Investigating the Mechanisms for Earth Fissure Development at Bicycle Lake Playa, Fort Irwin, California

Bicycle Lake Playa, located in the Mojave Desert of Southern California, has long been used as a runway for aircraft operations at the U.S. Army’s Fort Irwin National Training Center.  The development of a large earth fissure (~0.5 km length) across the airstrip in 2005 has led to concern over the future use of the playa as an operational runway.  The USGS recently began a study to determine the cause of critical ground failures at Bicycle Lake Playa and to provide reasonable predictions for fissure evolution in order to facilitate development of a successful mitigation strategy. The study approach includes (1) surface geophysical surveys for fissure characterization, (2) tape extensometer measurements for monitoring fissure movement, (3) radar interferometry (InSAR) for measuring subsidence, (4) monitoring of soil-water tension in the playa sediments, (5) laboratory experiments of desiccation-crack propagation under controlled soil-water tension, (6) laboratory analyses for investigating the spatial variability in soil properties, and (7) numerical simulations of playa hydrology.

Preliminary InSAR results indicate that subsidence rates are as large as 3 mm/month in the area north of the playa where pumping of ground water has resulted in ground-water level declines of as much as 15 meters since 1967. The presence of subsidence near the fissure is highly suggestive of differential compaction of the aquifer sediments as the mechanism for fissure development. However, fissure formation was coincident with a prolonged period of playa inundation and subsequent drying of the lake-bed surface.  Geophysical surveys and aerial photography have revealed an extensive macropolygon (or giant desiccation crack) network across much of the playa suggesting that the desiccation process also may be a plausible mechanism for fissure development.