Crews closed the area to traffic at noon.
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The closure affects a nearly 13-mile stretch of the highway between Paul's Slide and Mud Creek in Monterey County.
That area experienced the biggest mudslide in Caltrans history in May 2017.
About 6 million cubic yards of debris slid onto Highway 1 and out into the Pacific Ocean, creating 2,400 feet of new shoreline.
It took 14 months to repair the road before it reopened in July 2018.