Overlooking Puget Sound, the bluffs along Shilshole Bay from Golden Gardens to Carkeek Park are unstable and prone to landslides. On Thursday, January 6, a four foot high retaining wall above the railroad tracks running along the shoreline failed, allowing a small slide to close one of the two lines. Passenger rail service was halted for 48 hours.

Approximately 60 slides occurred across the city in 2005. Landslides are a yearly occurrence in Seattle. They are caused by a combination of precipitation, loose soil, and steep slopes, three things that are abundant throughout the city. Every winter the city organizes a series of landslide information meetings to raise public awareness of the problems and solutions.

On Saturday, February 25, around 60 city residents gathered in the cafeteria at North Seattle Community College to learn about landslides, and how to prevent them. Four city employees gave presentations based on their individual expertise. In addition, representatives from the American Society for Civil Engineers, the Association of Engineering Geologists, the International Society of Arboriculture, and Associated Building Contractors were on hand to talk one-on-one with people.

Water is the key ingredient needed for landslides. That was the principle message of the presentations. The US Geological Survey used data collected by the DPD's Seattle Landslide Study, and found that 15 days of cumulative precipitation followed by 3 days of intense precipitation offered ideal circumstances for landslides. As Bill Laprade, a geologist and vice-president of Shannon & Wilson, Inc., put it, "wet the ground, and then kick it off by adding a lot of water."

In Seattle, the landslide cycle begins in November and December, the city's wettest months. This soaking of the ground set up January as the busiest month for landslides, according to the Seattle Landslide Study.

The bluffs that run from Golden Gardens to Carkeek are particularly landslide-prone because of their geological makeup. The hills north of Ballard consist of a layer of clay topped by a layer of sand. Loose soil forms an unstable skin over these layers.

"Most of the sliding occurs at the contact between the sand and the clay," said Laprade.

The most common type of slide in Seattle is shallow colluvial. Laprade described it as a "thin or skin" slide. The loose soil sloughs off. These slides can occur suddenly, and carry debris flows, such as trees, rocks, and even structures.

The bluffs in the area are still settling from the last glacial period, according to Mark Mead, a senior urban forester with Seattle Parks and Recreation. They have not found their angle of repose, the natural shape a geologic feature takes on based on the material it is made of. Mead used a pile of sugar on a table as an example. Its sides take on a natural and stable slope, their angle of repose, after a given amount of sugar has slipped off of them. The bluffs continue to look for this equilibrium in part because of what they are made of, and partly because of the wave action of Puget Sound, which carries sand and soil out to sea.

An embankment protects the railroad lines from the wave action. Burlington Northern Santa Fe Railroad maintains the embankment and protects the tracks from landslides with a variety of measures. Over forty trains a day pass underneath the bluffs at Golden Gardens and Carkeek Park, according to BNSF. These include passenger trains operated by Amtrak and Sound Transit.

BNSF has experienced more slides than a typical winter in recent years, according to Gus Melonas, BNSF spokesman. Amtrak has halted service over ten times this winter. The most recent slide cost more than $1 million to clean up. The wall that gave way allowed debris to cover one of the two lines, but did not threaten any homes.

"Strategically-placed sensors played a critical role, along with human eyes" in detecting the slide, said Melonas. BNSF maintains the stability of the embankment through a variety of measures, including studies of hazard areas, retention walls, and culverts.

"Some of the things [BNSF has] done has enhanced the stability of the slope," said Laprade. He added, "You can't stop it because gravity still works."