Please advise on the best alternatives for a 20′ H x 80′ L retaining wall, where the soil is sandy, and the air salty. Method must be reasonably priced, as the county is footing the bill for excavating the property in error.
Respectfully, Shiela
I live on the Indian River in St. Lucie County, Florida, where it’s very hot, wet and humid most of the year. The county, in an effort to restore sections of eroded river bank and roadway from two direct hurricanes hits last year, excavated my undamaged property “by mistake”. Articulated Block have been laid to the high water line, therefore the retaining wall should not normally contact river waters. Our county engineers and their consultant have suggested one of the following 3 options:
1. Timber Retaining Wall, CCAA-treated wood timbers laid in a brick wall pattern, butt ends in one course offset from those above, w/ a 15 degree slope
2. Sierra Slope Retention System/ Tensar Earth Technologies, Inc., a “green structural alternative to retaining walls,” which uses a Geogrid Reinforcement and natural vegetation, for slopes 26 to 70 degrees. Low cost of 60% savings versus conventional retaining walls may be the leading factor for the county on this system and the one that follows.
3. A Sierra System/Tensar Earth Technologies, Inc, “basket system” stair -stepped design of welded-wire mesh form, erosion mat, surficial and primary reinforcement plus natural vegetation, for slopes over 45 degrees.
Thanks for your responses so far. Shiela
Edited 3/28/2005 5:47 pm ET by Shiela
Replies
The ONLY alternative for a 20' retaining wall is to hire a professional engineer. Cheapest will probably be poured-in-place concrete. There will be significant hydrostatic pressure against the wall which will require a drainage system and buttresses.
Mike
I disagree a little with Mike--if soil is sandy, drainage (to prevent hydrostatic pressure) shouldn't be a problem. Does your county have "sanitarians" to answer questions about septic tanks and percolation? Seems like they might have enough engineering background to design an adequate retaining wall. But on the other hand, rereading your post, 20' tall is pretty tall! Guess you should hire an outside engineering firm.
Danno, I made a couple of assumptions. Sandy soil often has layers of clay in it that trap water, and in 20' I would guess the soil is not completely uniform. It's an assumption, one that an engineer can verify. Code in MA is that any retaining wall over 4' has to be engineered.
Mike
Good points; and I sort of jumped in before I saw how deep (tall) this wall must be! Like you say, sandy soil may bring problems of its own besides drainage--like how to support the load and so on. Should have thought a little more before I said I disagreed with you!
Where are you located ?
On a hill by the harbour
Seem to me if the county screwed it up, they have their own engineers and should come up with a solution.
Seem to me if the county screwed it up, they have their own engineers and should come up with a solution.
**************I believe I'd want to run their solution past a qualified engineer. I've seen local governments come up with "solutions" that meet code -- barely.Moreover, as they'll be the ones building it, I'd require them to be responsible for repairs should it fail.As Boss Hog pointed out, they did screw it up in the first place. Look for the best solution, not the cheapest.Leon Jester
Welcome to BT, first off.
You asked a great question for a first post.
I'll agree with MM, that your retaining wall needs an engineer. Too many retaining wall are out there that don't have enough engineering in them. The footing for the wall will be critical, especially in sandy soil.
Probably a cast-in-place concrete wall will be fastest and cheapest--it just may not be the most attractive (I wouldn't expect the County to pay extra for appearance--but I'm cynical about anything with "County" attached to it).
Are you watching this go in, or is this on your property, or is this potentially your property? I'd be disinclined to buy a property the County "screwed up"--the potential for an "uh-oh" out of your control is just too high.
If it's on your existing property, what was the County doing that they created the "mess up." I'd have an attorney on retainer to keep an engineer company, both of whom would already have their favorite doughnuts being delivered to the County Engineer & Commissioner's Court, etc. spaces. Not that I've BTDT & lost the t-shirt or anything . . . <g>
I value the comments you, Danno, Mike, Leon, Boss Hog and teo have made regarding my retaining wall problem. I have posted a bit more information about the proposed solutions by the county. Like you though....I am very cynical. They are the cause of the problem, therefore I have little trust in their solution.
To answer your question: I watched them like a hawk while they worked in front of my home, but was out of town during Thanksgiving (they removed 10 mature trees) and Christmas (extreme excavation of land), when this happened.
Keep smiling! Shiela
they removed 10 mature trees
I'd be right tempted (even in Fla's "strong" county environment) to beat them up about the loss of the trees.
If there's any sort of landscape ordinance, I'd insist on compensation (existing trees are always more "points" than new).
