Drilling inside a mouth

April 2, 2018
Tight quarters calls for low-clearance drilling unit on Golden Gate Bridge

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.

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