Seismic Imaging of a Leachate-Recirculation Landfill: Spatial Changes in Dynamic Properties of Municipal Solid Waste

摘要

Leachate-recirculation landfills enhance waste degradation through percolation of leachate inside the waste mass. In situ monitoring of moisture distribution and changes in mechanical properties (stiffness) of municipal solid waste (MSW) is needed to optimize the safe and effective operation of these types of landfills. Geophysical methods, such as electrical-resistivity tomography, have effectively monitored moisture distribution during and after leachate injection. This study investigates seismic surveys to capture the changes in mechanical properties [shear modulus (via shear-wave velocity) and Poisson's ratio] of MSW to infer the extent of degradation and provide dynamic properties needed for seismic stability evaluation. To achieve this goal, a series of seismic surveys were performed at the Veolia ES Orchard Hills landfill, 15 km south of Rockford, Illinois, to image seismic velocity structure and the Poisson's ratio of a recirculation cell, compared with an adjacent newer landfill cell without leachate recirculation. Seismic data were collected using fan shot direct P-wave (compressional wave) surveys and S-wave (shear wave) surveys, conventional P-wave refraction, and the multichannel analysis of surface waves (MASW) method. The fan shot surveys employed a sledgehammer source on one side of the landfill and geophones on the opposite side, thus exploiting the landfill's topography and geometry to image waste to a depth of approximately 10 m. P-wave-velocity tomographic models and S-wave-velocity tomographic models from these direct-wave (through-pile) raypaths indicate a dramatic velocity increase below 5 m depth, perhaps indicating consolidation and compaction of the waste material. The ratio in P-wave to S-wave velocity ranges from 1.8 to 3.7, with an average of approximately 2.7, and Poisson's ratio ranges from 0.29 to 0.46, with an average value of 0.42 (standard deviation 0.024). In the upper 5 m S-wave velocity (～150-170 m/s) is higher at the recirculation cell than in newer portions of the landfill consisting of uncompacted waste (～110-120 m/s), as derived from analysis of surface waves. The landfill cap, which is present over the recirculation cell, but not over the new waste, may be at least partly responsible for this difference. S-wave velocities are very similar below 5 m depth, whereas P-wave velocity (from seismic-refraction profiling) indicates a slight decrease in velocity between the leachate-recirculation cell and newer portions of the landfill. The refraction and surface wave surveys, however, were depth-limited (less than 8-9 m), compared with the fan shots. Overall, this study showed that seismic surveys have the potential to monitor spatial and temporal variation of dynamic properties of MSW.