I'd step on them about decreasing the "natural" habitat and for increasing the potential damage from runoff & erosion.Occupational hazard of my occupation not being around (sorry Bubba)
20' high is pushing the limits for a reinforced concrete retaining wall. Absolutley turn down any timber wall option. They are just trouble down the line. If the area will be filled, then the stacked and filled precast walls can be economical. Drainage is straight-forward for this material and wall height.
You are correct to assume you want an independent engineer to check the town's design, however the bill for your independent structural engineer should be paid for by the town as well.
Good luck....that's not a mistake, it's rustic
I have built several styles of retaining walls and one of the main things is the ground under the base. Concrete retaining walls need to sit on a firm un disturbed ground. Thier are many styles of concrete walls some include layers of geogrid that is incorporated with the back fill. Hydronic pressure should not be an issue if scuppers are used. Also 20' is quite high I do not believe a wood timber wood be allowed unless it is terraced. I have done several miles of retaining walls along highways. But you should get engineering firm as several types of engineer such as structeral and soil
Thank you for the input. I have had great feedback from everyone, and I plan to take this information to an engineer of my choice and to the county engineer for resolution.
Do you have any experience with the Sierra Slope Retention System by Tensar Earth Technologies, Inc.? The literature I was given by the county pictures most of the product installed along highways, DOT projects, hillsides and commercial applications.
Very respectfully,
Shiela
when deciding on a slope please considder maintainance. also with the wall how do you get to the bottom from your property? up here most precast block manufacturers have engineers that will gladly visit your site at no charge.
I suggest invlolving a landscape architect. there is more to consider here than errosion.
This is a MONSTER retaining wall; hire a structural engineer (I am one, but not in your area).
Tensar works, but it must be engineered.
Luck.
Poured concrete will be expensive and will require an engineered design.
Forget timber - I can't think of a single engineer that would be willing to design a timber wall that high. What happens when it rots away? Major collapse?
I vote segmental, and many of the segmental wall companies either have staff engineers or can recommend a civil/structural firm that can help you. There are many different segmental wall companies out there - some local to you, some nationwide. Call around - start with the local concrete block plant or do a web search.
In any case, a wall that big (to repeat the consensus) NEEDS to be professionally designed.
Jon (PE, but not in Florida)
Here's something for you to look at. Our soils are clay and much of the river bottom is limestone bedrock. Loads of silt on top of that. Best of luck on your project.
Remodeling Contractor just outside the Glass City.
Quittin' Time
a bit bigger. These from the distance I took this from appeared like dry layed retaining wall block, only larger. Something an engineer would know about if you like the way it looks.
edit, well luka, i'll try again.
Remodeling Contractor just outside the Glass City.
Quittin' Time
Edited 3/31/2005 8:19 pm ET by calvin
uhhh Cal...That's the same pic twice.; )
The person you offend today, may have been your best friend tomorrow It is easy to be friends with someone you always agree with.
sorry for the lack of detail.Remodeling Contractor just outside the Glass City.
Quittin' Time
Calvin, that stepped design looks really neat, HOWEVER, I'm not going to volunteer to haul the mower up and down the steps once a week ;)
jt8
Don't accept your dog's admiration as conclusive evidence that you are wonderful. -- Ann Landers
I've seen walls of stacking pre-cast blocks approaching 20 ' high. They are remarkably stable, generally cost effecient and about as good looking as a retaining wall can get. It is beyond question that you need engineering on this. Whoever builds it will become the engineer simply by the act of doing it, it would serve you to get one who knows exactly what they are doing.
http://www.versa-lok.com
I live on the Indian River in St. Lucie County, Florida, where it's very hot, wet and humid most of the year. The county, in an effort to restore sections of eroded river bank and roadway from two direct hurricanes hits last year, excavated my undamaged property "by mistake". Articulated Block have been laid to the high water line, therefore the retaining wall should not normally contact river waters. Our county engineers and their consultant have suggested one of the following 3 options:
1. Timber Retaining Wall, CCAA-treated wood timbers laid in a brick wall pattern, butt ends in one course offset from those above, w/ a 15 degree slope
2. Sierra Slope Retention System/ Tensar Earth Technologies, Inc., a "green structural alternative to retaining walls," which uses a Geogrid Reinforcement and natural vegetation, for slopes 26 to 70 degrees. Low cost of 60% savings versus conventional retaining walls may be the leading factor for the county on this system and the one that follows.
3. A Sierra System/Tensar Earth Technologies, Inc, "basket system" stair -stepped design of welded-wire mesh form, erosion mat, surficial and primary reinforcement plus natural vegetation, for slopes over 45 degrees.
Thanks for your responses so far. Shiela
I have 15' of dirt behind the rear wall of my house, about 60' long. Cast-in-place concrete. Engineered, lots of rebar, not a big deal or particularly expensive.
