Mechanics and Rates of Subsidence on Alternative Landfill Covers in Arid Environments

Alternative landfill covers that rely on evapotranspiration (ET) to keep waste isolated from infiltrating water are becoming increasing accepted in arid and semi-arid regions for waste disposal site closure. However, questions remain about the impact of some phenomena, such as subsidence, on long-term cover performance. To investigate the mechanisms and rates of subsidence on ET covers, a 32-year-old landfill at Edwards Air Force Base, California was investigated. Although not formally constructed as an ET cover, soil of variable thickness was placed over waste cells when landfill use ended in 1976 and no maintenance has been performed since. Prominent surface expressions of subsidence include troughs along landfill cell axes and tensional cracks along cell edges. Infiltration into the landfill today is dominated by these subsidence fractures that allow water to bypass the ET cover and shallow waste layers.

Analysis of historic records allowed calculation of a range of settlement between 1976 and 2006 of between 1.8 and 2.4 m, with an average of 2.1 m. Historic accounts indicated much of this subsidence had occurred by 1991. Core soil samples from soil trenches and core tube samples from an auger rig were collected to evaluate the potential for additional subsidence. One-dimensional consolidation tests were performed on remolded bulk waste samples and on undisturbed samples of intermittent soil layers that were periodically placed atop the waste when the landfill was active. If left in its current condition, an additional 0.6 m of settlement could occur over 20 years. Subsidence potential was also calculated for one closure option using a 1.5 m monolayer ET cover to be built over the existing landfill, assuming maintenance of any new subsidence features. Under this scenario, 0.6 m of additional subsidence from loading from new cover material and continued decomposition of organic matter in waste cells could occur.