This is going to be expensive - http://www.wset.com/story/24424658/...ne-wall-gives-way-at-south-peak-in-roanoke-co From looking at the pictures I have a good idea what one of the causes of failure is, curious to see what others think.
Hydraulic pressure from water trapped behind it.
No drainage.
Ditto above. A good portion of my business was to install walls of this type and I received factory training to do it. They prescribed that there should be perforated pipe at the bottom of the wall run to daylight at some point with a 1' minimum depth of clear stone directly behind the wall protected by geotextile.
It certainly looks like a lack of drainage issue. But if you look, there is geotextile fabric in those pictures. This leads me to believe there may have been some stone in there too. Not sure on drainage. Not sure what the weather was like down there, but I wonder if its been cold and there may have been some freeze issue that compromised some construction/material/design defect? Now the bigger question is what would you do to fix this? I cant imagine trusting what is there and rebuilding it. I also cant imagine removing everything and starting over. Seems like it would make more sense to build a new wall in front of the old one and fill with stone?
Lack of proper sized stone backfill behind the wall and hydraulic pressure looks to be one reason the wall failed. However notice just the top portion of the wall fell. The picture shows some stone backfill in the reinforced zone directly behind the wall, maybe 4-6'. I had my suspicions that the water behind the wall that caused the issue, came from a storm pipe above. If you notice there is not much shoulder between the curb and the wall, not a lot of room for a large amount of water to enter behind the wall. There were two MSE walls that failed in the Birmingham area at two different shopping centers. One of the failures was due to a storm pipe that settled where it entered a stacked manhole, behind the wall. The water obviously leaked from the pipe and went directly behind the wall. The loading dock for the Circuit City settled about 4' due to the failure. Hayward Baker spent several months at that site, stabilizing that wall and they are currently working at the other site stabilizing a JC Penny. There is no telling how much that cost.. It seems a storm drain failure might be the culprit here as well - http://www.wdbj7.com/news/local/tra...ng/-/20128466/23883306/-/qk822qz/-/index.html I don't buy the "pressure in the pipe" pushed against the wall. There was a bad connection that allowed the storm water to exit the pipe and saturated the material behind the wall. The last major Keystone wall I built was 15' high at it's tallest and 187' long. We installed a sheet drain between the existing fill and the reinforced zone. The reinforced zone was entirely backfilled with #57 stone, horizontally 20' in some places. Proper draining is the life of these walls and special care needs to be taken when running storm sewer within the reinforced zone. There is no way I would have built that wall without having at least 10-15' of stone backfill horizontally behind it.
Im thinking its a lack of drainage. If there was crushed stone back there it seems as though we should be able to see some sign of it. Glad I'm not responsible for that mess, its definitely going to be expensive to fix!!
Good question and topic of discussion. I would be inclined to repair it, after all the site assessments have been made. If the bottom portion of the wall is in good shape, excavate the upper portion carefully and rebuild that section. If the storm pipe was the culprit, then a thorough investigation of the integrity of the storm piping close to the wall should be done and repair that first. Any manholes and storm piping adjacent to the wall should have had mechanical or concrete restraints to keep any pipe connections from settling, something we don't do when laying normal storm piping. Encasing the RCP in flowable fill or concrete close to the wall might have prevented this from happening. If you think about it, if they just laid the RCP like normal - rubber gaskets, bell and spigot, and joint number 2,3 or 4 out of the manhole settled enough to allow water to leak out, that would be more water than the wall is designed to drain in a concentrated area. Whatever happens, this is a very expensive failure that needs to be used as an educational tool for anyone doing this sort of work.
My suspicions are confirmed - http://www.wset.com/story/24437949/...alled-pipe-caused-wall-collapse-in-roanoke-co http://www.wnct.com/story/24434051/south
I agree about drainage and proper backfill stone. We have to do the same on poured concrete walls or they can fail. Not being familiar with this block system I first assumed the fabric we see still hanging in the picture was geotextile cloth laid against the inside of the block. If I understand correct this is actually "geo-Grid" that is installed in the fill every so many feet with the outer edge tying into the block for support ? www.keystonewalls.com/media/Technotes/bckfllmvmnt.pdf Interesting product www.keystonewalls.com/media/Literature/CS0950-RockyMtAirport.pdf
That's geogrid which is different from woven and non-woven geotextile cloths. It's specified at a certain interval horizontally, so many courses of block and gives the wall it's "hold back" strength. It's attached to the blocks with fiberglass pins and rolled out to the designed length in the reinforced soil zone. Some manufacturers use different styles of anchorage to the blocks, not just fiberglass pins but the walls I have built have all been designed that way.
Any idea what a repair like that would run? probably close to the original cost of the wall , if not more?
Thanks for the answer CM. My only experience with block walls were the smaller blocks that had a lip on the inside to lock each row together and we weren't going very tall with them so no tie system was used. Back to the wall failure in VA. Is it possible they are pushing past the limits of the product ? Man that wall looks 25 to 30 feet tall ! I just wonder even with proper drainage & backfill if the plastic geogrid tie system might still deteriorate & fail over time.
