4 February 2010
The Rivermist subdivision in San Antonio, Texas is on the move again – and it is a rotational retaining wall failure
Posted by Dave Petley
The woes of the families displaced by the slope failure at the Rivermist Subdivision in San Antonio, Texas have been exacerbated by a new phase of movement. AP Texas News reports that heavy rain has triggered additional cracking of the wall, and the movement of some debris. However, the houses at the top of the slope are apparently not moving.
I have been wondering why they are so confident that the wall won’t fail. In an earlier post I suggested that this is not a simple retaining wall failure, given the toe scarp – a couple of subsequent commenters agreed. These aerial images, from My SA news show that this is the case:
The key part of the image is actually at the bottom of the first image above, although once you have your eye in you can see these features in the other image too. This is an annotated version of that first image:
This landslide is clearly not a simple retaining wall failure. It is a rotational slip in the slope that has caused the retaining wall to fail. It is not surprising that they do not expect the wall to collapse during these rains as the wall is being rotated to lean backwards by the failure.
Of course, as per the comment below, ensuring that the retaining wall would not be affected by this type of rotational failure should always be a key part of the design process for such structures. I suspect that this will become a textbook example of the problems that can affect retaining walls.
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.
Great analysis as always Dave. But the designer of the retaining wall is still not out of the woods, as they should have undertaken a design check for 'external stability', which normally involves a slope stability analysis that considers a rotational failure that encompasses the wall, its foundations, and the retained earth. This is in fact a text book example of what the 'external stability' check should consider.
Dave, I don’t buy your observation of Toe Scarp as proof that this is anything but a simple retaining wall failure. I reference Web Soil Survey Analysis of the Hills of Rivermist (http://www.homeownersoftexas.org/Rivermist_Soil_Survey.pdf). This paper uses freely available government data to show that the entire subdivision is on very expansive clay soil that’s very limited for home building. USDA defines “very limited” as having limitations that generally cannot be overcome without major soil reclamation, special design, or expensive installation procedures, saying poor performance and high maintenance can be expected.Even discounting photographic proof that the wall was not properly engineered and/or constructed, one must question why Centex Homes built there in the first place and was allowed to by the City. Both the builder and the City should have known the risks and taken proper precautions. Not doing so was taking a huge business risk, and apparently their gamble didn’t pay off. I find nothing that comes close to suggests Centex is a victim of natural causes or anything other than the culprit in what should be called a crime scene. (See http://homeownersoftexas.blogspot.com/2010/01/texas-builder-at-center-of-san-antonio.html) The City of San Antonio shares some blame because of what seems like poor planning, a lack of restricted zoning, and a lack of permitting and inspecting. Texas homebuilders, without regulatory oversight and accountability, will naturally cut corners everywhere they can to maximize profit.
This reminds me of this particular place:http://www.youtube.com/watch?v=VB06kISmARYhttp://www.bing.com/maps/?v=2&cp;=pj3msb56zkhk&scene;=5889956&lvl;=1&sty;=o&where1;=33.2239777777778%2C%20-117.205786111111/#I wonder just how stable the ground under that large retaining wall really is.
Your diagnosis appears accurate to me. Was this area the subject of landfill?
Yes, it is related to landfill. Rather than cutting into the hill, and rather than building homes with tall foundations on a hill, many builders find it cheaper to level the land as Centex Homes did. The entire subdivision is on expansive clay soil, but we can't tell for sure from the photos what was used for fill dirt. It looks like black clay to me, but we may never know, because the builder quickly removed important evidence while keeping independent inspectors out.
I complete agree with your analysis. It's probably that the wall fail unexpectedly and cause a terrible catastrophe.
The entire subdivision is on expansive clay soil, but we can't tell for sure from the photos what was used for fill dirt
I agree with you. Soil stabilization is a technical factor that every engineer should focus on. Erosion is a common phenomena in San Antonia, both natural or man-made. But as you said, this problem is solvable. A compact soil settlement is needed to regroup the disarranged house. Great post! Perfectly explained.