By: Larry Trojak, Contributing Author
Like many major bridges in the San Francisco Bay area--the
San Francisco-Oakland Bay Bridge, the Benicia-Martinez, the Richmond-San
Rafael, the San Mateo-Hayward and the Carquinez Strait--the Golden Gate is
undergoing a three-phase renovation to meet seismic requirements. In the case
of the Golden Gate, that upgrade will allow it to retain its structural
integrity through a quake measuring 8.3 on the Richter scale--up more than a
full point from existing limits. For Drill Tech Drilling & Shoring Inc.,
Antioch, Calif., drilling subcontractor to the joint venture general contractor
Shimmick-Obayashi, that meant having to deal with low overhead clearances
during its retrofit of the south approach--a challenge that was met through the
use of a drilling unit made for just such situations.
A return to form
The current phase of the overall retrofit, Phase 2, includes
the south viaduct and anchorage housing, Fort Point arch and south pylons. On
the south viaduct, foundation footings are being increased in size by about
50%, tied to the existing footing and anchored into bedrock using 136 tiedown
anchors with design loads ranging from 308 to 1,970 kips.
After the Loma Prieta quake in 1989, engineers tested a
number of foundation systems for seismic retrofit and found that the most
effective way to handle a significant quake's effects is to immediately bring a
bridge back into its proper position. Without it, traditional structural
designs tend to rock upward in a quake and stay in that position. This method
was deemed to be far better. In addition, it was discovered that the anchors
also serve to act as a dampening agent, thereby providing an additional
benefit.
The tiedown anchors consist of a number of 0.6-in.-diam.
epoxy-coated 270 ksi steel strands bundled to provide the required load
capacity. The tiedowns up to 97 ft deep are grouted into holes drilled into the
bedrock. The upper 35 ft of each epoxy-coated strand is sheathed and greased to
allow this unbonded portion of the strand to stretch when seismic activity
applies loads in excess of the 1,200 kip lock-off load, thereby dampening the
pylon foundation's rocking motion.
Drilling in tight spaces
Equally important in the Phase 2 retrofit was the need to
enlarge the footings at the abutment and four bent locations to support the
temporary bridge supports while the permanent steel bents are replaced under
the approach spans. According to Drill Tech Project Manager Ian Stait-Gardener,
the location of the CIDH piles for the bent's footings presented the major
challenge.
"The Golden Gate Bridge, Highway and Transportation
District will invest approximately $400,000 to repair corrosion damage,
strengthen and preserve this national landmark by retrofitting the existing
steel structures, pylons and footings," he said. "To do this, and
maintain the specs that say traffic on the bridge cannot be disrupted,
temporary towers are being constructed on the enlarged footings on either side
of existing bents. Once the temporary towers are in place supporting the
bridge, the original steel bents will be replaced. Seismic isolation bearings
located at the top will serve to reduce the transfer of forces during seismic
activity."
At the temporary tower locations CIDH piles were required to
transfer the load on the enlarged footings through the backfill of previous
excavations to the bedrock. So, in addition to simply enlarging the existing
bent's footing, compression piles had to be drilled alongside the existing
footings with limited overhead clearance under the bridge.
"The majority of the Golden Gate sits on a substrate of
serpentine bedrock," said Drill Tech's Shannon Creson. "However, that
layer of fill atop the bedrock prompted the need to drill 44 CIDH piles into
the newly-expanded foundations all with a limited amount of overhead room--as
little as 24 ft in some areas.
Unique situations, special solutions
The equipment to which Creson specifically refers is a
LoDril Model LLMHT "Fat Boy," manufactured by Bay Shore Systems Inc.,
Rathdrum, Idaho, an excavator-mounted drill that delivers 36,000 ft-lb of torque.
Despite an attachment height of 19 ft, the tool is still capable of drilling to
depths of up to 80 ft. "There's no denying the drill's low profile allowed
us to keep this job on track," he said. "Using this type of drill,
mounted on a Hitachi 300 excavator, we were able to average three 24-in.-diam.
CIDH piles a day, at depths of 30 to 48 ft. And that included drilling,
splicing the cages, placing them and pouring the concrete. "
Creson said another feature of the drill came into play as
the project progressed. "Bay Shore recently introduced a Round
Auto-Locking Kelly Bar feature for its line of LoDrils and we immediately saw
the benefit it could provide us in additional down-crowd pressure at the
bridge. With the addition of the round locking kelly bars, I'd say our down
pressure was increased from 10,000 lb to about 30,000 lb and, because it is all
automatic, there was no need to insert or remove any pins as is often the case
with traditional square kelly bars."
Creson said that at the abutment the overhead clearance was
reduced to 11 ft. For that section, Drill Tech is using a unit they
custom-fabricated for onsite use.
Phase 3 awaits
Construction costs for Phase 2 of the Golden Gate retrofit
are estimated to be about $170 million with the total bridge rehab weighing in
at about $388 million. While that price tag seems hefty, it pales in comparison
to the $2.1 billion estimate for totally rebuilding the famed landmark.
Contracts for Phase 3, which will involve work on the suspension span, north anchorage
and main towers, have not yet been released for bid.
