After weeks of demolition and survey work, construction of the new Interstate 35W bridge is expected to begin Thursday. And while cranes are already gathering at the site, you'll need to look down, not up, to see the first step: Workers will be drilling at least 100 feet as they start work on the underground supports.

The drilling begins an intense 14-month process featuring roadways that will grow, piece by massive piece, from the main piers on each bank and jut out far over the Mississippi River until they're joined in the middle by the concrete equivalent of a golden spike.

The construction team, led by Flatiron Constructors of Colorado, has pledged to finish the bridge in 437 days -- the clock started ticking Oct. 15. The plan is to finish the $234 million replacement for the collapsed I-35W span by Christmas of next year.

The timetable's ambitions were evident last week in a printout of a bridge-construction schedule, which included about 900 items and gave new meaning to the concept of a desk calendar: Each page was the size of a desk.

The schedule was draped across the desk of Jon Chiglo, who is the Minnesota Department of Transportation's project manager for the bridge. Chiglo can't quite see the construction site from the high square window in his temporary office on the Minneapolis riverfront, but he has a bird's-eye view of the Stone Arch Bridge, a 124-year-old symbol of the durability that the builders of the new bridge say they're aiming for.

The I-35W bridge will be a bit like an iceberg -- not only will it be some shade of white, but it will extend farther below the surface than you might expect.

The first major drilling for the bridge will cut through 50 feet of soil, then an additional 50 to 70 feet into the sandstone below. The resulting hole, between 7 and 8 feet across, will be lined with steel, then filled with concrete that will form a shaft.

The shaft will be allowed to cure. Then they'll try to break it.

The point is to make certain that the shaft, which will be on the north side of the site, can withstand forces greater than the bridge's weight. Assuming it survives the intense pressures applied via hydraulic jacks, work then will proceed on the real shafts, all 32 of them. The test shaft will be abandoned and covered.

Obstacle course

With the old bridge removed, the site might look like a blank slate. But it's more like a living-room floor scattered with toys -- there are only a few places to put your feet.

In figuring out where to put the bridge's piers, engineers had to deal not with toys but with storm drains, a web of utilities, a railroad track, a place where river dredge spoils are dumped, and a capped Superfund site directly under the southern approaches to the bridge. "There is a lot going on," Linda Figg, president of Figg Bridge Engineers, said last week at a community meeting about the bridge's design.

But neither rain nor snow nor dark of night are expected to be obstacles to construction. Most activities can be done in winter, said MnDOT spokesman Kevin Gutknecht. He said they'll try to limit pile driving to daylight hours and minimize the noise from trucks backing up. Casper Hill, a spokesman for the city of Minneapolis, said the city has not yet received a request from bridge contractors for permission to do nighttime work.

The end result will be a bridge with 48,700 cubic yards of concrete and nearly 22 million pounds of steel -- one that will lift drivers 115 feet above the river.

Jim Foti • 612-673-4491

A NEW DESIGN, TECHNIQUES AND MATERIALS

Jim Foti • jfoti@startribune.com