The eastbound 625-foot-long Sprague Avenue flyover ramp to Highway 16 is almost gone. The single-lane bridge that spanned Tacoma’s Nalley Valley for 28 years has been reduced in the past week to one concrete island atop a couple of 35-foot piers.

An excavator with a long yellow arm hammered away Monday at the crumpled cement and heavy rebar that poked out of the two remaining piers.

Motorists are getting a close-up look at what’s going to happen to the entire Nalley Valley interchange over the next four years.

Commuters, especially those eastbound in the morning to Interstate 5, have been enduring delays approaching and transiting the valley since the $120 million Westbound Nalley Valley Project started two weeks ago with the closing of two of the four Sprague Avenue ramps.

With one of the ramps nearly gone, demolition begins this weekend on the 685-foot-long eastbound Sprague exit off Highway 16, a 100-foot-high flyover ramp that crosses over the top of both lanes of Highway 16.

“This weekend is a big weekend,” said Troy Watts, 39, of Puyallup, who is the field engineer for the project. The ramp will be gone by the end of the month.

With the two ramps taken down, Watts said work can begin on the new temporary east-west lanes for Highway 16 traffic across the valley.

Those lanes will be completed this fall, Watts said. All Highway 16 traffic then will shift south to those temporary lanes, he said.

“Half of this job is moving traffic out of the way to build the project,” Watts added.

Here are some things you might wonder about.

What happens Friday night?

Crews will take down a 60-foot section of the eastbound Sprague Avenue exit ramp that crosses over westbound Highway 16.

All westbound traffic on Highway 16 will be stopped Friday night at 10 p.m. The highway will open again at 10 a.m. Saturday.

At 10 p.m., trucks will dump 8 inches of sand on Highway 16 under the ramp. Thick metal plates will be laid on top of the sand to protect the roadway from falling debris. Construction lights will illuminate the area.

Huge excavators with hydraulic hammers and sheers then will cut apart the concrete and iron ramp between the two piers.

Westbound traffic will be detoured off at the westbound Sprague Avenue exit to 19th Street and directed west to Union Avenue to connect back to Highway 16.

What happens Saturday night?

The 60-foot section of the ramp that crosses over the eastbound lanes of Highway 16 will be taken down.

All eastbound traffic on Highway 16 from Interstate 5 will be stopped at Union Avenue on Friday night beginning at 10 p.m. The highway will open again at 10 a.m. Sunday.

Like Friday night, more sand will be dumped. Excavators will cut apart the concrete and iron ramp between the two piers.

Eastbound traffic will be detoured off Highway 16 at Union Avenue and then directed to 38th Street South in order to reach Interstate 5.

What about the basketball tournament?

The Class 4A state basketball tournament runs both Friday and Saturday evenings at the Tacoma Dome and the last game both nights starts at 8. Drivers who normally would use Highway 16 should be aware of the 10 p.m. closures and detours.

WSDOT has sent e-mails to participating schools making them aware of the closures and encouraging them to let their fans know.

What happens to the debris?

The concrete and rebar will be separated and recycled.

What are the next important dates?

With the two Sprague Avenue ramps torn down by the end of March, work will begin where they once stood on a new but temporary east-west route for Highway 16.

Traffic will be shifted south to the temporary eastbound lanes in the fall of this year. Traffic will be shifted to the temporary westbound lanes in February-March 2010.

Mainline traffic will stay there until a new east-west viaduct is built across the valley. It will open in 2013 and traffic again will be shifted back north.

In the fall of this year, the other two Sprague Avenue ramps will be closed: the westbound entrance to Highway 16 and the westbound Highway 16 exit to Sprague Avenue.

Sprague Avenue will be completely closed off to Highway 16 until new ramps are completed in 2013.

Why has morning commuter traffic slowed to a standstill through Nalley Valley?

WSDOT spokesman Jaime Swift said the closures of two of the four Sprague Avenue ramps two weeks ago was expected to slow down traffic on Highway 16.

Exactly why it is worse than expected during the morning isn’t really understood, he said. Two weeks hasn’t really provided enough data, he added.

Both Swift and Watts speculated that change is always difficult for motorists and they slow down until they get used to it.

More to the point, Watts said, is the lane re-striping on Highway 16 needed for the project. Eastbound traffic was moved from south to north closer to the median as it approaches the viaduct.

That shift, as well as the narrowing of the eastbound route through the creation of 2-foot shoulders instead of the normal 8-footers might be slowing motorists, Watts said.

“It’s a been a difficult lane restriction,” Watts said.

The good news is that the lane restriction will straighten out when the new temporary eastbound route opens this fall.

How can I keep track of the project?

Check out WSDOT’s project Web site at tacomatraffic.com.

Mike Archbold: 253-597-8692