The city expects to finally finish the Navy Pier Flyover by the end of this year, adding a tunnel through two Depression-era limestone bridge houses.

The construction will include months of lane closures beginning April 30 on lower Lake Shore Drive and the temporary rerouting of a portion of the Lakefront Trail.

The city started building the $64 million flyover for bikes and pedestrians in 2014, after it had been discussed for a decade. It has been delayed by various factors, including repairs on the Lake Shore Drive bridge over the Chicago River that ended up being more extensive than expected.

The first portion of the flyover, which opened in December, goes over Grand Avenue and Illinois Street. The final portion will carry the path over the river.

But the process will be complex, and will happen at the same time as repairs are being made on the two-level steel bascule bridge, built in 1937, according to city officials and engineers. Here's a step-by-step guide to what drivers, pedestrians and cyclists should expect in the coming months.

The two bridge houses on the east side of the bridge will be tunnelled through to make room for the expanded Lakefront Path. The existing path, center, runs between the roadway and the limestone towers. (Abel Uribe/Chicago Tribune) Construction and lane closures Lower Lake Shore Drive has has three two-lane sections, separated by low steel barriers. Closures for work on the bridge will begin Tuesday, April 30, said Chicago Department of Transportation spokesman Michael Claffey. About 19,500 vehicles use the lower drive on a daily basis, including 12,200 southbound and 7,300 northbound, Claffey said. Date What's closed What's open Tuesday, April 30 Two westernmost, southbound lanes Southbound traffic will use two center lanes Four weeks later Two center, southbound lanes Southbound traffic will use new western-most lanes Early summer Two easternmost, northbound lanes Center lanes will be divided into one northbound lane for traffic and one new, temporary two-way pedestrian and bike path. In the final phase of bridge work the pedestrian and bike path will be separated from vehicle traffic by a barrier wall, Claffey said. This path will be the connection to the Lakefront Trail over the river. The arrangement will continue until the end of the year. The northbound vehicle lane will take traffic to upper Lake Shore Drive, but the lane that takes traffic to Illinois Street and Grand Avenue, Lake Point Tower and Navy Pier will be closed. Claffey said people heading for Navy Pier or Lake Point Tower can use the upper drive instead, or take Columbus Avenue to Illinois Street.

Making room for wider Lakefront Trail Current trail: Lower Lake Shore Drive: 6 lanes of traffic 12 feet in sections, 8 feet at the narrowest points. Chicago River Sidewalk Expanded trail: At least 16 feet total throughout, up to 21 feet over the Chicago River. Lower Lake Shore Drive: 6 lanes of traffic Current trail over river: 12 feet in sections, 8 feet at narrowest points. Chicago River Sidewalk Expanded trail: At least 16 feet total throughout, up to 21 feet over the Chicago River. Current trail over river: Lower Lake Shore Drive: 6 lanes of traffic 12 feet in sections, 8 feet at narrowest points. Northeast bridge house Expanded trail: At least 16 feet total throughout, up to 21 feet over the Chicago River. Chicago River Sidewalk Southbound Northbound Southeast bridge house To continue the flyover over the river, CDOT engineers will cut part of the trail through the two bridge houses on the east side of the bridge, said Lawrence Mestan, resident engineer with the project's construction management team. Mestan, of the T. Y. Lin International engineering firm, said the city would not be cutting into the support walls of the bridge houses -- those are along the sides and will stay where they are. Cutting through the limestone: Jackhammers and saws will be used to cut through the Depression-era limestone bridge houses on the east side of the bridge, on both sides of the river. Southeast bridge house Tunnel through house Northbound lanes Bike Southbound lanes Pedestrian Two new lanes will be created through the bridge houses. North Lakefront Trail lanes will go through the houses and then on expanded surface. Southbound lanes will be on existing surface. Cutting through the limestone: Jackhammers and saws will be used to cut through the Depression-era limestone bridge houses on the east side of the bridge, on both sides of the river. Southeast bridge house Tunnel through house Northbound lanes Southbound lanes Two new lanes will be created through the bridge houses. North Lakefront Trail lanes will go through the houses and then on expanded surface. Southbound lanes will be on existing surface. Bike Pedestrian Cutting through the limestone: Southeast bridge house Jackhammers and saws will be used to cut through the Depression-era limestone bridge houses on the east side of the bridge, on both sides of the river. Tunnel through house Northbound lanes Two new lanes will be created through the bridge houses. North Lakefront Trail lanes will go through the houses and then on expanded surface. Southbound lanes will be on existing surface. Southbound lanes Bike Pedestrian

Moving bridge controls The city has been working on clearing out the ground floors of the bridge houses to make way for the bike and pedestrian path. In the southern bridge house, this will mean moving an entire wall of 82-year-old electrical equipment, Mestan said. The equipment will be relocated one floor up, Claffey said. Chicago Department of Transportation engineer Lawrence Mestan stands near a panel of electrical controls which will have to be moved to make way for an exanded Lakefront Trail Navy Pier Flyover. (Abel Uribe/Chicago Tribune)

Repairing the bridge Structural repairs to the bridge, which will include cleaning and fixing the mechanical parts that raise and lower the bilevel structure so that boats can pass through, will total $25 million, separate from the $64 million total cost of the flyover. These repairs will continue into 2020, Claffey said. The controls to raise and lower the bridge will still be at the top of the southern bridge house, in a windowed room with great views of the lake. Access to the giant steel wheels that take the bridge up and down is below street level. Work on the Lake Shore Drive bridge will include cleaning and fixing the mechanical parts that raise and lower the moveable bridge. (Abel Uribe/Chicago Tribune)