This would be considerably cheaper:
PAHS Designer/Builder- Bury it!
Edit: Forgot to mention that burying styrofoam blocks behind the wall is an effective way to greatly reduce the load on the wall. Learned about that method after I built.
Edited 3/28/2005 6:40 pm ET by VaTom
I'll chime in being that I am formerly an employee of the retaining wall company Teo gave the link for. Among other things, I use to design walls.
First of all, anyone who would blanketly say that poured concrete walls are the least expensive, doesn't know what they're talking about. Typically, segmental retaining walls are 2/3 the cost of a poured wall. He who mentioned that 20' is pushing the limits of a poured wall is right on, but I would add the qualifier "economic" limits. If a poured wall is designed right, it's going to have a huge cantilever footing underneath it that will be designed to offset the huge load of the soil that's behind it. Rarely is a poured wall cheaper than a segmental wall and it never looks as good (IMHO).
First, a lesson in retaining wall design. The physics are the same regardless of what type of wall you're talking about. You have to hold back a wedge of soil (between the wall face and the slope) that wants to separate itself from the stable soil that is the slope. This "active wedge" wants to slide down the slope (failure plane). Geogrid works by passing through this active wedge and "tying" the wall face(regardless of what that face is) and the active wedge back into the soil behind the failure plane. Done right, the entire area that has the geogrid all works together and actually becomes the wall. So instead of your wall being only 8" or 12" thick, it actually becomes 12 feet thick (the length of the geogrid). In a properly designed wall the geogrid does all the work and the wall face is only there for looks and errosion protection - it does very little, structurally.
I'd discourage anyone from building a 20' tall timber wall. Even assuming quality treated lumber is used (and won't need to be replaced in 12 years), a wall that tall is still going to need geogrid. They'll tell you that they'll put deadmen in the wall to tie back the face. In a wall this tall the deadmen probably wouldn't be long enough to get back behind the failure plane. Using geogrid with timbers is an option and probably cheaper than deadmen.
The second option you were given is not a retaining wall, it's a steepened slope. The geogrid still goes back through the failure plane but then it connects to a device that holds soil for planting. The "sierra" part of this system only stabilizes the face. If you don't mind the loss of land, and you like the look of a green slope this is probably a very good option if designed and installed correctly. The only real concern is maintaining the integrity of the face. I could forsee "weed" trees growing out of the face of this thing and climbing up a steep slope to cut 'em down is no fun- and dangerous.
The third option is gabion baskets. In this option you replace geogrid with mass. They'll work fine if you can put up with the looks of them. I've seen them around river banks where junk gets entangled in the baskets and they just don't look that great. Again, stuff will be growing out of them.
The advice you got about talking to engineers is good. Google retaining walls and start calling. I can also assure you if you call Versa-lok at 1800 770-4525 they'll provide you with all the info you need. Versa-lok has engineers on staff, but they don't sign off on anything. Even if you don't use their product they can point you to an engineer in your area that can look at all the options you've been given and help you make a decision.
Being that it was the county that screwed up, you should be giving them reasonable options of how you want this fixed.
Good luck.
While I don't think the town is willing to allow real options, I think the precast/geogride retained earth options are the best looking options, if a terraced planting is not preferred....that's not a mistake, it's rustic
OK, maybe I'm just a little confused about what is going on here. The county dug away a pile of soil from your property (Including 10 trees) and left you with a 20' tall by 80' long cliff. They now want to add a retaining wall to keep the rest of your property (and maybe your house) from sliding down said cliff.
Why don't they just put the dirt back? (Including 10 new large trees.)
Here in Maine there are several towns that get ravaged by flood waters at ice out time. The rivers get flowing with icebergs and take out anything in their way. They have been building retaining walls out of wire cages filled with bolders. The ones that I have seen are sloping walls, not vertical. The wire cages are made of something that looks like heavy chicken wire. The overall appearance is almost a natural rip rap looking rock wall. The cages are filled with bowling ball size rocks. Have you talked with the state soil conservation department? I don't know if this type of material is available in your area. It's certainly faster and cheaper than poured concrete. I'm sure the system is specifically designed for the type of flood conditions we have but it may work in other circumstances. Salt may be an issue with the cages. We have lots of rocks.
Beat it to fit / Paint it to match
By erroneously excavating the "point" on my riverbank, the county straightened the natural curve of both the shoreline and bank. They immediately laid down sheets of interconnected articulated block along the water line, so it is now too late to simply replace my property without artificial means of support. To do so would mean constant soil erosion and environmental damage to both the land and the intercoastal waterway.
I have received much thought provoking advice through this forum. Thanks a million.
;)Shiela
Shiela, please come back with the updates. Too often we don't hear the rest of the story.
thanksRemodeling Contractor just outside the Glass City.
