| Better Bridges
CalTrans Rebuilds Sacramento River
Bridge
Partners go fishing for ways to protect the
environment
and come up with a better bridge design.
by Karen
Stidger, Contributing Editor
 In the northern-most reaches of the central valley of California, less
than 100 miles south of the Oregon border, the California Department of
Transportation is reconstructing a 600-foot bridge on Interstate 5 which
crosses the picturesque Sacramento River. A little more than a third
completed, the three-year, $16.1-million CalTrans project boasts a
modified California bulb-tee design, the first of its kind to be used in
California. The unique design, consisting of sectional precast girders
made continuous, is the result of addressing a litany of construction and
environmental constraints.
The original parallel five-span hybrid steel girder bridge, built near
the City of Anderson in the early 1960s as part of the completion of I-5
from Mexico to Canada, suffered from fatigue and critical scour. CalTrans
knew replacing this bridge would be a difficult and lengthy task. A joint
constructability review process was used to troubleshoot and discuss all
foreseeable issues prior to design and work.
After two years of design planning and the bidding process, on February
20, 2001, CalTrans, District 2, awarded a three-year, three-stage contract
to Shasta Constructors, Inc. of Redding, California.
Bridging the challenges
 River protection. Minimizing the footprint to the pristine upper
Sacramento River is key. This effort includes protecting the fisheries
along with migratory birds that nest on the sides of the bridge, as well
as maintaining clean water in accordance with The Clean Water Act.
The largest river in California, the Sacramento River runs through
Anderson on its 381-mile serpentine path through the central valley on its
way to the San Francisco Bay. Anderson is home to Coleman National Fish
Hatchery, the largest salmon hatchery in the lower 48 states. Large
numbers of fish are released into the rivers in the Anderson area to
eventually return for spawning.
Seasonal runs of Chinook and Steelhead drive the bridge construction
schedule, limiting in-river time to a window of May 1 through October 15
each year. In addition, precautionary methods used during demolition
prevent debris from falling down into the river.
In spring, great numbers of swallows build their mud nests on the sides
of the bridge where the river provides a diet of flying insects. To avoid
interrupting the swallows’ mating season, nets are hung on the sides of
the bridge, forcing swallows to seek other nesting locations before nests
are built.
Because of sediment and alkaline by-products from concrete, water from
cofferdams cannot be redeposited directly into the river. Instead, during
the dewatering process, water is pumped into desilting basins where it is
cleaned and neutralized before being returned to the river.
River access. Starting May 1, an access pad of spawning gravel is built
halfway across the river. By October 15, phase work must be completed and
the gravel removed from the river. At the end of the project, fishery
overseers have requested that 58 cubic meters of spawning gravel be left
behind in the river. Over time, the water currents will wash the gravel
down the river to create natural spawning areas.
Bridge load. According to Mark Darnall, resident engineer for CalTrans,
“Structural capacity and concerns about the existing bridge placed
constraints on both design and physical placement of girders on the new
bridge.” Each individual span has three girder segments, put into place
by 100-ton cranes.
Highway traffic. Throughout the project, CalTrans maintains traffic
flow at normal capacities without stoppages. However, certain steps, such
as girder erection and pile driving require greater construction area
access, making traffic diversion necessary. During these construction
phases, Shasta performs work at night when traffic flow is lightest and
when limiting traffic to one lane each way causes the least amount of
disruption to drivers.
Noise pollution. The night work of pile driving creates extra noise
pollution for the surrounding residential areas. In each stage of
construction when noise levels are highest, Shasta notifies local
residents and offers them substitute lodging at local motels. Only a small
number of residents found it necessary to accept the contractor’s offer.
River traffic. In addition to maintaining normal highway traffic, river
access for law enforcement, fishermen, and rafters is preserved by
limiting the size of the gravel access pad in the water.
The replacement structure
Darnall reports the new bridge includes the following elements:
1. Pre-cast, pre-stressed Modified California bulb tee girders.
2. Segmental erection of four 47-meter spans, made continuous by
post-tensioning.
3. 156 segments — three segments per span per girder.
4. High-strength concrete, 8,700 psi. The project requires a pumpable,
flowing, high early strength mix using an M.B. Rheobuild 300 high-range
water reducer.
5. Eight-foot steel pipe foundation, driven below the floor of 30-foot
cofferdams.
6. Two-stage longitudinal post-tensioning, before and after deck
placement.
7. Transverse post-tensioning at pier diaphragms before deck concrete
pour.
8. Three piers instead of four, providing less obstruction in the
water.
Three-stage construction
 Each of the three stages in this project builds a double-lane bridge.
Stage one, started in April 2001, called for the construction of a
two-lane highway between the existing parallel bridges. The stage-one
highway provides lanes for traffic diversion during the subsequent two
stages. In the event of future highway expansion, the final bridge makes
six lanes available.
On May 1, 2001, Shasta Constructors started work in the river with the
building of the gravel access pad. From the completed access pad,
construction of the substructure and temporary girder supports was carried
out. Crews dug 30-foot cofferdams and drove piles into the riverbed.
Shasta Job Superintendent Randy Smith recalls the challenge of digging
through volcanic tuff with a long-armed excavator with pointed teeth.
Next, Tremie Seal was pumped to the bottom of the cofferdams, water was
pumped from the coffers to the desilting basin, and crews trimmed the
piles.
Once 100-ton cranes placed the concrete girder segments on the complete
piers, the closures and diaphragms were poured. A local cabinet shop
fabricated custom forms for the closures.
Longitudinal and transverse stressing made the girders self-supporting
so that the falsework and access pad could be removed from the river by
the October 15 deadline. When one deadline extension was required, the
crews found themselves working in high water as river levels were raised
to accommodate fall salmon runs.
Finally, the deck was poured and the final longitudinal path stressed.
 The second stage replaces the northbound bridge while diverting
northbound traffic to the stage-one lanes. During the demolition, the
contract requires a registered engineer to be on site at all times. Shasta
brought in Robert D. Prahl, retired CalTrans senior bridge engineer, as a
consultant. When the deck was removed to expose the 40-year-old girders,
Prahl said, “I was surprised the steel girders rebounded to their
original camber. Even the paint was still pretty much intact.”
In 1962, Prahl was CalTrans assistant bridge engineer on the original
Sacramento River Bridge project. The $5-million project included 24 bridge
structures and the sub base for the I-5 roadway covering the 13-mile
stretch between the cities of Anderson and Redding. Prahl retired from
CalTrans as senior bridge engineer in 1998.
Stage three, scheduled to begin May 1, 2003, will replace the
southbound bridge, switching southbound traffic to the stage-one lanes.
Finally, all three stages will be transverse tensioned together and the
railing and roadwork will be completed.
 The estimated completion date for the Sacramento River Bridge is August
2004. In the meantime, thousands of truck drivers, motorists, and
fishermen will hardly have noticed all the work, and the needs and
concerns of a long list of partners will have been met.
Darnall, who values the constructability review process highly,
comments, “Open communication with all parties involved helps to ensure
a successful project.”
Reprinted from Better Roads Magazine
August 2002 |
