Lebanon — Construction on the Route 120 sewer improvement project will close the southbound on-ramp of Exit 18 for almost a week starting next Wednesday.

The ramp will be closed between June 22 and June 28. During that time, drivers will be directed onto I-89 South using either Exit 17 or Exit 19.

Detouring onto Exit 19 will require people to drive northbound on the interstate from Exit 18, before getting off at 19 and turning around to get back on the interstate. That route is a 4.5-mile drive, according to city officials.

Driving to Exit 17 takes about 3 miles. From Exit 18, people can follow Route 120 to its intersection with Hanover Street, turning left and continuing past Colburn Park and continuing on Bank Street.

“During peak time, (traffic) is going to be a little challenging,” City Engineer Christina Hall said. “It’s always tough because of the volume of traffic at (Dartmouth-Hitchcock Medical Center). That’s the basic generator of traffic.”

Hall said crews initially intended to bore underneath the ramp, but work halted over the winter when they encountered boulders.

To speed up the project, she said, the state Department of Transportation now is allowing construction to avoid the boulders by cutting directly into the pavement.

Beginning on June 22, contractors will take up portions of pavement on the ramp, install sewer casing and then patch up the ramp to reopen. Hall said it’s likely crews will close a section of the ramp later in the summer to complete pavement work.

Over the last four years, the City Council has appropriated about $6.5 million to the Route 120 sewer improvement project, which is important to several developments along the corridor awaiting the line’s completion.

The 1.4-mile sewer line project was originally estimated to cost $2.1 million but has more than tripled in cost partly because construction of the line has encountered a number of unexpected problems.

Tim Camerato can be reached at tcamerato@vnews.com.