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."
