STABILIZATION OF A LANDSLIDE PRONE SLOPE IN LOESS SOIL WITHOUT GRADING AND WALLS

On a half-hectare piece of property in southwest Portland, Oregon on the east facing slopes of the Tualatin Mountains lies a house and surrounding property that has had recurring landslides after large rainfall events. The landslides are formed below a scarp which is between the house and Fairmont Blvd. below. Past events of debris moving onto the road have been recorded by the City of Portland in 1973, 1983, 1996, 2004, 2007, and lastly 2021. Sizes of the landslide debris over time have been generally about 1000 cubic meters. The slope is mainly ML (silt loam) soils of the Portland Hills Silt Formation which is very prone to small slumps and earth flows. The city has asked the owner to come up with a plan to stop the landslides from moving onto the road at the bottom. I have come up with an environmentally sound approach that will not require regrading of the slope and building of walls. First step was developing a collection system for the gathering of storm water from the house into a central system that then can be piped to the bottom of the slope in flex tubing. This prevents concentration of water on the slope that leads to the reactivation of past landslides. Secondly, I recommended planting of 50 Douglas fir trees (at least 1 meter tall) about 2 meters apart between the house and the road. Removal of English ivy was also recommended, and constant watering during the dry summer would ensure survival of the trees. The approach creates an environmentally enhanced site and costs less than extensive regrading and building of walls. I recommend this method for landslide prone slopes: collecting the storm water and then planting of deep-rooted trees to the bottom of the slope.