Earth Fissures and Infrastructure: A Case History at the Siphon Draw Detention Basin, Central Arizona

Land subsidence can severely impact infrastructure and alter existing floodplain designations by changing ground elevation, ground slope (gradient), and sometimes through the development of ground cracks, known as earth fissures, that can erode into large gullies. Due to the alteration of surface water flow, ground elevation, and ground cracking that can undermine foundations, subsidence poses a particularly high risk to water conveyance, flood control, and other linear infrastructure. The Siphon Draw Wash (SDW) Detention Basin in Apache Junction, Arizona provides a unique opportunity to observe the impact of an actively propagating earth fissure. Earth fissures were first identified in the area in the 1990s. In the mid-2000s, plans were developed to construct a basin and channel to provide flood control along Siphon Draw Wash. A series of land subsidence and earth fissure investigations were performed as part of the design process for the SDW Detention Basin. During investigations, the nearby Southwestern Earth Fissure (SWEF) extended over 200 feet overnight following a rain event. Later during the investigation a trench located at the termination of the fissure extension was flooded by another rain event. The SWEF extension terminated just upstream of the boundary of the basin. The need for a basin for flood control purposes at this location required that fissure mitigation measures be implemented. Mitigation strategies included constructing 2 16-foot deep by 2-foot wide by 30-foot long slurry cut-off walls along the fissure extension and placing an embankment over the fissure extension, to help prevent propagation of the fissure into the basin. In addition, cut-off walls and geomembrane liner were constructed along the southwest embankment of the basin, to help prevent the fissure from moving further southwest toward residential areas, in the event that the fissure propagates into the basin. Construction of the SDW Detention Basin and Meridian Channel was completed in 2010. An annual monitoring program has been implemented that includes evaluation of satellite-based interferometric synthetic-aperture radar (InSAR), real-time kinematic GPS survey, analysis of high-resolution aerial imagery, and annual ground inspection.