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OFR-08-04 Highway 87 Landslide Report

Highway 87 Landslide News Release


Figure 1. 21 March 2008 Landslide on SR-87


Figure 2.
Buckling along SR-87
Figure 3. Clay-rich Sediments of Slidemass


Figure 4. March 2008 Landslide (outlined in black), hosted in Ancient Landslide (outlined in red).

 

 

AZGS Summary :  “Reconnaissance Report on the Easter Weekend Landslide of 21 March 2008, SR-87, Gila County, Arizona.” by M. Diaz, B. Gootee and A. Youberg.

The Arizona Geological Survey (AZGS) has delivered to the Arizona Department of Transportation (ADOT) a geologic report of the 21 March 2008 landslide that temporarily closed Arizona’s State Route (SR) 87.

For a free, online copy of the report – including all text and figures – select the PDF link to the right.
For a printed copy of the 10-page Open-File Report, OFR-08-04, call or visit our Tucson bookstore (416 W. Congress; 520.770.3500); or in Phoenix, Explore Arizona! Outdoor Information Center (One North Central Ave., Ste #120, Phoenix; 602.417.9300)

Easter Weekend-2008 Landslide. On 21 March 2008, a landslide buckled SR 87 near mile marker 224, creating unsafe conditions and requiring the Arizona Dept. of Transportation (ADOT) to close the highway for six days (Fig. 1).

The landslide toe broke the ground surface for about 45 ft along the southbound lane of SR-87, uplifting and buckling that lane and displacing the northbound lane about one foot to the east (Fig. 2).

Landslide materials.  The landslide occurred in wet, clay-rich to sandy sediments (Fig. 3) that are part of a larger ancient (paleo) landslide deposit (Fig. 4).

Figure 5. Possible Other Ancient Landslides along SR-87

What triggered slope failure?  We are still uncertain, but heavy rainfall and water-saturated soils probably played a contributing role. On 17 March, 0.24 inches of rain fell in the space of 1.25 hours on nearby Mt. Ord; and this on top of 14 inches of rain already recorded in the 2007-2008 precipitation year. 

Newly Discovered Landslide deposits adjacent to SR-87. The AZGS geoscience team discovered evidence for three additional, and previously unidentified, ancient
(paleo) landslide deposits adjacent to SR-87 (Fig. 5). Further study is needed to scope the potential impact, if any, of these deposits on SR-87.

AZGS Recommendations | The AZGS report includes recommendations for evaluating the potential for future landslides near mile marker 224 along SR-87. Those recommendations are summarized below; for a complete list, please see page 8 of the report. Last, we should note that in the eight months since the March 2008 landslide, ADOT has made substantial progress in evaluating the causes and full extent of that event. With this knowledge in hand, ADOT is developing strategies to mitigate future landslides along SR-87.

  • Identify the full extent of the 21 March landslide
  • Establish the number and extent (i.e., map) of other paleo-landslides deposits along this section of SR-87
  • Assess the potential for slope failure, if any, of newly discovered paleo-landslide deposits along SR-87
  • Delimit the combined geologic-environmental conditions that are likely to lead to further slope failure

Acknowledgments | AZGS thanks the Arizona Dept. of Transportation and its contractors for lending support and information to this investigation. We thank, too, the Arizona Department of Public Safety for their contribution.

Related Links:

AZGS Preliminary Report: Landslide Closes Highway 87 South of Payson, Arizona

USGS: Landslides Hazards Program

 

ADDITIONAL INFORMATION:   For more information contact Arizona Geological Survey geoscientists, Mimi Diaz (602.708.8253; mimi.diaz@azgs.az.gov) or Michael Conway (520.770.3500; Michael.conway@azgs.az.gov).
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