05/2007 - Active landslide on Perrigo Heights

2007 was the year I started my blog

"Landslides Threaten Big Trees, Sewer Alignment at Perrigo Heights" - by Susan Wilkins, Redmond, WA.

Preface: The initial geotechnical engineering report on Perrigo Heights was done in 1997. It's possible that the slope along the access road where the slides are now occurring was much more stable back then. At that time, the report stated that the whole property was stable. That was 10 years ago. Since then, time, gravity and severe weather have destabilized the north side of the property and it is sliding and unstable. This article and photograph are simply pointing this out.

A series of landslides along the access road on the south side of Perrigo Creek has toppled a number of trees and has left a majestic big-leaf maple precariously leaning with its roots exposed on the steep hillside. The tree sits on what is called a bench slide, a whole piece of the hill that is moving down-slope as a unit. It’s an historic slide that has been occurring slowly for years. It probably started years ago when the City of Redmond cut into the hillside when it built the access road that runs along the south side of the creek. The effects of gravity and the heavy rains this past year has caused the soil under the roots to break up and collapse. The landslide begins at the top of the slope where caving with the classic “scooped out” appearance can be seen on the relatively horizontal section of the hill where the proposed sewer line for the planned Camwest Perrigo Heights Development will run. In some places, the sewer line boundary will be within 15 feet of the top of the collapse.

Next to the dangling roots, the splintered trunk of a large cedar can be seen on the slope; the rest of the tree lies 20 feet below in Perrigo Creek where it toppled during the January 2006 storm. The landslide that sent the tree into the creek left a huge pile of mud and debris on the roadway. It also exposed a previously hidden clay layer that runs through the hillside. Rainwater percolating through the overlying sandy soils built up on the clay layer causing the overlying sand layer to become saturated and to slide.

When the Redmond City Council approved the variance allowing the sewer line to run down the exceptionally steep, unconsolidated slope at the east side of the property, they never even considered that the hillside might collapse out from under the horizontal portion of the sewer line on the north side. Oddly, the area where the landslides are occurring were supposed to be the backyards of lots 26 – 29 in the original 36-lot plat of the Perrigo Heights development proposed by Camwest. Obviously, the landslide danger on the north side of the hill was missed or understated.

Further down the access road, fresh soil from small landslides that have occurred within the past two weeks can be seen. Clumps of ferns that once lined the top of the slope are scattered along the hillside and in the roadbed where they have landed after the small-scale slides.
Additional information on landslides can be found at http://landslides.usgs.gov/ and www.ecy.wa.gov/programs/sea/landslides/about/geology.html.

-- The contributing author, Susan Wilkins, has a degree in Geology from Wellesley College. She is a Redmond, WA. citizen, active in the local land-use issues impacting our environment