Quittin' Time
...and some pictures, please....that's not a mistake, it's rustic
I second the call for pictures.
The person you offend today, may have been your best friend tomorrow It is easy to be friends with someone you always agree with.
did u lose land?that is, land u own is no longer available to u?if so, should talk to a lawyer, could be taking of land without emmenint domain proceedings, giving u more leverage in what they do
bobl Volo, non valeo
Baloney detecter
Here's another option: http://www.redi-rock.com/It needs to be engineered and it uses geogrid as the guy so ably described above. The blocks are 4' x 3' or 4' deep x 18" high and weigh 2300# or so. Cost about $70 ea. The wall can include planters and such. They have top block with a lip for retaining dirt, so that when you're up top you don't see the actual cap block. We just did a wall abut 15' x 80'. The cost added up, but the alternatives were no better. And I did mention to get engineering, right?
Cloud, I'm curious.
I've seen Redi Rock before, but I've never seen a real wall. My understanding was that they pushed the manufacturing to redi mix companies and touted it as a way to use up left over concrete that they'd just throw away. Is this your experience and if so, were you happy with quality and color consistency? How was installation? I remember when they first came out they were pushing it as a system that didn't require grid, I was glad to hear they are now recommending it.Thanks.
Color consistency and quality are fine. Can't spot any flaws. They're pretty tough buggers, too, considering the way they were staged till they could be stacked. One excavator was making claims about no grid, but the company itself advised on an engineer and the use of grid, and I listened. Installation was done as professionally as I could have hoped for. Really is an impressive and imposing wall. Really like the "cap" block that hold dirt so ya don't see 'em. Other block systems are too small for that lil' trick.
Are you refering to gabion baskets? Something like this??
I did these myself this past fall. The one wall is 7.5' by 48' and the other is about 4.5' by 12'. They were very easy to do.
Could be an option...
Craig Z
Those are what I was referring to, I didn't know the name. I see they have a variety of forms and wire types. Easy to install and environmentally friendly. Some of the floods here can move car size boulders, which just add their own damage. I see a lot of retaining walls get pushed over in time, these look like they will last a long time. I imagine they can be covered with vines if so desired. They look pretty good just plain, much better than some behemoth wall of wood or concrete.
Beat it to fit / Paint it to match
Wire cages.The problem with wooden walls is eventual rot.Won't these eventually rust out ?
The person you offend today, may have been your best friend tomorrow It is easy to be friends with someone you always agree with.
The wire cages are made with 11 gauge galvanized wire. Pretty heavy duty stuff. I don't see them rusting through in my lifetime.
They also make a PVC coated wire for more corrisive environments.
A 20' high retaining wall needs to be professionally designed. Don't let the City give you 'choices', find out what has to be done and present it to them. There is nothing worse than having repair work done, in this type of situation, and having it fail at a later date. Be really certain right up front take no substitutes.
I agree with most of the advise you have been given. Get an registered PE for this problem. I think you need a structural PE if you are designing a concrete retaining wall. However, a good geotechnical engineer can design a modular wall. A geotechnical engineer can also sample and test the foundation for the wall which is as important as designing the wall. In your description of the problem you indicated that articulated block had been brought up the the edge of the water and the wall would apparently be constructed on top of that. You will need a good evaluation of the capacity of the foundation material before you ever think about putting a wall on top of it. 20 feet of soil and retaing wall is a tremendous load for the foundation, especially if it under water. Hope this helps.
Ray
If you go with anything other than a poured wall, I wouldn't be real concerned about the foundation sitting on good soil. From what was described, it sounds like this is going to be replacing a stable slope that had been removed. It's important to understand that segmental walls and even gabion walls don't add anymore weight than the soil that was removed, so if the slope wasn't shifting and sliding, chances are that the bearing capacity is fine. That can be a big advantage to these types of walls when bearing capacity is an issue. A common misconception is that segmental walls fail because of poor base preparation. Rarely do segmental walls fall down because of a poor base - they may settle and look bad, but they don't fall down. Most often walls fail because of poor compaction(INSTALLATION). Second to that is inadequate(no) geogrid reinforcement or the grid is not strong enough(DESIGN). And almost always the failure will be poor design or installation plus water.
With a poured wall, bearing capacity can be an issue because it is designed to transfer the load that it's holding back through the wall face and then down to it's footing.
Segmental walls are better designed by geotechnical engineers as opposed to a structural engineer. These walls stay up because of the mechanics of the soil rather than brute force of the concrete.
In case the message hasn't sunk in yet: SEEK THE ADVICE OF AN INDEPENDENT, PROFESSIONAL ENGINEER.Good luck.