A second deep shaft is being dug at Avondale and Lavergne avenues as part of the next phase of the project to replace the Lawrence Avenue sewer in Jefferson Park. View Full Caption DNAinfo/Heather Cherone

JEFFERSON PARK — Traffic is about to get even worse on Montrose Avenue with crews set to begin the next phase of a $20 million project to replace a 100-year-old sewer to keep Jefferson Park homes from flooding.

Montrose Avenue — which is already closed to traffic from Cicero to Milwaukee avenues — will be blocked at Central Avenue for a month starting Monday, Ald. John Arena (45th) said.

"I understand that this is a major inconvenience to the thousands of people who traverse these intersections daily," Arena said. "However the upgrade should significantly reduce the potential of major sewer problems and should help prevent flooding."

Drivers should avoid the area completely, Arena said.

Montrose and Central avenues will be closed for a month, while the intersection of Berteau and Central avenues will close Monday for two months, Arena said.

Westbound traffic on Montrose will be detoured on Cicero Avenue south to Irving Park Road, west to Austin Avenue and north to Montrose Avenue.

Traffic on Central Avenue will be detoured to Irving Park Road before heading west to Austin Avenue and north to Lawrence Avenue before heading back to Central Avenue.

Crews are replacing the crumbling sewer underneath Montrose Avenue that has flooded area basements during strong storms and connecting it to the Deep Tunnel system.

The project started more than a year ago when crews began digging up Lawrence Avenue to reach the sewers and Deep Tunnel.

Since December, crews have been working at Avondale and Lavergne avenues, Lawrence and Lavergne avenues and at Montrose and Lavergne avenues as part of the second phase of the project. Those intersections have been closed since the work began.

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