EVALUATING THE DRIVING MECHANISMS FOR EARTH FISSURE AND PATTERNED GROUND FORMATION AT FORT IRWIN NATIONAL TRAINING CENTER, CALIFORNIA

The U.S. Army’s Fort Irwin National Training Center in California obtains its water supply from groundwater aquifers in the Bicycle, Irwin, and Langford Basins. Groundwater has been pumped from the endorheic Bicycle Basin since the 1960s, resulting in a 30-meter decline in aquifer water-levels. Following the emergence in 2005-06 of a large surface crack (500 m in length) on the Bicycle Basin playa across an area used as an aircraft runway, a comprehensive study was initiated to determine the cause(s) of ground failure and develop recommendations for hazard monitoring and mitigation. In 2013, another large crack emerged at a separate location across a perpendicular aircraft runway. Although of similar appearance and dimension, its bifurcated nature suggested relation to a relict patterned ground morphology of giant macropolygon structure (macropolygons of hundreds of meters in length) observed from historic aerial photography.

A variety of methods were used to evaluate ground failure hazards in Bicycle Basin, including first-order leveling and other land surveys, satellite interferometric synthetic aperture radar analyses, geophysical surveys including electromagnetic induction imaging, water-level monitoring, and modeling of hydromechanical processes via continuum and discrete element method approaches. Integrating results allowed us to evaluate hypotheses for shallow versus deeper processes as the driving mechanisms for surface cracking. Our results indicate that both shallow and deeper processes are responsible for the observed ground failures. Shallow desiccation processes activated by playa inundation events with subsequent lake evaporation and volumetric shrinkage can lead to spectacularly large crack formation and macropolygon formation in the presence of regional, extensional tectonic stresses. Deeper processes with stresses driven by localized differential compaction from groundwater pumping are also necessary to explain the observations and appear to work synergistically with desiccation to accumulate stresses that lead to ground failure. Finally, we demonstrate that time-lapse electromagnetic induction imaging is effective for monitoring ground failure hazards, notably in areas where there are no visible indications of cracking or depressions at land surface.