Explosions along Interstate 90 are part of the $551 million project to widen the nation's busiest mountain highway from four lanes to six. At the same time, the state Department of Transportation is improving habitat by fashioning larger culverts and wildlife passages under and over I-90.

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HYAK, Kittitas County — Glaciers carved this mountainside over thousands of years. A highway crew needs only an instant to alter it.

An air horn blasts, and a countdown begins. Blasting caps transmit a shock wave into dozens of explosive-filled holes. The flashes combine to resemble a lightning bolt sprawled across the rock face. Dirt bounces upward. Rocks spray over the concrete barrier onto Interstate 90.

Then a second boom echoes from the green foothills on the far shore of Keechelus Lake, back to the blast zone.

These explosions are part of a $551 million project to widen the nation’s busiest mountain highway from four lanes to six. An average 27,000 cars, trucks and buses cross Snoqualmie Pass each weekday, and more than 50,000 on busy weekend days. The project will increase road capacity, provide straighter lines of sight and replace broken pavement.

Also, the slopes alongside the westbound lanes should be more stable, so the pass will close less frequently because of avalanches, the state Department of Transportation (DOT) predicts.

At the same time, the state aims to improve habitat by fashioning wildlife passages under and over I-90. The first three miles of construction from milepost 55 to milepost 58 began in 2010, two years earlier than the original plan. Costs are trending $42 million below budget on this stretch, built by Max J. Kuney Construction, of Spokane, said Brian White, a DOT assistant regional administrator. Bids will be opened this month for the next two miles east, from the snowshed to Keechelus Dam.

If the savings hold up, lawmakers in 2012 can consider whether to OK another couple of miles of construction, beyond Keechelus Dam. This would include one of the nation’s first freeway overpasses specifically for wildlife. Legislators last spring reserved $8 million toward design for this area.

Blasts are under way just west of the I-90 snowshed. The cliff is being lowered in layers, several feet per explosion. Last week’s explosion required about 7,000 pounds of emulsion, said Brandon Bair of Western States Drilling and Blasting. Workers pour veins of ammonium-nitrate putty into holes in the rock.

Before the dust settled Wednesday, Bair sprinted up the hill to confirm that all charges had gone off. Picking machines, front-end loaders and street sweepers converged at the curve, with the urgency of a pit crew in a car race. The weight of falling boulders shoved the concrete barrier into the highway, forcing operators to remove a pile before the lane could reopen about 90 minutes later.

The DOT’s immediate goal here is to smooth out a sharp highway curve. The new freeway will have 10-foot shoulders, as well as wide ditches to catch snow and boulders that otherwise would tumble into traffic.

Three women were killed in a 2005 rockslide west of the pass, and rocks have also fallen onto the roadway to the east.

Back at Hyak, new bridges are being assembled over water and shorelines. The longest span is 1,000 feet. When that’s finished, the old freeway will be torn out so that Gold Creek can flow unobstructed into Keechelus Lake, a man-made reservoir that feeds the Yakima River and farm irrigation.

The corridor is a connection between the Alpine Lakes and Goat Rocks wilderness areas, and for creatures to migrate as far as Mount Rainier and the North Cascades.

“This underpass is going to facilitate movement for everything from bull trout in Gold Creek, to mountain goats, and maybe we’ll be recovering wolverines,” said Jen Watkins, outreach coordinator for the I-90 Wildlife Bridges Coalition.

Wolverines have been found in other parts of the Cascades, closer to Mount Rainier and the North Cascades, so ecologists have some hope that they will show up north-south through the Gold Creek/I-90 area, if the passages improve.

White said I-90 passes through a U.S. Forest Service easement, adding to the environmental oversight. Environmental agencies and groups collaborated with the DOT in the design, similar to freeway crossings for wildlife in Montana and Alberta, Canada.

The whole Hyak-Easton plan calls for three wildlife overpasses and 21 locations where road bridges and culverts will be replaced to allow wider, more natural undercrossings.

The public is encouraged to report animal sightings, dead or alive, to I90wildlifewatch.org. Besides raising awareness, the reporting will help ecologists and the DOT measure how animals use the new crossings, Watkins said.

Meanwhile, this season of blasting and clearing will continue through October.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com