I tend to have that same train of thought TD, although my skepticism of the wall system would be how the geogrid is anchored to the blocks. A wall that tall has many opportunities for connections to fail. In my opinion, the geogrid would last longer than the life of the wall, buried in the ground away from sunlight. The weakness I see is the connection point to the blocks, if this connection is not kept then one just has a wall made of stacked CMU blocks.
I worked with Unilock Pisa II, it has a cast in keyway system to hold everything together. I did a wall with the brand pictured here once where the homeowner had already purchased the block, I didn't care for the plastic pin system. You were supposed to fill the voids with pea stone as well to give the wall some additional mass. Unilock is a solid concrete block. The geogrid resembled a snow fence, on walls over 4' high they were required to be engineered, which was provided free of charge by Unilock. The geogrid has fiberglass strands running in one direction, the other part of the grid is just a textile to hold the fiberglass sections at the proper distance. It's imperative that they are not confused or the wall will fail. The mesh was laid over the prescribed course of block almost all the way to the front of it so the grid was pinched in the keyway. The weight of the upper courses held it in place and it went back into the backfill a prescribed distance with compaction. Behind the wall was the clear stone I mentioned that had a layer of filter fabric between it and the backfill to keep the backfill from migrating. It was a fiberglass felt product.
This may not be a good comparison CM. But I have seen plastic field tile go bad over the years. Farmer called & said he had some sinkholes in the field so we took the skid loader & hoe to investigate. We found the plastic tile broke about every 16 to 18 inches . The whole dam field was like that. Farmer ended up re-tiling the field. I always thought plastic was good forever if kept out of sunlight. This tile would have been first installed in the late 1970's. I know plastic has come a long way sense then but on a big tall project like the block wall in VA. I would still question it. Like my dad always said..... "If you want something to last.... Build it out of concrete & steel". I'd be a nervous wreck working on the a project like the wall in VA relying on plastic to hold it all up. Heck .. even on a 4' tall concrete retaining wall I put a deadman in every 25 to 30 feet with drain tile cast in it. I'm old school & skeptical
I would agree on lack of drainage. on a wall that tall I would have considered put more drainage pipes in. Maybe one at the top for ground run off and half way down. In a time of a lot of rain it would take awhile to drain the water to the bottom and out a 4" pipe.
Sheet pile and tie-backs. Tear out that fragile block wall, call Hayward-Baker and have them drive sheets with tie-backs. They will last forever.
TD25C the geogrid spec'd in these walls is tough as nails and not your normal plastic. http://en.wikipedia.org/wiki/Geogrid Some more information- http://www.nilex.com/book/export/html/22 Who knows at this point what type grid the engineer spec'd for this wall but I believe the grid didn't contribute to the failure -the leaking storm drain was the culprit. These walls are designed to handle water on top and behind them as a system but not designed to handle a point flow being introduced behind it.
Yeah CM , that's interesting about the geogrid and I agree its tougher than the plastic of yesteryear. Yes it's amazing how quickly moving water can under mine fill damaging a structure. Hey... I think I found some overhead views of the wall & facility. Scroll down & check out the first photo under Residential updates October 2013 http://www.southpeak.net/photo-galleries/
You have to believe they would have pitched the pavement away from the wall as well to control runoff. I agree with CM here too.
Thanks for sharing that TD. I come from a real estate development background and cannot believe that they spent that much money in retaining walls, excavation and site improvements for that project. How in the world did that make sense on a pro forma?
They sure did spend some bread on retaining walls CM. I was thinking about moving there as I heard they had a job repairing a wall
The whole entrance looks designed to give people on the highway something to look at. They sure got a spectacle now. If it was anything other than gaudy attention-getter it would have taken 1/10 the money and blocks to put it in like this. The middle section seems to be a staging area for some pretty hefty storm water bits. If that the same stuff laid where the blow out occurred i'm surprised any of it is still standing.
I was thinking the SAME thing as well. Is land that hard to come by in that area that they would build there? With it that close to the highway it doesn't seem like "prime" area either.
I think if I owned that property I would be stressin' about when the next part was going to fail. That had to have been crazy money!
It wouldn't have worked for what they wanted to do but gaudy it is. There is a single and multi family component to the project that runs the ridge line. The area by the highway is the commercial section. This property has some serious grade changes.:cool2
repair ? I've never used keystone blocks so just taking a stab at an idea. I assume due to damage & overlap of the blocks that possibly every block inside the red line would have to be removed to start a repair . I just wonder if a guy could install a ground anchor system (green dots) that would tie into the new wall where the geogrid ripped out? www.williamsform.com/Ground_Anchors/Manta_Ray_Soil_Anchors/manta_ray_soil_anchors.html
Yair . . . Showing my ignorance again. I have never seen that type of wall built so high. Until I saw the photo of the complete project I couldn't understand the topography and why such an edifice was necessary . . . and I still don't understand. (grins) On a more serious note. It is difficult to tell from the photo but to my eye the wall to the left of td25c's photo in the #28 post looks to be uneven and this is just the first season or so, the project appears to be far from completion. My concern would be how it will stand up to ten years of floods and rains. I reckon after this it would be uninsurable and a huge liability. Cheers.
