Two lanes of the northbound 5 Freeway near Castaic will remain closed through Tuesday as Caltrans engineers assess damage to retaining walls caused by the Route fire.

The two open lanes of the freeway extend for about three miles north of the State Route 126 juncture near Parker Road up to about Vista Del Lago Road, officials said. All southbound lanes are open.

The Route fire, which was first reported Wednesday, has burned more than 5,200 acres, according to the Los Angeles County Fire Department. It was 87% contained as of late Saturday night.

Caltrans is advising drivers to take the 101 Freeway or State Route 14 as an alternative on this heavily-travelled Labor Day weekend.

Wooden lagging used in steel barriers for the freeway’s retaining wall were burned by the fire, according to Caltrans spokesperson Lauren Wonder. Parts of the wall were still smoking Saturday.

Wonder said the steel “looks OK,” but she said compacted dirt directly below and behind the barrier was destabilized and beginning to slough off the banks.

Wonder said engineers were scheduled to arrive Monday to examine the damage and test the structural soundness of the wall.

🚧 🚗 NB I-5 two right lanes closed near #RouteFire #Castaic. Retaining walls burned; structural engineers must inspect wall integrity before any load is put on lanes. Queue is 11 mi and delay is 90 mins. PLEASE use #US101 or #SR14 instead of I-5. pic.twitter.com/bFGtgtx9aT

“We want to put truck traffic on the freeway and we don’t want a scenario where the corner of the road could slip off,” Wonder said.

Wonder said the timeline of the closure would depend on the extent of the damage. She said the long-term forecast for a full freeway opening was “to be determined.”

Caltrans will also replace damaged and destroyed guard rails and signposts, and clear dirt that seeped into nearby storage drains. Emergency contractors have closed portions of the freeway since the fire began Wednesday.

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Andrew J. Campa covers the Eastside, Boyle Heights, Echo Park, parts of the San Gabriel Valley and the Southeast Corridor for the Los Angeles Times. He previously worked at the Glendale News-Press, Burbank Leader, Whittier Daily News and Pasadena Star-News. Campa is a Cal State Fullerton and Pasadena City College alumnus.