Need Residential Retaining Wall Ideas 2

[xjken]

Hello. My property currently has a failing wood-tie multi-tier retaining wall on a slope that needs replacing ASAP. I did much searching and research and have come across many ideas but am looking for guidance on which to go with. COST is a BIG concern, however I want to replace this wall and not have to worry about it ever again...or at least 30 yrs.

Please give me some thoughts...
Current wall to replace is about 1200sqft face total. Three walls each roughly 4 feet high by 100 feet long. They vary in distance from eachother and towards one end, they begin to get closer to eachother then finally curve into eachother and finish off the last 25 feet as one high wall holding back the corner of my side/back yard.

New Ideas were:
1. pumping in concrete to make a single concrete wall about 10 ft high a few feet down the slope from the existing walls, then filling in and grading the property down to this new wall height.

2. three (or two) new tiered modular block walls with geogrid, such as Keystone, Anchor, Unilock or similar.

3. These "Green Walls" "Vegetated Walls" or "GeoWeb" such as the stacked bags or the block that has a large batter that is filled with dirt and plant seeds. This would either be tiered as well or just make one large slope.

I would probably need a fence for the slope idea, or a wall over 5 feet high, so that would need to be factored in to the cost. I did searches and am coming across prices from $20 to $60/sqft.

If someone recommends the Green Wall or Slope idea... I can't seem to find this product in my area?? (I'm in the New York Metro area).
Any help would be greatly appreciated! I am on a budget of about $40k MAX and am hoping for it to be much less than that. Thanks.

Ken

 

[PEinc]

Can you just install three tiers of 4'high Redi Rock (or similar) walls in front of the three original walls? Leave the original wall in place. At only 4' high, the Redi Rock walls should not need geogrids.

http://www.PeirceEngineering.com

 

[xjken]

PEinc,
I looked into something similar, some contractors call it "Mafia Blocks" but I don't think I can get a machine in there to carry these in. Can a bobcat carry these in to the back yard?
Any thoughts on the total $/sqft to use this product?
thanks! Any other suggestions?

 

[PEinc]

Those blocks are not very good looking but are cheap in comparison. They are unreinforced concrete blocks, often 2'x2'x4', made from left over concrete. Most have raised keyways on top and recessed keyways on the bottom. You can buy these blocks without the top keyway for the top course of a stacked block wall. The blocks weigh about 136 to 140 PCF depending on the manufacturer's aggregate. At 138 PCF, a block would weigh 2208#. Check what a Bobcat can lift and carry.

Attached as an example is a data sheet for these blocks as available from a Philadelphia area concrete supplier.
I have designed a few of these wall, usually for temporary use, but I don't have an installed price. The price would greatly depend on your available access. It may be easier to set the blocks with a hydraulic crane (picker).

www.PeirceEngineering.com

 

[Ron]

Go with segmented block. Very nice looking and not all that expensive. You can do it yourself, also.

 

[PEinc]

Installing the geogrids may be more than he wants to do if he needs to remove the existing walls to install the geogrids.

http://www.PeirceEngineering.com

 

[Ron]

Geogrid is not necessary for a 4' high wall.

 

[xjken]

Thanks Ron and PEinc for your input.
Ron, If I did replace the walls with the segmented block at 4' high I will need to do the three tiers again. I was reading up on this stuff and some articles mention "global ??" which I believe has something to do with looking at the whole area to determine the structural requirements. This three-tier wall would only be 4' each one, however it is built on a fairly steep slope... the ground in front of the lowest wall will not be flat approaching the base of the wall. This makes a difference, correct? Also, I read the walls need to be minimum 2x the height stepped back from each other. Geogrid, I believe, may be needed in this case. Any thoughts?

 

[PEinc]

Three low walls without geogrids should be cheaper to build than one or two taller geogrid walls that could need temporary sheeting to be built. If you are allowed to move the bottom wall out a little, you may be able to position each of the other two replacement walls so that you have your 2x setback. Global stability refers to the overall stability of the entire hillside, including the three walls.

Ron is correct that walls with smaller segmental blocks would look nice and are easier to handle, but the design height of a wall also includes the portion of the wall that is buried below the finished grade in front of the wall, usually at least the height of one block. Therefore, a 4' high wall without geogrids may really be only 3'-4" or less in exposed height. With bigger blocks, you can go higher without geogrids. Without geogrids, you could have less excavation (i.e. gouging into the hillside and disturbing the existing walls), less backfill, and probably no temporary sheeting.

http://www.PeirceEngineering.com

 

[Ron]

PEinc...nice explanation.

 

[fattdad]

to accommodate 12 ft of grade change it seems to me you just can't make three 4 ft tall block walls without geotextile reinforcement and say that's good. I wouldn't do that at all!

any suggestions from this forum seem to make no sense until we see a cross section that extends to either side of this "wall" by a distance of 20 ft or so. Also, it'd help to know the soil type.

