CEMENTING THE FUTURE

The reconstruction of the Cypress Freeway in Oakland, California, required some of the most extensive and innovative uses of soil-cement mixing in the United States. The subsurface conditions at the project site involve very dense, highly permeable sands down to a depth of 23 m, interrupted at depths of 6-9 m by a layer of silty sand with occasional clay lenses. Below the sands rests a stiff, thick layer of clay known as the Yerba Buena mud, or Old Bay clay, which is essentially impermeable. A continous cutoff wall penetrated the Old Bay clay layer. Approximately 2,300 linear m of soil-cement walls were installed. Engineers used three types of soil-cement walls to resist the lateral pressures of earth, water, and surcharge: cantilevered, structurally reinforced walls; tied-back structurally reinforced walls; and earth berm unreinforced walls. Because the Seventh Street seal slab is under permanent uplift, the slab required permanent tiedowns. Engineers chose soil-cement piles as the most effective and economical solution. The construction of a Seventh Street off-ramp also posed unique problems, because the ramp would pass less than a meter from two existing Bay Area Rapid Transit (BART) columns, which are supported on shallow spread footings. Engineers demolished existing BART foundations to make room for new ones. Because conventional foundation systems such as driven piles or drilled piers could have caused excessive settlements and lateral deformations, the engineers used soil-cement mixing to construct 10 shafts 2.4 m in diameter. These "soil-cement cylinder piles" were designed to resist both axial and seismic loads.