Details on Lansing bridge demonstration shaft progress
(Left) Crew members working on the Blackhawk Bridge replacement project were in attendance to talk about the progress made over the past month. (Right) A model of the current Blackhawk Bridge was shipped to the Meehan Public Library in Lansing, Iowa, for display through July. (Steve Van Kooten/Courier Press)
By Steve Van Kooten
After the lights were switched off at the Meehan Library a video played on a view screen against the windows. The video flickered and the audio popped like an old drive-in movie before a crane holding a large piling appeared. A rhythmic banging pumped through the speakers.
“They use that hammer to drive those piles into the ground,” Anden Lovig, HNTB Engineer, said. “I’m sure most of you have heard that if you live in town.” Several audience members murmured in agreement.
“You can hear it in DeSoto down on the ridge,” one onlooker answered.
The piling, a 52–foot long 16–inch diameter pipe, slowly sank into the ground as the video concluded. Paul Lindsay, Senior Field Inspector, who measured the blows–per–minute and strokes–per–foot to calculate the depth the piling had penetrated, stated the piling was being driver past 112 feet in search of a hard layer of rock. The sandstone or other solid layers were deeper than initially expected, according to Lindsay.
On Feb. 8, members from the Blackhawk Bridge replacement project held their fourth informational meeting at the Meehan Public Library in Lansing, Iowa. Along with Lovig and Lindsay, Travis Konda, HNTB Project Manager, was in attendance. Rather than a hands-on activity, Lovig stated the meeting would show several videos to illustrate the project’s progress through January.
“It’s been just at a month, and there’s been significant work accomplished,” Konda said. “A lot of it has been at or below ground, so it’s hard to see these things.”
Lovig brought up another video, which showed a crane bringing a mixture of rock and water from a metal casing and disposing of it on the ground. The sandstone underneath the river was not as hard as granite or limestone and could not be excavated after the rock was cut with a core barrel.
“A core barrel is basically like a gigantic hole saw that has carbide teeth on the tip of it, and as it’s rotated, it cuts into the rock,” Konda said. The barrel cuts a “plug” of rock that usually can be excavated; however, in this case, a separate rock auger had to be brought in to further grind up the material. The rock could then stick to the auger’s bit for excavation. Once the sandstone was brought to the surface it looked like coffee that sat out too long in a motel lobby.
After the demonstration shaft was drilled out and cleared of everything except water Kraemer North America, the construction contractor, built a rebar cage to be inserted into the hole. The cylindric amalgamation of metal rods was more than a 100 feet long and weighed over 100,000 pounds. On video a time-lapse showed a crane crew raising the shaft from the ground to a vertical position to be inserted into the shaft. The entire process was completed in “a little over an hour,” according to Lovig.
“You have to remember this cage is held together with wire—it’s not welded. The actual structure is tied together,” Konda said. He further noted that rebar “does not like” to be welded and could become brittle.
At one end of the cage a metal ring wrapped around the circumference of the rods, which Konda said was “an example of effective engineering” from Kraemer, whose engineers designed the ring as a lift mechanism for the cage. “I’ve seen them [engineers] use chains and all kinds of things to do this.”
“Once the cage gets picked up it’s welded to a load cell,” Lovig said. “That alone is 109,000 pounds, and that’s basically three hydraulic drums.” The load cell, which would only be used for testing the demonstration shaft, utilizes mechanisms to measure the strength and bearing of the shaft to assess the construction contractor’s means-and-methods before work on the actual drive shafts commenced. “It’s to ensure we get the necessary capacities from these shafts.”
“Once these things [cages] go into the ground, it’s very difficult to remedy an issue,” Konda added. The load cell fulfills an important function with built in “tattle-tales” to alert engineers of problems or aberrations that need to be remedied.
In the next set of pictures, a concrete tremie transported concrete from trucks to the shaft. Lovig said the process wasn’t as simple as dumping ordinary concrete mix into an empty hole because there was still water present in the shaft.
“You wouldn’t drop it [the cage] the night before. If something happened—if the rock collapsed—you wouldn’t be able to remove the cage out of the hole,” Lovig said. Instead, the crew had to pour concrete immediately after the cage was inserted into the ground.
“We can’t just drop concrete into the water,” Konda said. “It would disintegrate.”
Konda explained that below the 127–foot steel can be used for the shaft an additional 36 feet had been drilled at a slightly smaller diameter (8.6 feet vs. nine feet). The additional drilling left the rock exposed for the concrete to bind with it. After that a plugged steel pipe led with a mix of concrete, water polymer and retardants would be lowered into the hole. The plug would eventually burst from concrete being poured into the pipe and displace the water already in the hole and allow the concrete to solidify.
The next “Let’s Talk Bridges” meeting will take place on March 14 at Lansing’s public library from 5–6 p.m.
