Crews are preparing to replace the 142-year-old water main under a stretch of Adelaide Street East downtown, the city's chief engineer says.

Michael D'Andrea, executive director of the city's engineering and construction service, says work officially began on Tuesday with crews locating the buried infrastructure before excavation starts. The project will cost $1.4 million.

The Adelaide Street East water main, which runs between Jarvis and Parliament streets, has reached the end of its service life, he said Tuesday. Replacing the aging water main with a PVC pipe larger in diameter will improve water service in the area, but it will mean lane restrictions, starting on Wednesday, while the work continues until the end of December.

"It's been good infrastructure, some of the best infrastructure that we have in the ground, when it's all said and done," D'Andrea told CBC's Metro Morning on Tuesday. "But it's served us well, over 140 years, and it's time to replace."

Michael D'Andrea, the city's chief engineer, says: 'Certainly, the downtown core presents some real challenges for us because it is costly when we're digging up and replacing some of this old infrastructure with some of the new. It's extremely impactful. We have plans underway to provide the necessary servicing for the future growth.' (CBC) According to the city, the project is part of a $720-million plan to maintain and improve infrastructure in Toronto. The city has about 6,100 kilometres of water mains and about 11 per cent, or 670 kilometres, is more than 100 years old.

Coun. Jaye Robinson, chair of the public works and infrastructure committee, has called the Adelaide Street East water main "vintage infrastructure."

D'Andrea said the pipe itself is thick and the water main is "sandcast," meaning the pipe was made in the mid-1800s from molten iron poured into a sand mould.

"In this particular case, actually, we are replacing this old infrastructure with larger-diameter pipe to service our future growth needs in the downtown core," D'Andrea said.

A water main break flooded a stretch of Yonge Street downtown earlier this year. (Tony Smyth/CBC) D'Andrea said the city makes decisions to replace water main pipes based not only on age, but also on their condition and number of breaks. He said the city is looking "to get ahead of the curve, replace this infrastructure before it begins to impact us quite significantly."

Another major water main replacement project is slated for Jarvis Street, from Queen to Dundas, starting next week. On that stretch of Jarvis, city crews are planning to replace two vintage water mains and to reconstruct and resurface the road. The project will cost $2.3 million.

He said the city tries to combine water main replacement work with road resurfacing to minimize the impact on neighbourhoods.

Downtown core presents 'real challenges'

"Certainly, the downtown core presents some real challenges for us because it is costly when we're digging up and replacing some of this old infrastructure with some of the new," D'Andrea said. "It's extremely impactful. We have plans underway to provide the necessary servicing for the future growth."

The city has about 1,400 water main breaks a year, he said. Replacing aging infrastructure is "critically important," he added.

Work on Adelaide Street East will be done in two phases, D'Andrea said.

The first phase begins at Parliament Street and continues west to Princess Street. During the first phase, traffic will be reduced to one lane eastbound and one dedicated bicycle lane.

The second phase will start when the first phase is done, likely sometime in late September.

At times, he said there may be a short term disruption to water service as the main is replaced but residents and businesses will be given notice.

The city will post signs to give directions to drivers, pedestrians and cyclists and warned there will be delays and an increase in traffic in the area and on nearby main and side streets.

Crews will work from 7 a.m. to 11 p.m. Monday to Friday and 9 a.m. to 7 p.m. on Saturday. Some work will be done after hours and on Sundays if need be.