The first rockslide
Without warning, a rockslide of gigantic proportions thundered down the mountainside Sunday, Nov. 9, 2003 at 6 a.m., just east of Newhalem. The size of the landslide was estimated at two to three million cubic yards. The slide slammed into Highway 20, took out portions of roadway and guardrail, and cut off access to the small Seattle City Light town of Diablo in Whatcom County. Enormous boulders and debris in excess of 100 tons, including a 50-ton boulder, were left on the highway blocking both lanes.
Understanding the situation
Use the first picture on the right as your guide. When looking at the picture, Newhalem is less than two miles to your left and Diablo is to your right. The steep ridge at the top of the picture divides Afternoon Creek (on the right) and Falls Creek (on the left). Most of the rock that came down, rushed down Afternoon Creek. However, Highway 20 sustained the most damage at Falls Creek, where the 50-ton boulder and other debris crushed and blocked the road.
The hillside
Crews have been monitoring the hillside since the November slide. Shortly after the first rock slide WSDOT and the Federal Highway Administration determined a solution to fix the hillside and make the highway safer for motorists.
What caused the massive rockslide on Highway 20?
WSDOT engineering geologists and Federal Highways Association (FHWA) both attribute the massive rockslide to the record amounts of rainfall we received in October, 2003. Shortly after the soggy weather in October, November brought freezing temperatures. The rockslide happened early in the morning November 9, which is the coldest part of the night. The freeze and thaw conditions that were occurring at that time, are what is believed to have brought down the hillside.
The second rockslide
Within four days of crews removing the rock and debris from Highway 20 to create an emergency access lane for Diablo, a second slide came down Falls Creek, covering the road and blocking the emergency lane. Weekend rain, November 16 and 17, washed smaller rock and debris out of Falls Creek. Maintenance crews were able to clear the rock and debris and reopen the emergency access lane to Diablo by Thursday, November 20.
December 17 rockslide
On December 17, 2003, a small rockslide occurred above Afternoon Creek. The material fell into Afternoon Creek and did not reach the highway below.
December 19 rockslide
On Friday morning, Dec. 19, 2003, a very large rockslide occurred above Afternoon Creek. It is estimated that 50,000 cubic yards of rock fell into Afternoon Creek. Large blocks of rock (50 to 60 ft) were visible in Afternoon Creek after the slide. The rockslide proved that the visible signs of deterioration and data gathered from prisms on the hillside were correct.
Monitoring equipment
State-of-the-art electronic devices, borrowed from the United State Geological Survey, are being used to monitor movement on the unstable hillside. USGS has established regularly updated sites that show the data received by the geophones. In the past, these devices were used to monitor the movement of the earth around volcanoes.
We are taking advantage of all available technology to assess and monitor conditions on the mountain. Our geologists and surveyors rock-climbed and dropped from helicopters to safely install a series of survey prisms on the slope of the mountain. These markers allow us to watch and monitor mountainside movement.