Western Australia Chapter

Geotechnical Investigation of Mount St. Helens’ New Roadway after May 18, 1980 Eruption

George B. Deardorff, Golder Associates

The May 18, 1980, eruption of Mount St. Helens was the deadliest and most economically destructive volcanic event in the history of the United States. Nearly 230 square miles of the forest was blown over or left dead and standing; 57 people were killed; and 47 bridges, 24 km (15 miles) of railways, and 298 km (185 miles) of highway were destroyed. A mushroom- shaped column of ash rose thousands of feet skyward and drifted downwind, turning day into night as dark, gray ash fell over eastern Washington State and beyond.

In 1982, President Ronald Regan and the U.S. Congress established the Mount St. Helens National Volcanic Monument and authorized some US$200 million to investigate and reconstruct an upgraded two-lane highway along the North Fork of the Toutle River into the very mouth of the steaming crater. The original roadway was destroyed during the eruption.

In 1986, the Seattle office of Golder Associates was awarded a contract to investigate the geological/geotechnical features that would be encountered along several proposed alignments for the new highway. Winter conditions in the mountainous terrain; hundred year storm events along the Toutle River; and the remoteness and crater-like conditions with destroyed timber along the alignments presented many obstacles that were successfully overcome. Design options for final design and construction were established as a result of the investigation.

The presentation will highlight an overview of the volcanic landscape and present project photographs, a short video documenting the project and comment on present day activities associated with the volcano.

Engineers Australia members participating in AGS technical sessions can record attendance on their personal CPD logs. Members should refer to Engineers Australia CPD policy for details on CPD types, requirements and auditing guidelines.