I think if you have a failing timber wall (i.e., one that worked for a while and then "failed"), you could expect a similar design life if you just redid what you have. Whether you use block or timber would make little difference. If you want a better performance, it'd be good to know what happened. It sounds like you are the victim of failed cohesion and a wall that was stable only when the soil's cohesion was there to assist. I'm thinking you have some clay on either side of your wall and forming your slope. I'm thinking you need some mechanical stabilization (i.e., geotextile) to reconstruct your wall.

f-d

¡papá gordo ain’t no madre flaca!

 

[PEinc]

For this particular situation, I wouldn't build a three tier, segmental, small block wall without geogrids. That's why I suggested the larger blocks, like RediRock. From my experience, the main problem with timber tie walls, especially in multi-tiered applications, is that they are (were?) usually built by landscape contractors and without any proper design - and they eventually rot. Rebuilding a properly designed and pressure treated, tiered, timber tie wall or a small-block, tiered, geogrid wall would probably be more expensive than a tiered, big block wall placed in front of the original tie wall.

fattdad is correct. More information is needed to make a proper decision. For now, we're just throwing out ideas.

http://www.PeirceEngineering.com

 

[cvg]

keep in mind that if you go anything higher than 5' high, you will probably need an engineer to submit calculations, stamped for your permit. And frankly, in my opinion a 3-tiered, 12' high wall should do the same thing. Check to see what the permit requirements are.

You are willing to spend $40k and perhaps have a landscaper design it? How long will it last? Will you be willing to spend another $40k in 3 years when it fails?

 

[xjken]

cvg,
No way!...I wouldn't leave this up to a landscaper to design. I am a PE and am getting ideas from here and my own research and am going to come up with some type of design and then review with a civil engineer for final review and stamp.

PEinc,
I like your big block in front of the existing wall idea but am questioning this... The bottom wall can be built in front of the old wall, but the second tier would wind up on top of or close to the old first tier wood wall. I'm thinking this old wood will eventually rot and create a weak base for the very heavy new wall above it. I don't know if that will work.

 

[oldestguy]

In a municipal area I have see something that works, if designed and very cheap.

Materials testing labs readily have to discard tested concrete cylinders.

The original owner of Milwaukee Testing built some nice walls at his house using these cylinders. The last I knew they still stood.

Where I used to work we had many a homeowner coming by to take our tested cylinders,since we did not break them, once they started to yield.

Otherwise the labs have to dispose of them.

 

[PEinc]

That's hard to address without seeing the actual geometry. However, as you build and backfill the first, lowest wall; you may be able to remove the top foot or two of the lowest, original wall and then fill in its location with well compacted granular material or concrete. Then repeat for the next tier.

http://www.PeirceEngineering.com

 

[Dozerman56]

I'd suggest contacting your local Contech sales engineer. They represent a number of different systems.

 

[civilman72]

Generally speaking, all retaining walls over 4' in height are considered "structural retaining walls" and require a structural engineer to certify. In order to avoid this requirement, and keep costs down, stepped/tiered retaining walls (with 4' max height for each wall) have become popular. However, without the proper setbacks (2x height is a good example) between the walls, these walls really act like a structural retaining wall. Lots of landscapers/contractors install these walls without the proper setback, or consideration of soil types and these walls usually fail.

I'm not a geotech, but I've designed (grading plans for) numerous tiered retaining wall. From my experience, there's a big difference in retaining wall design if it's for a fill slope or cut slope. When you have a tiered retaining wall holding up a fill slope (as example, this can be done at the lower portion of someone's front yard, to extend the flat area of yard), you usually do not have to consider reinforcement (geotextile) behind these walls.

But when you "cut" into an existing hill and try to use a tiered retaining wall to hold up this cut slope, geotextile/geogrid is almost always required. In order to avoid additional excavation for the geogrid, very often soil nails are used for reinforcement.

I think this may be why you are getting contradicting statements on whether you need reinforcement or not - it depends on if it's holding up a hill, or a flat area of fill. It sounds like from your description that you are trying to retain a hill. As previously stated, having a cross section of the existing conditions and soil types would certainly help.

The use of the large retaining wall blocks could help you avoid reinforcement, but I agree with the construction issues you mentioned above if you attempt to keep the existing walls in place.

I think it's prudent that you are trying to do a lot of this work by yourself (and with some minimal input from this forum). But, I would recommend that you pay a good local geotech/structural engineer for a few hours of time - you may be surprised how quickly they can determine the most cost-effective design for your situation. Just make sure you give them some site photos, soil types (if you can dig some holes and take photos), and an existing cross-section.

 

[PEinc]

Whether you are holding up a hill or a flat area has nothing to do with the need for geogrids. The need for grids is based on the wall height, the soil types (retained and internal), grading behind the wall, and global stability.

The discussion of the need for grids in this thread is based on the actual height(s) of the tiers, the available bench widths (1x, 2x, etc.), and the choice of small blocks vs. big blocks. For tiered wall on an existing hill, installation of grids would most likely be more involved and expensive than no grids.

http://www.PeirceEngineering.com

 

[civilman72]

"Whether you are holding up a hill or a flat area has nothing to do with the need for geogrids."

I understand it does not determine the need for geogrids, but holding up a flat fill or a steep grade directly correlates with two of the design items listed: slope stability and grading behind wall. So actually, where you are placing the wall has a lot to do with the need for geogrid.