The idea is to prevent the kind of landslides that occurred at this site near Castle Rock, Wash., on the railroad's busy double-track mainline between Portland, Ore., and Seattle, Wash., in February 1996. That winter, after record precipitation, the saturated soils gave way, burying the tracks under 1.5 to 3 m (5 to 10 ft.) of mud and another 1.5 to 3 m of tangled, uprooted trees. The landslides blocked heavy freight and passenger traffic for five days.
Like most areas of Washington west of the Cascade Mountains, long spells of rainy weather are the norm. In the previous few years, wet conditions had caused smaller traffic-blocking slides in the Castle Rock cut in the previous few years. The rainfall events that precipitated these slides far exceeded the norm. What's more, the massive slope failure in 1996 and subsequent failures in 1998 were far from typical. The five-day average precipitation preceding the 1996 landslides was the highest since records were first kept in 1931. The four months preceding the slides were also recorded as the wettest four-month period on record. Landslides again occurred at the site in December 1998 following heavy rains and the wettest combined November and December precipitation on record.