THE CANTERBURY CASTLE LANDSLIDE, PORTLAND, OREGON: A CASE HISTORY OF WHEN IT IS GOOD TO BUILD ON AN OLD LANDSLIDE SITE

This landslide first occurred as an earthflow on a vacant lot next door to the Canterbury Castle house in Portland’s West Hills on February 8, 1996. The house is on the national historical register and was built in 1938. It is located at the intersection of Fairview and Bennington Roads just below Canterbury Lane about 4 km west of downtown Portland. The failure happened in a soil of loess after 15 cm of rain had fallen in three days. The Dimensions of the earthflow were: scarp: 10 m; length: 30 m; width: 20m; volume: 6000 cubic meters; slope angle: 35 degrees; side scarps: 2-3 m. The cause of the slide was increased water in the soils from storm water runoff from Canterbury Lane at the top of the scarp. The first mitigation during the next summer was a 10 m high and 20 m wide I-beam wall at the scarp at a cost of $70,000. The next November another heavy rainfall caused hydrostatic pressure in back of the wall to “blowout” the structure and cause another earthflow of 3000 cubic meters. Lack of a water collection above the wall caused the buildup of stormwater and the cause of this second slide. The following summer the whole slope was frozen using an extensive pipe system and freon gas. Eventually in 2001, rockfill was placed at a depth of 8 m across the site and a new house built on it with a proper water collection system to stabilize the slope.