When it opened to traffic on Feb. 29, 2016, drivers quickly realized that a new interchange in Antioch, Calif., made their commute smoother and more direct. What wasn’t so obvious was the effort that went into earthquake-proofing the connection between State Route 160, the Antioch Bridge, and Highway 4.
That insurance was largely made possible by a construction and design team that equipped the bridges with corrugated steel isolation casings. These casings essentially allow columns of all lengths to move uniformly and prevent stress concentrations in those that are shorter.
Formally known as State Route 160/Highway 4 Direct Connector Ramps, the $50 million project was a joint effort of the Contra Costa Transportation Authority (CCTA), Metropolitan Transportation Commission (MTC)/Bay Area Toll Authority (BATA) and administered by the 20-employee CCTA. Begun on March 24, 2014, the elevated interchange transfers 10,000 daily commuters from gridlocked City of Oakley streets to free-flowing ramps on Highway 4 and SR 160.
“Drivers accessing the Antioch Bridge had previously been forced to follow a U-turn that dumped them onto the congested city streets,” says CCTA Construction Manager Ivan Ramirez. “The new connector ramps allow them to remain on the freeway.”
Regional capacity expansion
The connector ramps are part of a colossal $1.3 billion undertaking, launched in 2010 by CCTA and partners, to widen the Highway 4 corridor between Pittsburg and Antioch in eastern Contra Costa County and extend a Bay Area Rapid Transit (BART) line to Antioch.
“CCTA is making a major investment in this part of the county because that’s where most of the population is going,” says Ramirez. “This project complements all our other investments on Highway 4.”
Two connector ramps link westbound Highway 4 to northbound SR 160 and southbound SR 160 to eastbound Highway 4. A 12-foot auxiliary lane to SR 160 in both directions between the State Route 160/Highway 4 Interchange and the East 18th/Main Street Interchange was also built. The new structure crosses the Highway 4 median at a height that can accommodate a future BART extension.
Isolation casings earthquake-proof bridges
The term “isolation casing” may be less familiar to civil engineers elsewhere in the U.S., but it’s common on the West Coast due to the region’s seismic activity. Essentially, the casings are used to allow columns to be the same length regardless of the terrain.
Maintaining uniform column length in areas with varying topography is critical. “If there’s a seismic event, the bridge is going to tend to move,” Ramirez explains. “The isolation casings allow the bridge to move uniformly.”
In contrast, traditional bridge design focuses on making columns efficient by allowing the contour of the ground to dictate their length. During an earthquake, the shorter columns would cease to move, absorbing the bulk of the seismic load and either collapsing or causing irreparable damage.
The isolation casings were installed on all three new bridges. Two straddle an active railroad line and another a flyover ramp traversing SR 160 and Highway 4.
The interchange design was provided by Rajappan & Meyer Consulting Engineers Inc. with Geocon Inc. working as a subconsultant to engineer the foundations.
Change order saves time, money
The construction team initially planned to use temporary casings to drill for the piles and columns. But temporary casings often get stuck and are left in place, wasting material and labor. There are also times when the design dictates that the casings remain in place.
Malcolm Drilling wanted to increase the diameter of the temporary isolation casing to support the load imposed by its heavy drilling equipment. The contractor submitted a change order requesting a switch from a 13-foot-diameter permanent to 15-foot-diameter temporary casing that would essentially become the permanent casing.
CCTA approved the change order, realizing substantial savings that was split between the contractors and the agency.
Pipe supplier Pacific Corrugated Pipe Co. asserts: “This project demonstrated that corrugated steel pipe provides flexibility by varying profile and gauge combinations to meet site-specific project requirements, which included providing the necessary resistance to the soil and equipment load pressures. Selecting corrugated steel pipe in lieu of the thicker and more commonly used non-corrugated steel shortened lead times and allowed CCTA to meet its critical timing requirements.”
Handling massive pipe sections
While corrugated steel is considered lightweight and easy to handle, the sheer size of the pipe presented unique challenges. The diameters, 9 feet and 14 feet, were so large the pipe barely cleared the doorway at Pacific’s fabricating plant in Antioch.
The company used up to three forklifts to shuttle the pieces about the plant. The sections were then loaded onto a stretch step-deck trailer, hauled to the interchange, and offloaded by crane.
Ultimately, the company delivered 430 feet of 168-inch, 8-gauge, 26- to 66-foot sections and 200 feet of 108-inch, 10-gauge, 19- to 46-foot sections.
Meanwhile, Malcolm drilled into bedrock to prepare for the casings while workers fabricated and spliced rebar cages up to 70 feet long onsite.
Two cranes installed the cages, which served as a form for the 13-foot-diameter piles. The hole was then backfilled and slurry applied to the outside of the pipe to make it a permanent part of the foundation.
The next step was to set the isolation casings atop the piles. A smaller-diameter column was then inserted into the 15-foot-diameter casing. The void between the casing and the column allows the column to move during a seismic event without resistance from the soil.
The project was delivered on time and $1 million under budget. Ramirez describes CCTA as a partnering agency and believes this has a lot to do with the success of this project.
“We partner with strong applicants, we select construction management firms that partner, and we engage the contractors,” he says. “This is a great project and we love to showcase it.”
Brian M. Fraley is founder of Fraley AEC Solutions LLC, a marketing communications firm for the architecture, engineering, and construction (AEC) industry based in Pennsylvania. Visit fraleysolutions.com; e-mail [email protected].
