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November 18, 2004

A past blast: building the road to St. Helens

As the volcano roars to life again, engineers recall the challenges involved with rebuilding parts of the 48-mile Spirit Lake Memorial Highway.

By DAVID FINDLEY and BOB PLUM
Golder Associates

Photo courtesy Golder Associates

A drill crew in 1986 worked along the steep slopes of the Toutle River Valley to reconstruct the highway destroyed by the 1980 eruption of Mount St. Helens.

When Mount St. Helens blew (or rather puffed) its way into the world's headlines this autumn, many Pacific Northwest residents thought back to the mountain's previous major episode on May 18, 1980.

It was one of those defining moments in many lives, and "Where were you when …" conversations dominated local offices for weeks.

The volcano also provided many defining moments for the people who dealt with the aftermath, particularly those of us who were involved in rebuilding the Spirit Lake Memorial Highway, or state Route 504. This 48-mile road takes visitors from Interstate 5 to the last road-accessible viewpoint overlooking the volcano, Johnston Ridge Volcanic Observatory.

The original Route 504 was buried under hundreds of feet of volcanic avalanche and mudflow deposits.

For the surveyors, engineers, construction workers and state Department of Transportation staff who worked on rebuilding the road, it was a major challenge as well as a career highlight. How often in one's career is there an opportunity to build an entire new roadway along challenging slopes in the sights of an active volcano?

The Seattle office of Golder Associates was responsible for geotechnical exploration and engineering of 17 miles of road from Hoffstadt Bluffs to Coldwater Creek. Other companies involved in the repair of the road included PC Exploration for the exploratory drilling and sampling, and Exploration Products, who provided the remote base camp and life-support services.

A seven-year wait

While many tourists have traveled the road to view this natural wonder, perhaps few remember that rebuilding the road had to wait seven years after the 1980 eruption.

One reason is that the road became a political football. Derided by many as "the road to nowhere" because there were no longer any residences or commercial buildings along it, there was considerable pressure to not rebuild the road.

Another reason for the delay is that nobody knew for sure if the mountain was just taking a breather until its next eruption. Despite all the science leveled at them over the years, our ability to predict volcanoes' behavior is far from perfect.

When additional federal funding became available, the new route would not go to Coldwater Lake as originally planned, but stop at Johnston Ridge, the current site of the Johnston observatory. This location is named after David Johnston, the geologist whose final radioed words "Vancouver, Vancouver, this is it!" provided warning of the 1980 eruption. Johnston was perhaps the first of the 57 people who perished in the blast.

The new road would be relocated from the valley floor to the south-facing slopes of the North Fork of Toutle River for two main reasons: The old route was covered by over 100 feet of unstable volcanic debris, and rebuilding through the valley would put the new road squarely in the sights of the next eruption of the mountain.

Surviving base camp

The Golder team members involved in rebuilding remember the challenges of the remote base camp from which we operated — in extreme weather conditions including winter snowfall — from January to May 1986. To access the remote borehole locations, we had to use bulldozers and other heavy equipment. For other difficult locations such bridge piers in steep narrow drainages, we used helicopters.

During the field phase of the project, all staff — drillers, field geologists and engineers, design team members and administration — were housed in a 40-person field camp. This site included living quarters, office space and support equipment. We also had a soils laboratory on site to perform soils index testing and point-load testing of cored rock samples.

As it happened, the evening before "kick-off," the region was hit by a hundred-year storm that washed out the Toutle River Bridge, which had provided the main access to the alignment. Access now required a 20- to 30-mile detour.

To overcome this problem, a sacrificial bridge was constructed with an old railroad flat car that could be easily repaired and replaced if subsequent washouts occurred, and they did.

One challenge specific to this volcanic terrain was with wet, saturated volcanic debris deposits. While much of the volcanic debris is solid enough to support a roadway, there were anomalies, including "back eddies" in the flow of volcanic material, which were unstable. In an earthquake, these areas might liquefy into a soft substance that could swallow a structure built on the surface.

As luck would have it, a major back eddy occurred at the site of the first major bridge across Coldwater Creek, the outlet of Coldwater Lake on the section of the road we were working on.

Engineering for a quake

Dealing with this involved drilling a series of holes down 150 feet to the base of the volcanic material, packing the borehole with explosives, and setting off our own miniature earthquake to "densify" the material and make it able to support the bridge abutments.

While we were fairly certain of success, some observers thought the blast would cause a major collapse of that part of the ground, and so there was quite an audience for the event. Perhaps to onlookers' disappointment, the explosion produced the result that engineers had expected.

A complication in this event was Coldwater Creek, which was a product of the rearranged topography of the mountain following the 1980 eruption, and which had become home to a significant fish population. Our contract for the blast required an attempt to protect the fish by scaring them away from the part of the creek closest to the blast. We were to do this by having an employee, just before the blast, run through the creek, splashing loudly to drive the fish into safer waters.

We complied, although we fully expected that the fish would fail to understand the evacuation warning, and simply hide under some convenient rocks until the commotion was over. Whatever the fish thought or did, we checked after the blast and did not find any floating on the surface.

Making sure that the highway would be safe and durable, Golder and its business partners carried out substantial research and testing. Facets of the project included:

Drilling and logging over 400 borings to evaluate soil and rock conditions for cuts, embankment foundations, seven major bridge foundations and several thousand feet of retaining wall along the route.
Mapping engineering geology, hazard assessment of landslide, debris flow and avalanche hazards along the route.
Mapping surface hydrology features for culvert and highway foundation design.

The cost to rebuild the highway was $165 million. It has proved a tremendous success with tourists and, consequently, local businesses that depend on tourist traffic.

The future of the road depends on the mountain's own plans regarding eruptions.

Spirit Lake Memorial Highway is not the only road in the state running through terrain that could be affected by a volcanic eruption. Mount Rainier is one of the most likely-to-blow volcanoes in the region, and there are previous mudflows from Rainier that extend to Puget Sound, over 40 miles from the mountain's peak.

Residents of Orting and Puyallup live on historic mudflows coming from Mount Rainier. The community of Orting has recognized the potential danger and developed evacuation plans, installed a system of warning sirens and regularly practices school evacuations.

Mount St. Helens and other volcanoes dotting the state are a reminder that nature is a powerful force, and sometimes there is nothing we can do but live with the consequences — and rebuild when we can.

David Findley is an associate and Bob Plum a principal in the Seattle office of Golder Associates.

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