22 December 2008

Landslide on State Route 87, Arizona

Posted by Dave Petley

Arizona Geology, the Blog of the Arizona State Geologist Lee Allison, has a new post highlighting the reactivation of an ongoing slope failure on Highway 87, which links Phoenix with the town of Payson. Movement on this slope was first noted in March 2008, whereupon emergency works were initiated to try to reduce the immediate hazard. Nonetheless, the road was closed for six days at that time.

The failure is quite unusual in terms of its morphology, as the image below shows (image from the Arizona Department of Transport – click on the image for a larger view on their web site):

The slide is clearly rather large (look how far back from the road the cracks entend), and fairly deep-seated and complex, given the heave in the road (image from the same web site):

After the initial slide in March the Arizona Geological Survey published a reconnaissance report about the slide, which can be accessed from a dedicated website here. This report notes that the slide is part of a much larger, ancient complex of landslides extending over a kilometre down slope (see image below from http://www.azgs.az.gov/hazard_info_hwy87_2008.shtml):

The slide, which first occurred in a period of heavy rainfall, is located in wet, sandy sediments with clay rich layers (classic landslide materials). The emergency repairs seem to have consisted on some regrading of the slope together with some reinforcement of the road area, supplemented with a state-of-the-art monitoring system. It appears that the latter has successfully detected the recent movements, which have occurred in another period of heavy rain. Larger scale mitigation works are planned for the Spring.