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Alameda Corridor Trench Project
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Freight traffic in operation - the completed
project.
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The Port of Los Angeles and Long Beach intermodal connection to
the Los Angeles rail hubs predicted a doubling of the current 100
million metric tons of cargo now entering the ports each year. The
new Alameda Corridor, which serves the ports, will initially handle
60 trains per day and have the potential to accommodate 100 trains
per day when a third track is added in the future. Travel speeds
in the new corridor are two times greater that when the line was
situated at street level. This project demonstrated financial and
contractual innovation as well as technical success. Most certainly,
it serves as the model for others to follow.
The Alameda project consolidates four single-track routes into
the port area as one. Previously, trains as long as 1.5 miles had
to negotiate 200 street-level crossings and maximum speeds were
barely 20 miles per hour.
During trench construction, a bypass track was constructed alongside
the proposed trench to provide uninterrupted rail service. The trench
is fitted with drainage troughs adjacent to the walls to handle
precipitation. The design was based on a 50-year flood occurrence
and incorporated pumping stations of 12,000 gallons per minute capacity
at each end of the trench. The 51-foot-wide clearance between the
trench walls was originally designed for two tracks and a service
roadway. It was constructed, however, with three tracks to accommodate
the expected increases in rail traffic. The 24-foot-6-inch overhead
clearance allows railroad signals to be suspended from an overhead
unistrut. Also, adequate clearance is provided in the event the
line is electrified in the future.
Annual traffic volumes may exceed 150 mgt per year, thereby limiting
the opportunity for track maintenance and requiring careful selection
of track components. Special studies of the track section were initiated
due to concern over early degradation of the ballast and the track
components. These studies investigated optional materials and ballast
depths under a stiff track modulus environment. The concrete trench
floor slab supports trackwork that is laid on an 18-inch ballast
bed with concrete ties spaced at 20 inches on center with 136-pound
continuous welded rail on fasteners with rubber rail pads. Ideally,
the track modulus should not exceed 10,000 psi. To achieve this,
however, the design engineers considered adjustments to the pads,
ballast depth, ballast material, and other components of the track
structure. Studies to determine the actual track stiffness are ongoing.
The results will undoubtedly benefit future trench projects.
Alameda designers chose several contiguous bored pile configurations
based on the conditions of the site. Typically, 3-foot diameter
piles spaced 4 feet apart were bored to a nominal depth of 14 feet
below the proposed trench invert. The piles were cast in the drill
hole leaving a 1-foot soil face between the piles. In some situations,
4-foot-diameter piles spaced 5 feet apart were constructed based
on design loads. This type of wall construction was the predominant
method on two-thirds of the 10-mile trench. The remaining one-third
of the trench was composed of a secant pile wall due to high groundwater.
Secant pile walls were spaced 0.8 to 0.9 pile diameters apart. Primary
piles are secanted by secondary piles, thus providing a closed structure
to act as a barrier where the groundwater table is high.
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Placement of the sidewalls was performed in sections.
| When the contiguous walls were completed on each side of the 51-foot-wide
trench, the trench was partially excavated to 16 feet below grade
to allow for construction of a heavily reinforced concrete cap beam.
The cap beam essentially locks the piles together and was constructed
to accept precast octagonal concrete struts that span the trench.
The struts are spaced 25 feet on center. Following the installation
of the struts, the trench was excavated to design depth. During
excavation the deflection of the wall was monitored. A concrete
bottom slab ranging from 1 to 4 feet thick was then cast in place.
The variation in thickness was necessary to counteract hydrostatic
uplift. Next, 6,000-psi shotcrete, 6 inches thick, was gunnited
onto the pile wall and trowelled to provide a smooth face. The shotcrete
was reinforced with welded wire fabric and attached to the piles
with “L” dowels, previously set in drilled and epoxy-filled
holes.
At intersecting cross streets spanning the trench, the piles were
terminated approximately 24 feet below invert to achieve greater
bearing capacity. Here, 14-inch precast slabs set on elastomeric
pads were placed on cross girders to form a roadway bridge deck.
A 5-inch-thick finished deck was then poured on the precast slabs.
Because the invert grade at the ends of the trench slope up to
the ground surface, the use of struts across the trench was not
possible. Therefore, cantilever walls were constructed at each end
of the trench. In all cases, deflections were calculated not to
exceed 1 inch.
Project Credits
Owner: Alameda Corridor Transportation Authority
Design-Build Team: Tutor-Saliba Corp., O&G Industries Inc.,
HNTB Design-Build Inc., and Parsons Transportation Group
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