Dynamic Numerical Evaluation of Landfill Perimeter Levee on Liquefiable Subgrade Mitigated with Cement Deep Soil Mixing

A multi-phase liquefaction mitigation program in support of a vertical expansion of the Newby Island Sanitary Landfill (NISL) on the southern margins of the San Francisco Bay has been ongoing since 2012. The landfill, including the site perimeter levee, is founded in soft, estuarine soils, and the proposed waste fill thickness of 87 m (285 ft) is relatively high, as is the M7.1 design earthquake peak horizontal ground acceleration (0.41 g). Site-specific evaluations showed that the stability of the landfill-subgrade system is governed by a series of potentially liquefiable sand lenses underlying the site. In situ cement deep soil mixing (CDSM) through the levee was constructed to mitigate the expected excessive seismic displacements in underlying liquefiable layers. To date, design and construction of three mitigation phases have been completed along 1,000 m (3,280 ft) of levee. Slope stability and seismic deformation evaluations for mitigation design were performed using a state-of-practice de-coupled approach based on limit equilibrium and Newmark principles. This paper presents results of preliminary dynamic numerical analyses of the NISL mitigation using the two-dimensional finite difference-based computer program FLAC. Preliminary deformation results from the FLAC model were comparable to those from the conventional de-coupled approach, providing a reasonable degree of confidence in the numerical results. Given the versatility and more realistic physical behavior of the FLAC model relative to the de-coupled approach, this numerical approach may be considered for optimization of future NISL liquefaction mitigation phases or similar projects.