There's engineering, and then there's ENGINEERING. There's construction, and then there's CONSTRUCTION. Much of that engineering and construction was implied as the piers of the new Stillwater bridge were being built in the river. We poked around them a few times last summer. Barges and casings surrounded much of the underwater work, so you got an idea of the size of the job, but not the sophistication. Hidden was some of the scope and new techniques being employed to build the foundations of the structure that is intended to serve more than 100 years into the future.
If you're interested in the details of design and construction, the DOT has an interesting site that explains much at: http://www.dot.state.mn.us/stcroixcrossing/design.html
The ENGINEERING and CONSTRUCTION is obvious, however, when you look at the design that utilizes pre-cast segments that will be fitted together to form the bridge deck. Those segments will be assembled much like a giant jig-saw puzzle when they're hoisted high in the air to the top of their very tall piers. The smaller versions that will make up the on and off ramps from the riverside are being made in a casting yard located at the bridge site. The bigger versions are being made in a yard built directly next to Holcim's sand and gravel mine on Grey Cloud Island on the Mississippi River outside of Saint Paul Park. All of them will start moving into position soon as the next major step in the bridge making process.
Mike McCormick, husband of Bonnie's riding friend, Colleen McCormick, works for Aggregate Industries, a local division of Holcim, the global manufacturer of cement and construction minerals headquartered in Switzerland. He used his unique access to give us a tour of their sand and gravel mining operation. As a bonus, we also had the chance to step aboard the tow boat that ply's the Mississippi with sand and gravel barges from the mine to distribution yards upriver.
As we approached the mine, we saw a huge warehouse-looking building off to the right. It was constructed exclusively by the bridge construction company on Holmium's site to serve as the casting house for the segments. Sand and gravel moves directly from the dredging equiment on a conveyor to the ready mix plant located next to the casting house in an integrated manufacturing process. Many of the castings had already been made and were stored, in order of their installation, in the yard. Each one-of-a-kind segment is cast in the order they will be placed on the bridge, with a size of around 48 feet wide by 18 feet tall by 10 feet deep and weighing about 180 tons.
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| Tubes Along the Top Hold Steel Cables |
They each have a unique, form-fitting pattern cast into the two faces of the casting, with plastic-lined holes all along the top (covered in red tape in this photo). Those holes make way for the interconnected steel cables that will reinforce the fit under tension, anchoring the bridge all the way back to the pier.
The challenge lies in the fact that Grey Cloud Island is about 35 miles from Stillwater, with no feasible means of transporting them by land. Instead, the 650 segments will be loaded onto a barge at about 10 at a time, and pushed down the Mississippi to Hastings, Mn, and then up the St. Croix river to the bridge site. Thats a lot of movement through the narrow passages of Kinnickinnic State Park and Catfish Sand Bar just outside of Afton, and through all that recreational marine traffic.
We're looking forward to several visits to the bridge to watch the progress, and to watch the barges deliver these behemoths to their final resting place. I'm hoping our Holcim tour guide and his wife will join us on occasion.
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