On 7 September 2002, two large sinkholes opened suddenly on a major, five-lane roadway in Tucson (Arizona), USA (Figures 1 and 2). The exact cause of the sinkholes is still controversial, but the sinkholes were apparently related to failures near 90° bends in a 35-year-old, 6-m deep, 1.1-m diameter sewer line known as the outfall interceptor.

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