The grade immediate below the track structure is called the subgrade. It was disturbed by an excavator during a recent construction project conducted adjacent to the track.
Good track is easy to take for granted, but a safe and reliable railroad depends on it. And good track requires a stable subgrade, especially within the 1:1 live load zone beneath the track. (The 1:1 live load zone is the area that extends downward from the ends of the ties at a 45-degree angle.) Steep slopes, poor soils, seasonal flooding, adjacent uses, earthquakes, and heavy annual precipitation are all among the conditions that must be considered to ensure stability. So when recent construction activities were found to have affected the subgrade at Snoqualmie Falls, immediate steps were taken to protect the track and trains.
A rock slope was constructed to stabilize and strengthen the subgrade for more than 200 linear feet. Geotextile fabric was placed below the rocks to improve performance of saturated soils.
First, for trains already operating, speed was immediately reduced to a crawl to reduce dynamic forces on the track and subgrade. Second, a licensed geotechnical engineer was called in to inspect the subgrade below the track and determine if and under what conditions trains could continue to operate. Third, rehabilitation work was performed to mitigate the issues discovered in the affected area using a design that was reviewed by other engineering professionals.
A new rock slope below the track at Snoqualmie Falls.
The long-term rehabilitation plan was developed by the Museum’s geotechnical engineering consultant at PanGEO Inc and reviewed by other knowledgeable professionals including Museum volunteer Dave H., who is a civil engineer – thanks Dave!
The design uses “heavy loose rock,” which are large rocks 30 inches and more across. It relies in part on prior stabilization work performed when soil anchors were driven under the track and a retaining wall was installed at the toe of the slope. To address the current issue, the slope on the subgrade below the track was covered with geotextile fabric and the heavy loose rocks were keyed into the sloped using an excavator. This design places several hundred tons of rock on the subgrade slope and restores integrity to the 1:1 zone beneath the track. However, work did not stop there. Areas affected by construction activities that were outside the 1:1 zone but within the 2:1 slope of the railway grade are being strengthened with a foot or more of quarry spalls three to six inches in diameter.
Work took three days to perform and was completed between scheduled trains. What is most impressive to Spike is that the total elapsed time between discovering the issue and implementing final rehabilitation was just four days! Trains are continuing to operate and will resume track speed when authorities determine that stability has been fully restored. So a stable subgrade once again supports good track.
