PORTLAND – A key phase of the $135 million Maine Turnpike widening project got under way Tuesday as crews began filling a half-mile valley just south of the Scarborough-Saco line.

“The dip,” where the highway drops 29 feet from the crest of one hill to the next, has been blamed for traffic jams and accidents along the busiest section of the toll road, one that carries about 85,000 cars and trucks each day.

The plan is to partially fill the dip so the highway will drop only 19 feet. Engineers calculated that it would take 64,000 cubic yards of fill, most of it from a Dayton gravel pit, to raise the road’s elevation 10 feet.

Because work on the dip starts on the northbound side, the Maine Turnpike Authority began diverting northbound traffic into the southbound lanes early Tuesday morning.

To fix the dip, workers first added a third lane and a breakdown lane to the southbound side of the highway. That provided enough room for the lane diversions.

The heavy traffic in the area is expected to make the project one of the most difficult during the five-year turnpike widening, which began last year.

Even with months of planning by the authority, its engineers and contractors, some delays and other problems are anticipated.

“It’s been a big deal for us,” turnpike spokesman Daniel Paradee said Monday. “Hopefully, it won’t be a big deal for the average driver.”