The third and final phase of the construction project to seismically upgrade Vashon’s north-end dock will begin Monday, and ferry system officials say it could cause delays.

The project will last five weeks and will involve closing one of the dock’s two ferry slips from 7:30 a.m. to 3 p.m. Monday through Thursday.

“It will not affect weekends,” Washington State Ferries (WSF) spokesman Ian Sterling said.

The slip closures will be necessary so crews can brace the portion of the dock farthest from the island. Traffic will continue to be loaded via one lane, but Sterling said two-lane off-loading will be able to happen by the evening commute due to construction wrapping up by 3 p.m. on weekdays.

Greg Beardsley, head of Vashon’s Ferry Advisory Committee, said the construction will definitely “mess things up.”

“But there’s no way around it, really,” Beardsley said. “They have to close it to get to where they need to get to.”

Sterling said this phase is expected to be the most disruptive for both vehicle traffic and ferries, seconding what WSF’s Leonard Smith said at a public meeting on Vashon late last month. The pedestrian walkway will remain mostly the same, with a few detours around construction equipment, Sterling said.

“There could be some delays,” Sterling said before explaining that WSF recently conducted a test of the one-slip system with no major issues.

WSF officials estimate that the third phase of construction work should be complete by May 25. The entire project is expected to be done by July, Sterling said.

The major construction project to update the 1950’s dock has been ongoing since last summer. Construction has entailed installing 10 braces that have been placed along the dock along with replacing 200 feet of wooden pilings with steel ones.

The project cost $15 million and is funded through a combination of federal grants and state money.