Cypress Viaduct Reconstruction

The I-880 Cypress Viaduct through west Oakland was constructed to replace the original Cypress Viaduct which collapsed in the 1989 Loma Prieta Earthquake. The reconstruction took place on an entirely new alignment, which has had effects on significant archaeological deposits in the area. The $1.13 billion dollar construction project was divided into seven concurrent contracts, including seven miles of bridge structures. Portions of the structures are standard Caltrans post-tensioned concrete box girders, steel plate girders up to 7' deep, an orthotropic steel box section and even a small section of precast prestressed concrete girders.

Geotechnical Innovations

Lightweight fill using lightweight cellular concrete and polystyrene blocks.

Using ultra lightweight material, designers were actually able to apply a negative surcharge to the San Francisco Bay mud material, which has low compressive strength and a potential to liquefy and amplify seismic forces. Because of the need to construct the freeway quickly, a more traditional approach of applying a soil surcharge was abandoned in favor of utilizing the lightweight fill. A lightweight cellular concrete pad 2 ½ ft thick was placed as a subgrade for the polystyrene blocks, then polystyrene blocks were stacked up to approximately 10 ft in roadway fills and bridge approaches.

Bathtub Section

A depressed roadway section was constructed at, and in some locations below the water table, requiring special geotechnical features in the design. A bathtub type section, with slurry walls and a concrete slab to form a water barrier was constructed to support the roadway. In-situ soil cement piles provided tensile strength to withstand buoyant forces on the slab. Uplift forces were quite significant, considering that the lowest point of the roadway is approximately 25 feet below the water table. A proprietary slurry wall system by Seiko Construction provided an expedient method of quickly installing the retaining walls. Because the contract for several of the Cypress jobs, including this particular contract, were A+B contracts with significant time incentives, the speed of this method became very important. However, the same system has been used on other Caltrans projects that were bid by traditional competitive bidding terms, so it appears to be a cost effective option.

The Seiko system is unique because of the three augered drill rig, which, like other slurry wall processes, places the slurry as the drilling progresses. The benefit is in the speed with which the walls can be placed with three augers working concurrently.