A proposed city project to replace a 70-year-old sewer line in east Lawrence may be dirty business in more ways than one.

City crews recently discovered that a sewer line in the 1500 block of New Hampshire Street has crumbled to the point that it must be replaced. But crews also discovered something else: The city never obtained a sewer easement for the line that serves dozens of homes.

As a result, garages, gardens, backyard playgrounds and large trees are now atop the buried line. That has city engineers scratching their heads over how to bury an 8-inch pipe 10 to 12 feet underground without tearing up a mature neighborhood.

“We would prefer not to have to remove structures,” said Philip Ciesielski, assistant director of utilities for the city. “That’s not in anybody’s best interest. We’re looking for alternatives, but we may still end up in a bad spot.”

City commissioners are expected to approve at their weekly meeting on Tuesday a contract with BG Consultants to study different options for the project. For example, a new route could be chosen for the sewer line, although that could require individual lines to homes to be rerouted. Ciesielski said it also may be possible to bore underneath garages and other structures. The study also will have to consider how to maintain sewer service to the homes during the construction period.

“Hopefully, we’ll have two or three options to pick from,” Ciesielski said.

Ciesielski said the process will involve public meetings with residents in the area to get their feedback on what works best. The process also could end up including real estate negotiations. The city will want an easement for the new sewer line, which means the city will have to negotiate with property owners.

Ciesielski said the city did not have an easement for the sewer line because the city’s process for acquiring easements back in the early part of the 1900s wasn’t “real strong.”

He said the issue comes up occasionally, but often the city has been able to avoid digging up and replacing aging sewer lines. Instead, the city uses a process where it can send a device down a pipe that creates a new inner layer for the pipe. This line, however, already has collapsed too much for that process to work.

The city has responded to six sewer backups in the last year related to the line.

The city also will ask BG Consultants to come up with a plan for replacing a crumbling section of line in the 1100 block of Delaware Street. The city does not have an easement for that line either, but Ciesielski said that area includes much more open space for the city to work.

Ciesielski plans to meet with neighbors at both locations during October, and hopes to start construction before the end of the year.

A total cost for the project hasn’t been determined. The engineering work by BG Consultants — which is expected to cost about $70,000 — will come up with an estimated construction cost.

Commissioners meet at 6:35 p.m. on Tuesday at City Hall, Sixth and Massachusetts streets.