Designed for seismic stability

Dec. 28, 2000
On Oct. 17, 1989, the Loma Prieta earthquake struck the San Francisco and Monterey Bay areas. Measuring 7.1 on the Richter scale, the earthquake was the largest to occur in the San Francisco area since 1906. The epicenter was located along the San Andreas Fault, north of Watsonville, Calif. Tremors were felt as far south as Los Angeles, east to western Nevada and north to the Oregon-California border. It caused widespread damage and forced the closing of 18 bridges including the San Francisco-Oakland Bay Bridge, the Struve Slough Bridge and the I­p;880 Cypress Street Viaduct.

The Cypress Street Viaduct was California's first continuous elevated, double-deck freeway and a major north-south artery leading to the San Francisco-Oakland Bay Bridge. It was a reinforced-concrete structure with post tensioning in some upper level bent caps. Its collapse resulted in the deaths of 42 people and its loss has hampered the movement of traffic resulting in severe congestion.

The Governor's Independent Board of Inquiry found that, "soft ground on the border of the San Francisco Bay amplified ground motions more than anticipated by current codes." The soft ground, made up of Bay Mud or Artificial Fill, was one factor in the collapse. Soil of this type is susceptible to liquefaction during an earthquake. Liquefaction occurs when earthquake shaking increases the hydrostatic water pressures in the soil. When the soil "liquefies" it losses its strength and ability to provide foundation support leading to the collapse of structures (see Soil Liquefaction Major Threat in Earthquakes, November 1987).

Another factor in the freeway's collapse, according to the Portland Cement Association (PCA), was the lack of continuity and confinement of rebar. Four, 4-ft long connecting dowels at the top and bottom of the columns supported the upper deck of the freeway. The dowels could not hold up to the bending motion when movement at the top of the column lagged behind motion of the piers' foundations. This pressure on the dowels caused the columns to buckle at the top and bottom and collapse (see Pier Continuity Blamed for Earthquake Collapse, December 1989).

After the collapsed freeway was demolished and the rubble removed all that remained was a grassy strip of land between two local streets. The new Cypress Freeway follows a new route, extending north and west from the I­p;880/I­p;980 interchange to a fork, at a reconstructed West Grand Avenue interchange. Here the freeway splits into two branches. One branch connects with the Bay Bridge toll plaza and the eastern approaches of the San Francisco-Oakland Bay Bridge. The second branch continues west to join up with I­p;80 in Emeryville, Calif. The new Cypress will have three lanes in each direction and be a total of three miles long. Work also includes improvements and repairs to local streets.

Before construction could begin, the design of the Cypress had to take into account the seismic activities of the area and the soft ground geology. The work was divided into seven separate design contracts-lettered A through G. Two were designed by Caltrans engineers and the other five were contracted to private consultants. During the design phase a seismic philosophy took shape, the goal of which is to design a structure to withstand a collapse under the maximum credible earthquake with limited repairable damage. Any damage should be repairable with minor interruption to traffic. Structures also were required to meet minimum seismic performance as a stand-alone frame.
The design of the foundation systems for the structure was critical because the rerouted road would pass over soil types that affect the construction of piles. The disparity in strength among the four soil types-Artificial Fill, Young Bay Mud, San Antonio Formation and Yerba Buena (Old Bay) Mud-caused concerns about the constructability of driven piles.

Seismic considerations also dictated the selection of pile type and capacity. In order to limit inelastic response to plastic hinging of ductile columns, the connecting structural elements, and the foundations, were designed to resist the shears from the columns. The combination of these forces' strength and the local geology of the soil required a ductile-pile system with the ability to handle large axial loads and shears while sustaining large horizontal displacements.

A pile test program was implemented by the California DOT's (Caltrans') Consultant Contract Management Branch of the Division of Structures in collaboration with the Office of Structural Foundations to address these concerns and aid in the design of an earthquake-resistant foundation system.

The test program studied 24-in.- to 42-in.-diam steel-pipe piles at four locations, representing the four different geologies, and the different foundation conditions anticipated along the planned route. These pipe pile sizes were chosen because of the local inexperience in driving large-diameter pipe piles. A 16-in. pipe piling was incorporated into one of the design contracts; however, there was no need to test it because of prior experience working with this size. The pile wall thicknesses varied from 0.5- to 0.75-in. depending on the driving stresses encountered. Pipe piles were filled with concrete and reinforced with steel to meet the structural support demands for the new freeway.

Caltrans has been using large, diameter steel-pipe piles on an increasing amount of projects because of the superior lateral stiffness and ductility these piles provide. The test program would help Caltrans better understand issues involved in the use of large-diameter steel-pipe piles.

Large impact hammers were needed to perform the driving, and various impact hammers and one vibratory were tested during the program. The hammers used included Delmag models D30-32, D46-32 and D62-22, a Foster IHC S-90 double-acting hydraulic hammer and a Kencho VM2-25000AII vibratory hammer. In addition, a 175-ton crane, a support crane for the pile stabilization, a crane-mounted auger drill and offshore leads were used.

There were a few minor problems encountered during the test. High groundwater levels were a constant problem. Water pumped from the site could not be directly returned to the water system because of possible contamination. Instead water was stored in tanks to await treatment. A section of right-of-way for the new freeway was a rail yard acquired from the Southern Pacific Railroad. Some pipes sank and disappeared into the soft soil while pile-driving in the rail yard.

At the test site near the Bay Bridge toll plaza one of the piles broke free of its support crane while being driven. It leaned over the car pool lanes and began to sway back and forth. Fortunately, the crane operator was able to catch it before an accident occurred.

The monitoring and analysis efforts conducted during the tests helped fine tune the piling design. Knowledge gained from the tests about foundation installation and long-term performance was provided to the prospective contractors in order to aid in the facilitation of the bidding process.

Overall test results indicated the low-displacement piles perform better during installation with the larger impact hammers. In some locations the pile length was reduced because it was found that structural loading was much less than the predetermined capacity. The tests resulted in a modification of standard specifications and the development of new specifications.

The test program also allowed for a significant saving in money. Mike Brenner, formerly with Nova Group-the contractors involved in the testing-stated, at a Deep Foundations Institute seminar, "The test pile efforts helped save millions of dollars."

About the Author

David Banasiak

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