Challenge

Newby Island Sanitary Landfill, located on the southern margins of the San Francisco Bay, is founded in soft, estuarine soils, and the stability of the landfill-subgrade system is therefore governed by a series of potentially liquefiable sand lenses underlying the site. The Landfill needed remediation options in order to mitigate the expected excessive seismic displacements.

Solutions

GLA provided design and construction services for multiple mitigation phases at the landfill starting in 2012. In order to design a mitigation plan, our team used pseudo-static limit equilibrium slope stability and Newmark-type seismic displacement evaluations developed through a state-of-practice FLAC model to assist in evaluating distribution of stress, strains, and displacements. The team developed this model through evaluations and investigations on the site, including site-specific soil profile shear wave velocity measurements.

Using the FLAC Model, we designed and constructed an innovative mitigation system of in-situ stabilization by cement deep soil mixing with the site perimeter levee to increase composite ground strength. GLA has implemented this mitigation system through multiple phases.

Results

Site response analyses performed by GLA post-mitigation have shown that our mitigation system successfully intercepts the weak large, lateral center plane of the levee, reducing displacements significantly. Overall, our mitigation program significantly reduces liquefication-induced lateral deformation in the levee and related slip along the side-slope liner to within tolerable limits. Our state-of-practice models have ensured that the Landfill can continue to run in environmentally sustainable ways despite the significant challenges presented by the instability of the land, all while providing Republic with cost-efficient methods of mitigation.