with 48" of vertical height in the wall with a 24" horizontal section in the footing installed on 12" centers. (just some info from past jobs, please consult with your own p.e. blah blah blah.)
make sure you stone the backfilled section. waterproof the foundation wall before this.
carpenter in transition
"are the steps themselves going to offer strength in preventing the walls from pushing in ?"
If they are done right they sure will. You got the right idea with your footings and walls. as far as the stairs are concerned. he's gonna need rebar doweled from wall to wall for each step as well as rebar tied in going up the throat of the stairs.
That along with the footing done as he's proposed it won't go anywhere. I've worked on some 12' retaining walls and foudation walls close to 20' in those cases the engineer has called for butresses. but at 6' no way he will need anything like that.
But like everyone has said, it should be engineered.