Building the St. Croix Crossing: Everything Under the Water

Building the St. Croix Crossing: Everything Under the Water

The St. Croix River Valley is experiencing
a long-awaited addition with construction of the new St. Croix Crossing. The bridge
will connect St. Joseph, Wisconsin, and Oak Park Heights, Minnesota, and replaces the
Stillwater Lift bridge. Construction began in 2013 and the bridge will open to traffic
in fall 2016. The bridge’s foundation include an element called caissons and these caissons extend up to
140 feet below the water. As of today, all bridge work below the water is complete. The early foundation contractor used a precast coffer dam system that is the first time we’ve used this system in Minnesota. To get the bridge foundations started, crews
floated in a pre-fabricated steel framing system and then poured a concrete deck topping
that served as a work platform. The platform included four large circular holes. These
holes served as a guide for placing the casings which are hollow and made of steel. There
are four caissons per pier column and ten pier columns in the river. The casings which is one part of a caisson–
were placed through the platform openings, extended into the water and muck down to the
bedrock beneath the river bottom. Before reaching the bedrock, the casings passed
through up to 25 feet of water, up to 87 feet of muck, 2 feet of sand and gravel and 2 feet
of soft sandstone. The next step was removing all of the muck
and gravel from inside these 40 steel casings that extend to bedrock. Crews used many tools
to excavate the material, but the water remained inside each casing during this work. Still, the foundation of the bridge needed
to extend deeper into the rock. Crews drilled an additional 25 feet into the bedrock to
secure each foundation. They used massive drill bits that measured up to 7.5 feet in
diameter to break up and remove the rock. The process removed 2,500 dump truck loads
of sand, muck and gravel that went to a landfill. The caissons are roughly 125 feet below the footing. So about 150 feet below the water. The piers themselves are a little bit taller than that. On the Minnesota approach about 100 feet to the bridge deck plus 67 up to the top of the towers. On the Wisconsin side its a little bit higher than that, where it’s about 140-160 feet to the deck level with 67 feet of tower above that. Once these large pier casings were clear of
all materials, crews installed reinforcement bar cages and pumped concrete to create solid
foundations for the new bridge. The next step was to build a footing above
the caissons. The footing is the transition component from the caissons to the pier column. In order to safely move forward, crews needed
to dry out the work area and remove all the water. To do this, they installed a box-like
structure around the pre-fabricated deck. They then lowered this structure until it
reached the river bottom. Once in place, the structure is considered a cofferdam.
What’s really interesting about this step is that the pre-fabricated deck that crews
used as a work surface up to this point became the base to build the footing at the river
bottom. Next, a three and a half to four foot thick
layer of concrete is poured underwater. This is called a seal. Once the concrete seal hardened,
crews pumped out the water from inside the cofferdam to have a dry working area. The removed water was filtered and returned
back to the St. Croix. The dried work area allows crews to construct
a reinforced concrete footing on top of the concrete seal. There are 10 footings in total,
or two at each of the five river piers. Crews then constructed a column above each
footing in the river. Together, the ten columns will support 400,000 tons of weight. With the pier columns now above the river
surface, crews removed the cofferdam walls, allowing the water to surround each column. Those underwater foundations will certainly last the expected 100 year duration or the bridge and quite possibly well beyond that. Each of the 10 columns stood about 16 feet above the water’s surface at the end of 2013 A lot of the work done in December of 2013 was in extremely cold temperatures. Many days were below zero. The river froze up. The river froze up. The river is not a very fast flowing river–it’s more like a lake–so the ice was forming quite rapidly. That marked the completion of the work below
the water. Next, crews will work to extend the pier columns up to their full height which will mark the first time the public can see the bridge take shape.

6 Replies to “Building the St. Croix Crossing: Everything Under the Water”

  1. Brilliant!  We have a boat at Sunnyside and have been watching the construction, this is the first time we actually knew what was going on.  Keep it coming.

  2. I suppose if your looking east at the Wisconsin shoreline, the bridge would seem like a blight. However…..if you are looking west to the Minnesota side of the river….the whole panoramic is a blight to behold.

  3. After years and years and years of delays it is great to see this much needed infrastructure FINALLY moving forward……

  4. The first time I went over that bridge, I nearly had an anxiety attack, because I have s fear of heights. 😞I was coming from Wisconsin headed toward Oak Park Heights, and all of a sudden this big white beast with cables snuck up on me. I was like,what the heck is this!!!! Then I had no clue what lane I was suppose to be in and what I kept saying, “I just want to go to Walmart!!”
    I made it, but then I soon realized,I have to cross the bridge again to get home. 😳😳 but I made it, barely.

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