Installation of rammed piers up next for City Hall project

Sheet piling efforts will be wrapping up as construction activities at the new City Hall site enters the next phase this week.

A vibratory roller will be moved to the site and work will begin on the rammed aggregate piers, which are designed to bolster the pilling system.

The roller along with the aggregate piers will create vibrations near the area, according to the city’s representative Mike Cronquist.

“The rammed aggregate pier work will be accompanied by a moderate increase in construction traffic and noise,” Cronquist said. “And, although the tamping activity creates a sound similar to that of a large jack hammer, it is not expected to be excessive.”

To install the aggregate piers, a 2 1/2 foot diameter hole will be augured to a depth of 15 feet and filled with compacted gravel. After the hole is drilled, it is packed with engineered base aggregate in layers of 8 to 12 inches at a time, with a large hydraulic tamper. The sequence is repeated until the hole is completely filled with a solid column of gravel. Nearly 400 piers are planned, according to Cronquist, providing a support system to hold the weight of the City Hall building and parking structure.

If the installation of the piers goes as planned, the job should be done within three to five weeks. Following that concrete work will begin on the site.

Cronquist said the city has engaged a local engineering firm to monitor the extent of any disturbances caused during the course of work.