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ADOT crew with backhoe on US 191 July 2021 - J. Cook

U.S. 191 in Cochise County damaged by extensive ground fractures

U.S. 191, the main north-south highway in central Cochise County, was recently closed due to damage caused by collapse along large surface fractures during a monsoon storm. Local slumping and collapse near milepost 53 southwest of Willcox began during heavy monsoon rains on the evening of July 5, 2021 (Figure 1). The Arizona Department of Transportation (ADOT) closed a portion of the highway to traffic and began to assess how to repair the roadway. Joe Cook with the Arizona Geological Survey (AZGS) and Brian Conway with the Arizona Department of Water Resources (ADWR) visited the site on July 8th to assist ADOT and Cochise County personnel with understanding the extent of the hazard beneath the highway and in the desert nearby (Figure 2).

Figure 1 A and B
Figure 1 (L). Figure 1. Slumping across Highway 191 due to collapse along an underlying soil crack. ADOT has dug holes through the asphalt to assess the extent of subsurface damage and to repair underlying voids.
Figure 2 (R).  Figure 3. Brian Conway (ADWR) stands next to an open crack alongside the damaged portion of U.S. 191. This crack appeared freshly formed and was approximately 2m (6 ft) deep in some areas.

The damaged portion of Highway 191 lies within the Sulphur Springs Valley, which is riddled with earth fissures and giant desiccation cracks (GDCs, Figure 4). Earth fissures are tension cracks that develop due to land subsidence caused by groundwater withdrawal. GDCs are large surface cracks that form in soils that contain certain salts and clays which expand when wet and contract when dry. Repeated wetting and drying of these soils can lead to the formation of large polygonal crack networks resembling mudcracks but with polygons hundreds of feet across (Harris, 2004). The area surrounding Willcox Playa contains many fissures and GDCs, sometimes in the same areas. AZGS has suggested that the formation of earth fissures through areas with GDCs can lead to enhanced erosion along GDCs and deepening of preexisting cracks (Cook, 2011).

Map of GDC
Figure 3. Overview of polygonal cracks and AZGS site observations. Polygonal GDC networks are present along both sides of U.S. 191 and some new cracks have formed since this aerial photo was taken (2016). Areas 1 and 2 are zones of wetted soil where rainwater infiltrates. Yellow dots represent observed surface cracks on July 8, 2021. Not all photo-interpreted cracks were visited.

Both land subsidence and GDCs are present at the damaged portion of U.S. 191. ADWR has documented 38 cm (15 in) of subsidence in this area from 2010 to 2021 (ADWR, 2021). The cracks undermining the roadway are part of an extensive GDC network, but it is possible that ongoing subsidence is causing deeper cracks to form and these are enhancing the GDCs. Nearby earth fissures have repeatedly damaged the intersection of Cochise Stronghold Rd. and Dragoon Rd. (AZGS, 2019). The current subsidence rates in the Sulphur Springs Valley are the most dramatic in all of Arizona with some areas dropping by more than 16 cm (6 in)/year (ADWR, 2021). If groundwater withdrawal continues at current rates, continuing formation more earth fissures is likely.

Posted by Joe Cook (7/19/2021)

Cover image: ADOT investigators probing US 191 for buried voids.

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