Contractors working for the Washington Department of Transportation started blasting work on the 20 million-pound rock looming above state Highway 503 near Milepost 38.
Pacific Blasting and Demolition started blasting at about 2 p.m. Thursday afternoon.
Observations with LiDAR technology of the hillside show that the slab continues to move above the roadway.
The blast was the first of many planned to remove the rock. Following the blast, crews will remove loose debris and look for fracture points. Based on their observations, crews will develop new demolition strategies and bast more next week.
WSDOT officials believe the rock slab came loose from a slide that occurred March 13, initially blocking one lane, but an hour later, covering both lanes.
The agency is still unsure when the highway will be reopened.
