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

The first landslide occurred as an earthflow on a vacant lot next door to the Canterbury Castle 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 4 km west of downtown below Canterbury Lane. The 6000 cubic meter earthflow happened in loess soil after 15 cm of rain had fallen in three days. The cause of the slide was increased water in the soil from storm water runoff from Canterbury Lane at the top of the scarp. The first mitigation (next summer) was a 10 m high wall at the scarp ($70,000). Another heavy rainfall the next November caused hydrostatic pressure to "blow out" the wall and cause a second slide of 3000 cubic meters because of the lack of a water control system above the wall. The following summer the ground was frozen, a 8m rockfill was emplaced, and a new house built on it with proper water collection system. Total mitigation for 5 years was $300,000. Even with the building of the house on the lot, cracks developed in the castle. Development of a crack map and crack monitoring show an unsupported chimney is causing the stress to the castle.