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The Mukilteo Ferry Terminal Super Column - Construction Snapshot

Photo credit: Monica Blanchard

As part of the Mukilteo Ferry Terminal (Phase 2) Construction project, Manson’s Mukilteo project crew recently completed construction of an overhead loading structure utilizing a “super column” to accommodate pedestrian boarding. The overhead loading structure (OHL)is a unique design that changes elevation to accommodate varying sizes of ferries docking throughout the entire tide cycle. This movement is accomplished through the use of a large hydraulic cylinder. Manson installed what the project crew nicknamed the “super column” to support the hydraulic cylinder and structure. [See illustration below for a depiction of the “super column” which includes a drilled shaft and large steel structures.

To start, the crew built a work platform able to withstand loads from the chosen drill rig using a vibratory hammer to drive 24 in. diameter falsework piles. After installing the piles, the main structure was picked, set, and pinned to the piles. Some of the challenges encountered during the platform design were limitations on allowable temporary pile locations and available sizes and lengths of falsework beams. The team constructed the main platform structure prior to mobilizing it to the site, thereby minimizing time spent on the job site.

An opening was left in the platform to act as a template for the casing install and a vibratory hammer was used to drive a 10 ft.-6 in. diameter casing down to grade. Once the casing was installed, Manson flew — slang for hoisting with a crane — the drill rig onto the platform and Manson’s subcontractor, Condon-Johnson, began drilling. Manson supported Condon-Johnson throughout the drilling with hook support and disposal of spoils and slurry. The shaft had to be drilled 16 ft. below the tip of the casing. The slurry elevation in the casing was maintained at a minimum of 10 ft. above the outside water level creating enough head pressure to keep the hole open until the concrete was poured.

Once the driller reached the final depth, the rebar cage was tripped and set in the hole. To trip the cage, the project team engineered pick points located where the cage was reinforced with additional bracing to prevent buckling. Three pick point locations were needed for the 115-ft. long cage and multiple rolling blocks were used to allow one hook to grab the cage in these three locations. Another hook, designed to withstand the entire cage weight when vertical, grabbed a support at the top of the cage. The flexibility of the cage makes this a unique and challenging lift. The project team studied the loads in depth to understand how the cage would want to react before starting the lift, and the rebar cage was tripped the night before to ensure the pour tasks could be completed in only one day.

To prevent creating abnormalities in the concrete, the next steps had to be completed continuously in one day. First, the drillers cleaned out the hole one final time to remove any sediment that settled out of the slurry overnight. Next, the cage was set in the casing to proper elevation and the pour began. The concrete was then deposited at the bottom of the hole using a tremie, essentially a watertight pipe with a conical hopper used for underwater concrete placement. Slurry was removed from the top as the concrete displaced it. It was important to keep the tremie submerged in concrete until the very end to prevent voids, or cavities, and minimize the amount of bad concrete at the top of the shaft.

Once the concrete cured, the shaft concrete was tested for voids by using a test known as crosshole sonic logging. This test uses an ultrasonic signal to detect possible defects in the concrete.

The project team made a secondary pour up to the required elevation after chipping the top until sound concrete was reached. Next, the column was poured and a steel cylinder seat was grouted on top of the column. Following this, a 6 ft.-7.5 in. interior casing was set on top of the secondary concrete and the annulus space between the interior and exterior casing was filled with concrete.

All this work was done 40 ft. below the top of the casing in a confined space. Access was a challenge, since every added piece reduced the amount of remaining space to work. By managing the work sequence and designing varying access options for different stages and elevations, the team was able to successfully install each piece. The tolerances on the “super column” are extremely tight in order to make sure all parts are functioning as designed. It was a challenge to verify that all tolerances were being met at each step of the install. Large steel pieces were used in the design in order to attach the overhead loading cab to the foundation structure. The steel pieces were machined and fit tested to confirm proper fabrication prior to being shipped to the site.

Once onsite, the two-piece bearing housing was shimmed off of the concrete that was poured between the casings. The gap between the bearing housing and inner casing was grouted and the lift shaft set inside the bearing housing. Surveying was often used to check that all tolerances were being met and temporary supports were used to keep pieces in their proper positions prior to the final installation. Once the lift shaft was installed, the “super column” was complete and the project crew was able to move on to completing the overhead loading structure.

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