26 January 2010
The San Antonio retaining wall collapse – did the developer have a permit?
Posted by Dave Petley
Reports suggest that ground movement has ceased in the Rivermist subdivision, which suffered a fairly spectacular retaining wall failure over the last few days. Attention is now focussing upon that retaining wall – and in particular on why it failed. This is a pretty key issue given the potential liabilities and its magnitude – the wall is 1000 feet (>300 m long).
Press reports note two interesting aspects of this accident:
- The City of San Antonio has released a statement that says “The hillside collapse yesterday within the River Mist subdivision that damaged several homes was a result of an improperly constructed retaining wall, as well as improper compaction of fill on which homes were constructed by the developer, Pulte Homes. The developer did not obtain the required City construction permits for the retaining wall that collapsed.”
- Some reports suggest that this is the second time a retaining wall has failed at this site: “One neighbor who was among the first homebuyers in the subdivision set among rolling hills on the outskirts of San Antonio said he was initially told no homes would be built on the crumbling ridge because it was too steep. Romeo Peart, 32, said one retaining wall failed several years ago before the current one was built and homes were constructed above it” (Washington Post).
I should also add that I am not sure that this is a particularly simple retaining wall failure, based upon the image below (from here):
In particular, it appears that the slope / small retaining wall below where the man in the green jacket and hat is located has also failed. I wonder why?
Finally, I thought I’d embed this video, at least in part for the picture shown before the video starts, which is a somewhat surprising image in my opinion:
http://www.kens5.com/v/?i=82656182
The video itself provides an interesting overview and a verification that a section of the wall had needed rebuilding on a previous occasion.
Dave Petley is the Vice-Chancellor of the University of Hull in the United Kingdom. His blog provides commentary and analysis of landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.
Dave, I forwarded your email on to my parents, since they would have better answers for you– I am sure if they get some time, they will email you– in fact, I am sure the comment from RC is my Dad. Haha.I did want to comment on this though, "Reports suggest that ground movement has ceased in the Rivermist subdivision" and I say this is purely BS. My mother got up this morning and reported to me that there are cracks opening further north on this wall! She is really worried now because the cracks have gotten as close to her house as they CAN get. In fact, last night, the area next to her house was barricaded off just the three houses in the furthest area of the cul de sac were unavailable. Today, her whole street is barricaded again.As for the wall being rebuilt in the past– yes, it has been. I saw the construction with my own eyes. Here is some interesting info that my mom told me after attending the meeting: people have been complaining for weeks about things like 2X4 boards coming through their walls…. and Centex told them to "water their lawns more". For YEARS when it rained the streets would be VERY MUDDY in the areas below this hill. The residents thought it was from construction, but it was actually dirt washing out from behind this wall!People complained about groaning and creaking to Centex and that it looked like their yards were sloping… and Centex told them to water their lawns more. No joke.At the resident meeting, the President had NO idea about this wall being rebuilt, no clue about any of the complaints. This mess gets worse by the hour.
Has there been any report as to what company was actually responsible for designing and installing the retaining wall?
As a design engineer, I can tell you a retaining wall of this magnitude should have been designed by a structural engineer with geotechnical information from which to make his judgments. When it all comes out in the wash, I bet we'll find out either that the wall wasn't done by a licensed engineer or it was done by some incompetent. DJ
Is there another meeting set up for residents in the neighborhood? I live on Ranchwell Cove and am very concerned now about property values plummeting in this area. I keep on finding out after the fact that there were meetings. Any help would be appreciated.
You know what Vern? Greed's a terrible thing, it'll eat you alive.
As a civil technologist I agree it seems unlikely there was a site specific geotechnical engineering design to this backfill and wall.Offhand it looks like poor backfill drainage leading to saturated soil with soil piping weakened the slope leading to a slumping failure with wall intersection. The preceeding comments indicated muddy runoff from this area indicating that either existing soil drains had failed or the slope had soil piping undermining the backfill/wall.A proper geotech design likely would have placed graded and compacted backfill material with filtered soil drains and a stronger retaining wall probably using anchors. Soil drains work well, given an engineer designs for the application.All the backfill/wall, including apparently stable areas, should be assessed by a professional geotechnical engineer who may advise a properly designed retrofit be constructed.If there are continuing rains getting this assessment done ASAP would be highly prudent.
As a geotechnical engineer with experience with slope stability, this looks like a global translational failure to me. And for those that think the wall "should have been designed by a structural engineer with geotechnical information", you missed the boat. That slope didn't fail through the wall, it failed beneath it. Literally, the wall went along for a ride! That may – or may not – put the wall designer in the clear. Only time, and a careful review of the documents, will determine the designer's culpability in this case.All too often, residential developers "piecemeal" the geotechnical engineering design work. As a result, no single study addresses all of the site risks. This may be a part of the problem for this development and failure.What happened? While I could hazard a guess, I won't. Let's wait and see what facts are uncovered. I've (symbolically) put my hunches down on paper, to be revealed at a later date.Who is at fault? Again, it's too early to tell. One thing is clear: only the lawyers will win. Everyone else – the developer, homeowners, engineers, and contractors – will lose.I will watch this with a great deal of interest.
As a wall contractor in Austin, I'd like to see design specifications for ANY gravity wall system. You have design criteria for MSE walls but I have yet to see any calculations that work for a gravity wall. Pictures of that wall show the base to be approximately 4' in width…Not even close to viable.
I've been frequently involved in construction of LockBlock gravity walls while working with geotech engineers who used slope analysis software to design them. Typical LockBlock wall:http://www.lockblock.com/design.htmlI can't recall the old DOS software, it's been a few years, but a quick Google search reveals there's still lots out there.In each case there was a geotech engineer involved in design and construction since base and backfill material gradation, compaction and drainage was so important. I haven't seen a situation where a generic design would safely apply given unique site characteristics.
While the question of whether the wall was properly designed (and by whom) is important, construction methods are critical. For years many owners/developers have gotten away from having an engineer of record monitor the construction of their project, and it's particularly important when looking at critical elements such as a wall like this. One thing I noticed right off the bat is the presence of the partially undermined manhole structure sitting cockeyed in the one failure trench. I'm curious whether the outfall sewer from than manhole ran beneath the wall, and the trench was not properly backfilled. That very easily could have contributed to the extent of the failure.
My analysis of the free USDA Web Soil Survey data available for this site (http://www.homeownersoftexas.org/Rivermist_Soil_Survey.pdf) shows that most of the native soil is expansive clay with high shrink-swell characteristics. On top of that they added 20-30 feet of fill dirt, which also appears to be expansive clay, but we may never know for sure since much if it has been hauled away (evidence tampering in my opinion).The claim that the slope failed BENEATH the wall may be true, but it's because the wall was built on expansive soil that goes some 8 feet deep, and the wall apparently didn't have a footing that went that deep. So, pressure from tons of fill dirt likely squished the native dirt out from underneath – like squeezing one end of a water balloon.The USDA describes the native soil as "very limited" for building, largely because expansive clay can expand 30% when saturated and exert up to 15,000 pounds per square foot of pressure. That's like standing a cement truck on your bathroom scale.Follow our growing collection of videos, photos and articles on Hills of Rivermist at http://homeownersoftexas.org/Rivermist.html.