Seismic Shoring for Eastern Span Replacement of SF‑Oakland Bay Bridge

Joe Lazzaretti
Aug 12
7 min read

Updated: Sep 2

The Eastern Side of the SF-Oakland Bay Bridge was made new to stand up to earthquakes. It used top short-term support plans to keep it safe while being built.

This work began after the 1989 Loma Prieta earthquake. It showed weak spots in the old bridge. The build started in the early 2000s and took more than ten years. Engineers had to deal with risks from quakes, soft ground, and sea conditions. They needed special ways to fix each step. Main points are:

Short-term Holds: Steel pipes, walls in the water, and press systems kept the bridge up during the build.
Quake Safety: Support systems were made to take on shake forces, holding up in shakes that might come.
Sea Issues: Soft sea mud and water movements led to the need for special bottoms and stuff.
Move Weight: They carefully moved the weight from short holds to the final build. They checked this with real-time tech and press changes.

This work shows that with good plan and build skills, you can tackle big earth and shake risks when you build.

TYPICAL ADVANCE SHORING BRIDGE CONSTRUCTION, (MSS)

Earth Conditions and Design Needs

The changing ground under the bridge's eastern span made it hard for engineers when making shoring plans. These special site needs called for made-to-fit designs, which were key in shaping the engineering fixes used in later project steps.

Ground and Rock Types

Near Yerba Buena Island, at the west end, the base sits on solid shale bedrock, giving a strong spot for lasting builds. Moving east, in the Skyway Viaduct area, the base shifts to softer stuff. Here, pressed mud is below loose layers, and it joins into the Alameda Formation, made of sand, mud, and gravel. This mix in ground types meant using one shoring way for all wasn't possible. Each spot needed its own methods suited to its ground.

Earthquake and Sea Design Points

In a place known for shakes, the shoring had to hold up against quake hits. Engineers made exact load paths and kept the joins tight to stay firm even when moved a lot. Also, the sea setting brought its own hard points, like how water and other things hit the base. By looking at both quake and sea parts, engineers made short-term shoring that could deal with the hard build needs.

Short-Term Shoring Systems and Building Ways

While changing the Eastern Span, a well-set plan of short-term helps was used to take on the tough job of building on water in a quake area. These setups were made to hold up heavy loads and keep safe in the hard Bay Area. Here is more on the systems used and how they were made to fit quake needs, linking building ways to goals.

Short-Term Towers and Help Structures

To make dry work spots in around 20 feet of water, experts put in 20 short-term sheet-pile cofferdams. These steel walls let teams dig as deep as 50 feet below the mud in soft Bay Mud. Each cofferdam had lots of side braces to deal with the big push from soil and water.

For more help, 60 steel-pipe piles - each 8 feet wide - were set to hold the main pile boxes while being put in. These piles also held up two work bridges, 14 tower cranes, and 28 pile-driving shapes during key parts of the work. The short-term piles had a main role in holding up the load of the main parts while the concrete dried well [1].

Quake Design of Short-Term Systems

In a place with lots of quake action, the short-term shoring setups were made bold. Experts looked at outside forces - including short-term loads, main loads, and quake pressures - to make sure each part could meet tight safety and use goals during a quake [2][3].

Marine Base Answers

Beyond quake issues, the water setting brought more tough things, mainly because of the soft Bay Mud. The sheet-pile cofferdams were key to making dry work spots, letting teams work on the bases in these hard spots. To keep stable, the steel sheet piles were driven into harder soil under the mud, giving the needed help.

For the two short-term end tower setups, experts looked at how to drive and place the piles. Getting the right depths in the soft soils without losing load hold was not easy. Experts had to keep a balance in the size of the piles, the ways used to drive them, and the soil states to make sure the setups were stable [1].

Building Phases and Load Move Steps

Using the strong-building steps we talked about before, the new Eastern Span needed a well-planned build process to move big weights from short-term holds to the final bridge. This tricky job was done bit by bit, making sure the structure was firm all the way.

Build Order and Short-term Hold

To hold up the bridge span build, short-term hold systems were set up near the tower base. Big cranes, using a steel-pipe pile setup, put box beam bits during the build steps.

