ecology block retaining walls w/ geogrid

[soiset]

Because it looks like a simple, inexpensive retaining wall, I get a lot of requests from homeowners and some builders to design eco-block retaining walls. Many of them, especially the homeowners, incorrectly believe that eco-blocks can effectively form tall gravity walls, even to ridiculous heights. One thing they are correct about, though, is that eco-blocks are cheap, or at least cheaper than segmental wall systems like keystone. In our area, I have found that with clay backfill, eco-block walls are only reliable to 4' above ground (the first block is buried 1', and there is a 1' cap block) and maybe 6' with granular, freely draining backfill past the angle of internal friction, and with a 6:1 batter. To go any higher, especially without any batter, would require geogrid. Does anyone have any experience or insight into the design of an MSE/ecology block wall? How effective is the friction grip on the geogrid by the blocks?
Thanks

 

[Focht3]

The grip is tenuous. Any shifting and the grid will no longer be connected to the wall. And the amount of friction available is limited by the wall height anyway.

If your client is going more than about 4 to 5 feet, insist on a segmental wall system. Stick to your guns - you're on the right track.

Do you have much of a problem with the clays washing between the eco-blocks?

 

[soiset]

We haven't seen any instances where clay backfill has leaked thru the wall, although I can see why you might wonder. One reason that I am concerned about the security of geogrid in an eco-block wall is that the blocks made locally are not very uniform, and, when stacked, can have gaps between their horizontal surfaces which may cause them to pull away from the geogrid.
I'm thinking that maybe a bolted conxn with cable and a deadman might work, but then I'd have to worry about corrosion.
Damn I hate cheap clients, they always make me think.

 

[jimbeck]

The geogrid suppliers have several articles on this subject, showing test results. Cornerstone, Allen Block and others have websites with information.

 

[soiset]

I've looked, but have never found any of the articles to which you refer. Could you provide me with a link?
Thanks,
Chris

 

[MSEMan]

We have tackled this problem a little bit differently in the past. Have you considered using a separate facing system such as galvanised or (depending on how cheap your client is) black reinforcing steel mesh with a geogrid MSE retaining wall? After constructing the wall you can face it in ecoblocks (or any other kind of blockwork) using brick work ties. It may also be cost effective to shutter the front face, use the MSE side as permanent formwork and cast a thin CIP facing?

Good luck.

 

[jdmm]

If eco-blocks are like Lock Blocks (2.5'x2.5'x5')concrete blocks then I can help you. We use them frequently for both temporary (preload fill support) and permanent walls generally up to about 15 feet but as much as 30 feet for temporary walls using geogrid.

 

[soiset]

MSEMan,
I have some questions re your post. When you say "reinforcing steel mesh" are you referring to a pre-fab mesh that would normally be used in a concrete slab? What designation mesh or what size bar would you be talking about? Can you give me some more details on how this would be built? What do you mean by "shutter?"

JDMM,
Ecoblocks are blocks manufactured locally out of return concrete, probably the same as your "Lock Blocks" except they are 2x2x6, with semi-cylindrical locking sections (male at top, female at bottom) They tend to suffer from lack of uniformity, i.e. there are often gaps of an inch or more between the blocks when stacked, which could affect geogrid effectiveness (pullout). I'd appreciate any insights you could offer.

Thank you,
Chris

 

[ivanlocke]

I posted questions about lock/eco blocks previously:
thread255-43311

We did a quick test stacking two blocks with a piece of geogrid inbetween. The geogrid was Tensar UX1500HS. Using a a 16" wide piece of geogrid I wove a piece of barstock through the free end and we pulled with a backhoe. With no filler the the geogrid pulled out easily as you mentioned because you do not get a tight fit between the blocks (and the geogrid was placed between interlocks so they gave no contribution). The next time we put the geogrid down and mixed up a cement grout and trowled it over. I think we did it on a Friday, so it would have cured over the weekend, 3 days. Pulling this time ended up pulling the blocks over, so >3000lbs/ft connection strength. More than I needed.

Hope this helps.

 

[rockchip]

Geogrid reinforced, or MSE applications using large concrete blocks are feasible. I have designed several in the Pacific NW, and to heights of 20 to 27 feet, although I am aware of some as high as 50 feet. These are done with a product called Ultrablock, a 2½' x 2½' x 5'cast concrete block with interlocking features very different from eco-blocks. They have done their homework on the geogrid connection and pullout strength, having had Bathurst conduct full-scale pull-out tests (results are available from Ultrablock)using typical geogrid products. Tensar and other "slick" or "stiff" geogrids do not work as well, and they find that woven products like Stratagrid and Synteen work much better at conforming to the block interlocking lugs.

 

[jimbeck]

Cornerstone is at risistone.com, and Allan Block is at allanblock.com

They do provide computer programs for their products; but these can be a bit difficult to operate. If used, they should be verified by independent calculations. I have seen reports and design procedures from geotextile vendors; but this is some time ago. Check with Nilex, or Layfield plastics in Calgary, Alberta, Canada.

I will look for the references, and advise; but not this week. Sorry.

 

[rockchip]

You can use a commercially available software product called SRW Wall available from the National Masonry Concrete Association to evaluate the internal stability of large facing blocks and geogrid. The program (and some others as well) allow you to input the size and characteristics of the facing blocks as well as the geogrid reinforcing properties.

 

[layfieldman]

SRWall is very nice to use especially because many grid manufacturers have connection data with many of the commercially available modular blocks in a SRWall data format. What that means is that if you load up the data file for that block it will have inputed the block dimensions, and connection strength between the block and geogrid. Then just enter the dimensions, click a button and it will give you a preliminary geogrid spacing and pullout length you need and a full analysis for internal stability. Mirafi has all of their data downloadable from their website.