Blackhawk Bridge January meeting updates progress
The Blackhawk Bridge project has progressed with Kraemer North America demonstrating means and methods. (Steve Van Kooten/Courier Press)
The Meehan Memorial Public Library in Lansing, IA, hosted the third “Let’s Talk Bridges” meeting with representatives from the Blackhawk Bridge replacement project on Jan. 11.
The meeting was attended by Clayton Burke, IowaDOT Project Manager; Travis Konda, HNTB Project Manager and Structural Engineer; and Paul Lindsay, Lead Inspector for the project. Several members from the Kraemer North America were in attendance as well.
Burke introduced several videos for the audience. The first set showed a temporary casing driven into the ground with a vibratory hammer, which weighed nearly 100,000 pounds, according to Burke.
“The way that hammer works is there’s giant, hydraulic motors that are spinning an eccentric weight, and these eccentric weights are working together so that, as they come around, the downward force comes together from the acceleration from the off-set weight. And it shakes its way into the ground,” Konda said.
Another video showed an auger digging out material from the casing.
“This is essentially how they dig out the rock,” Burke said. “It’s this repetitive dig it out, bring it out, spin it, dig, bring it out, spin it.”
Burke stated the necessary digging was shallower on the Iowa side of the bridge: “On the Wisconsin side, the rock is much deeper than on the Lansing side. It’ll go all the way up—some of the houses are only eight feet on the Lansing side, but then, underneath the river, it dives down to 120 feet deep.”
Burke stated the shafts in the middle of the river will have a 12 feet diameter; however, the demonstration shaft has a nine foot diameter. Burke stated, as in the last meeting, that the display shaft was necessary as proof of means and methods for the construction contractor and the project as a whole.
Konda stated there would be sensors cast into each shaft that will relay information to measure the force put on the bridge from the surrounding environment.
Another question asked about earthquake impact on the bridge’s structural integrity, which Konda answered, though he stipulated he was not an expert on the subject. “Here in Northeast Iowa we are at a very low ground acceleration, and that’s what we are concerned about: how fast the ground accelerates relative to static [ground], right? So, if you’re out in, like, California or near the New Madrid fault in Southern Illinois, you’re seeing very, very high 0.5-0.9 g... Up here we’re probably like 0.1-0.2 g. So, when you’re at that low level of earthquake, it puts you into a design category where we’re actually able to design the structure to remain elastic,” Konda noted the current bridge would have more of an issue with earthquake displacement, but said it would still take a “sizable earthquake” to doing any permanent displacement.
The presentation also included a demonstration of cofferdams using a fishtanks and a miniature excavator provided by Kraemer North America.
The next meeting will be on Feb. 8 at the Meehan Memorial Library in Lansing, IA, from 5-6 p.m.
