6 June 2016
Granbury, Texas: a lakeside landslide that threatens an apartment block
Posted by Dave Petley
Granbury, Texas
Over the weekend an interesting, but to those involved distressing, landslide developed at the town on Granbury in Texas, threatening a four-storey apartment block. NBCDFW have a couple of decent images, including this one, of the landslide:
The landslide at Granbury, Texas via NBCDFW
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This image, showing most of the failed slope, is from Fox 4 News:
Granbury, Texas landslide via Fox 4 News
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It is hard from the image to know what has happened at this site. It is clear that the retaining wall at the toe of the slope has buckled and displaced outwards. Did the slope fail, displacing the wall, or did the wall fail, triggering collapse of the slope? Or some combination of the two? There may be a hint in the Google Earth imagery, which seems to show something a little odd. This is the site, taken in late 2014:
Google Earth imagery of the site of the landslide at Granbury, Texas, taken in 2014
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In this image the lake / river level is much lower than it is at present. There appears to be some sort of structure below the wall – is this a toe weight, or some sort of erosion protection? The image seems to show that this body has large black structures. An initial interpretation might be that these are cracks and fissures, but I am unsure. If so they are very large (and similar structures are seen elsewhere on the riverfront too). I have no idea what these represent – an artifact on the image? Or signs that the toe of the slope was damaged? Sometimes landslides like this occur when the toe of the slope is damaged, unloading the foot of the landslide, decreasing stability. Then, as pore pressures rise during heavy rainfall, the slope fails from the toe upwards. But it is very hard to tell based only on these images, and no form conclusions can be drawn in this case.
I am sure that this will be investigated properly, so it’ll be interesting to see what emerges. In the meantime, the news items report that the insurance company has declined to cover the costs of the damage, which could be as much as $1 million. The owners of the property have set up a GoFundMe page. As of this morning this had raised $5 of the £2 million target.
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.
Note the 1 pre- and 3 post-2014 repairs to the concrete road to the left of the apartment building.
A comparison of the Fox and Google Earth photos shows that something happened below the current water line. In the GE photo, the wall above the “toe weight” (if that is what it is) is a straight line. In the Fox photo, the center portion of the sea wall is at or below the water line, while the dock or landing on the left is still above it (and appears to be askew). It would be interesting to know what the topography is below the water line. The multiple retaining walls suggest that the ground under the structure was built up and is not the natural slope. If those were cracks in the GE photo, someone should have seen this coming.
Kool landslide my doooood
So was anybody killed durring this?
Joey, no one was hurt or killed. I believe the GoFundMe was organized by a relative of one of the displaced persons, to help him with relocation costs. Although it still shows up on imagery, the building was recently torn down.