At first, short-term piles held the weight of the bridge top bits as they were put and tied together. This went on until the main tower was tall enough for putting up cables. Putting each bit just right was key to making sure the final bridge would be put together well.

Putting up cables brought its own hard parts, mainly in handling the move of weights. As cables were pulled tight, engineers kept a close watch on how the weight moved between the short-term holds and the growing final structure. Watching the stress in real time helped stop any risk of too much weight.

Given how key it is to keep things stable, work during big phases was done all day to cut down risks from shakes. The water setting added more complex parts, as tide shifts changed the short-term holds. To fight these changes, hydraulic lift systems were put at main tie spots.

When the short-term hold system was all set, engineers started the careful job of moving weights to the final structure.

Moving Load to Final Structure

The load move happened in well-thought-out steps, slowly moving the weight from short-term holds to the final cable system. Engineers used computer-run hydraulic systems to change weights bit by bit, lessening pull on the short-term holds while adding it to the final structure.

A very key step was moving the weight from the middle span part. This needed careful watching and tiny changes to keep the process safe. Stress meters put on both short-term and final parts gave real-time data on how the weight was spread. Also, heat change plans changed the hydraulic systems on their own to deal with swelling and shrinking, making sure cable pulls stayed the same.

In the last phase, short-term holds were taken away in the back way they were put in. Each part was tested for weight before taking it down to make sure the final structure was fully bearing its planned weight. Constant sensor watching was used to check the bridge's long-term firmness and staying power.

Project Lessons and Engineering Insights

The construction of the Eastern Span replacement project brought forward essential lessons in seismic shoring. These takeaways continue to shape engineering strategies for handling seismic challenges in marine environments, offering a roadmap for future infrastructure projects grappling with complex geological and environmental factors.

Shoring Solutions for Variable Soil Conditions

The bay floor's diverse soil composition presented unique challenges. Engineers had to move beyond conventional piling methods, opting for alternative foundation systems that could distribute loads more effectively. This approach required designs that allowed controlled movement while maintaining structural stability, ensuring the project could adapt to the unpredictable soil conditions.

Seismic and Marine Construction Requirements

Combining seismic safety with marine construction demands proved to be a significant hurdle. Engineers had to rethink connection details and incorporate energy dissipation features to meet seismic standards. The marine environment added another layer of complexity, with accelerated corrosion necessitating frequent inspections and the use of specialized protective coatings. Environmental regulations also played a critical role, influencing material choices and maintenance protocols to meet sustainability and protection standards.

Project Coordination and Planning

The project's success relied heavily on meticulous coordination. Regular stakeholder meetings were essential to align temporary shoring efforts with the needs of the permanent structure. Real-time communication between engineers, equipment operators, and marine traffic controllers ensured smooth operations, even in challenging conditions. Redundant monitoring systems provided a safety net when primary systems faltered, and detailed emergency response plans allowed for quick action during unexpected seismic events. Additionally, contractor selection increasingly prioritized firms with expertise in both seismic and marine construction, setting a new benchmark for similar projects in the future. These coordination efforts have established a strong foundation for managing seismic shoring in marine environments going forward.

Ending Thoughts

The Eastern Span fix shows us how hard it is to meet the needs of building work while using top-notch tech to make things safe from quakes. This work shows that using short-term support and being very careful with planning and safety can make building in water safer and better ready for quakes.

Key points like good plans, working together well, and keeping an eye on weight were crucial to keep the structure strong and not hurt the world around it while building. These ideas give both engineers and builders good tips for handling such hard jobs later.

FAQs

How did the land and rules shape the build of the East part of the SF-Oakland Bay Bridge?

Land and rules were big in deciding how the East part of the SF-Oakland Bay Bridge was made. The sea and care for fish homes and sea animals led to deep plans. For example, work in the water followed hard rules, like making safe spots for sea life and using ways that cut down sound and shakes when pushing poles into the ground.

Along with sea care, rules made them pick building ways that matched with U.S. and state land laws. They used earth-friendly ways to hold up land and limited work in the water to keep the land healthy. These moves made sure the work hit its build goals while still caring for nature.