I was thinking that if failed because it was new. It seems like all the retaining wall failures I've ever seen are newly installed and then they get hit with an unusually heavy rainstorm before the soil has had a chance to "settle". If the wall can survive a few months it seems like the soil doesn't turn to soup so fast. Obviously this is mostly smaller and less professional walls than the one in question.
I don't think it's an issue of the grid ripping out of the ground but rather a connection issue at the block. If you look to the left of the hole in the wall you can see the blocks "hanging" in the air, with the grid still holding them. If you look closely at the arial photo, this isn't just a retaining wall, it was built to carry the road to the top of the mountain. There is another smaller wall on the inside and the distance between the two walls looks to be 50' wide max. As tall as that wall is and having two walls on either side, I would wager the grid is continuous from face to face. The grid embedment length is a function of the height among other parameters.
I don't think the geogrid pulled out from fill either CM . When I made the comment "ripped out" I was thinking about it breaking & pulling loose from the pin location. I suppose it's possible the fiberglass pin itself failed as well. I think Keystone also offers a stainless steel pin. I noticed the elevated road the other night when I found the picture. Kind of has a Great wall of China look to it . http://en.Wikipedia.org/wiki/File:Greatwall_large.jpg
Scrub Puller; Here is another example of a segmented wall located in the state of Florida,USA. It was in a subdivision I was staying at. These walls are quite common there.
Yair . . . Tinkerer . Thanks mate. They may well be around over here . . . just not in my area. It just amazes me they are built so steep with little step back. The only walls I have had anything to do with were only about twenty/thirty feet high and built a bit like this . . . . Cheers.
The short story from around these parts: The explanation provided as best I can relate is the typical steep sloped soil face with no retention will fail in a slip condition similar to a J shape in elevation view (section). Thus by adding the layers of the reinforcing grid, one can control or redirect the size of the J failure to an amount that is essentially minimal/trivial. In other words, a stack of little J shapes as opposed to one big J shape. The block wall 'just' controls surface erosion and absolves any minor surface profile, maybe deals with the little J forces. The required reinforcement distance behind the wall into the fill and the required depth of layers/lifts is a function of the soil mechanics. Which can be greatly affected by a loss of friction due to a "water transient". Curiously: Clips from the fabric to the interlocking blocks (mentioned in another post above) or wrapping a lift like a burrito (not mentioned above) are not typical solutions in our locale. Perhaps these variants are for different manufacturers. Although the Gravel is supposed to be provided within the block cells and in the immediate area behind the block. Directional Fabric of a specified strength into the slope about 10 or 12 feet is generally speaking 'common'. Fabric Layers about 21 to 24 inches apart (think little Js). Previous lift compacted, tested, and then struck flat to required lift elevation using a squeegee (no teeth marks) in preparation for the next layer of grid is the basic procedure. For work stoppage, place half a lift then next day work to tie together and complete the lift as opposed to dry out or rain wetting at the fabric layer. Hand work with rammers near the gravel/block area and then concern with fall protection too. * From the pic(s), wrapping the layer like a burrito may help reduce the amount of displaced material if there is a problem. But this may preclude layering the fabric or keying into the block. * Also regarding the idea the longer the soil sets after installation, the less likely for a problem....tend to agree. This is probably due to internal forces equalizing and re-establishment of soil particle bonds if there is clay. In sand, we know where a fairly tall but quite stable wall for a few years (ie no cracks to pavement etc etc), was exposed to a broken fire hydrant water rush drainage and it laid over like a logger sawing a tree. (regarding)re-excavation of the geotensilefabric is a quite interesting event. Tears, shards, half torn out clumps, frustrated laborers etc etc. and anxiety of damaging the remaining reinforcing are typical events in excavating this soil and fabric composite material.
It seems a storm drain failure might be the culprit here as well - http://www.wdbj7.com/news/local/tra...ng/-/20128466/23883306/-/qk822qz/-/index.html I somehow overlooked the news video link you posted CM1995 . The developer didn't seem to worried about it on camera. They talked like it would be repaired in two weeks. I sure am curious about how they went about the repair job ? Do they dig it all up & install new geogrid ? Or somehow splice in to the existing grid without digging it all up ? :beatsme
I don't see any weep holes anywhere... which interests me... I always wonder what the construction details look like when I see guardrail inside of what I believe to be the reinforced zone of an MSE wall. It seems to happen fairly frequently, but I feel that it could jeopardize a lot. My personal opinion is that many PE's have swayed to relying too heavily on MSE technology and are implementing it in situations where older systems would serve more appropriately. I agree with the storm drain concept, but I also believe that the ADS N-12 spec is a soil tight spec... not water tight. I often wonder whether storm systems are intended to work primarily on the "path of least resistance" concept, or be truly water tight.