A big vibrating hammer has run into an underwater obstacle while driving metal sheets deep into the Mississippi River bed to prepare for installation of new gates on the Coon Rapids Dam.

The work is part of a $16 million upgrade to make the 100-year-old structure a nearly foolproof Asian carp barrier. The state-funded project, slowed a bit by high water this spring, is aimed at stopping the invasive species from slipping past the dam into the upper Mississippi River watershed.

But there was a hitch as workers built a cofferdam that will keep water out of the work area. Although the hydraulic hammer packs a 200-ton punch, it was thwarted by an underwater monster that stopped the steel sheets from plunging 22 feet into the river bed. The "monster" was apparently some old concrete slabs that are too massive to bust or remove, said Dennis Jonjack, project engineer for contractor Edward Kraemer & Sons.

It took a day to figure out the size and locations of the concrete hunks, Jonjack said. Workers poked into the river bed with poles. Test piles were driven down by the 20,000-pound hammer head suspended from a 130-foot-tall crane. In the end, the 550-foot-long coffer dam was extended about 25 feet farther upstream to avoid the underwater concrete; it was finished Monday..

The process of getting the sheets into place for the cofferdam was tricky itself. The crane would dangle a sheet, weighing up to a ton, next to a worker on a mobile platform; the worker would guide it into the grooved top slot of another sheet already in place.

With the cofferdam finished, pumps will suck out river water from the enclosed area so workers can install the new steel gates for the regular dam. They will replace patched rubber bladders that are used now. When deflated during high flows, the bladder gates can allow carp to swim over the dam, said Jason Boyle, dam safety engineer for the state Department of Natural Resources.

"It will be a definite improvement in the effectiveness of the barrier," Boyle said.

The 18-foot high dam is owned by Three Rivers Parks, Hennepin County's park system, which has agreed to let the DNR oversee the rehabilitation.

Kraemer & Sons, of Plain, Wis., won a $10.8 million contract to install nine steel gates and repair the concrete apron below the dam during the two-year project. The state paid Rodney Hunt Co. of Orange, Mass., $3.8 million to produce and deliver the gates. Stanley Consultants of Minneapolis was paid $900,000 to do designs, plans and specifications. Adding $200,000 for DNR administrative and oversight costs brings the total so far to $15.8 million, leaving $200,000 for contingencies.

Kramer began the work by cutting a short, temporary road through the woods northwest of the visitor center in Coon Rapids Dam Regional Park to get 350 steel piles, cranes, a barge, tugboat and other equipment to the site. Jonjack said workers mined 64,000 tons of sand and gravel from a grassy field near the new road.

Dump trucks hauled the material to river's edge, where it was used to build a 200-foot-long, 20-foot-wide causeway to Dunn Island to carry equipment to the worksite. The sand and gravel will be returned to the mined pit and topsoil will be respread when the project is done, Jonjack said.

It will take a week or so to drain water out of the cofferdam and dump about five feet of gravel on the mucky riverbed to stabilize it so equipment and men can install the 8-foot-tall gates. Five gates will go in this year. Boyle said the longer gates will weigh about 45 tons each. The remaining four gates will go in next year.

While the gates are installed, work will begin on a second cofferdam on the downstream side so apron repairs can be made. The five gates are expected to be installed by November. The rebuilt, 1,000-foot-long dam is designed to last 50 years.