3829 SHORE AVE 2018-01-02 MF Import

surface backwasting failures. If these conditions can be reduced or eliminated. the rate of backwasting should be reduced. Of particular interest is the area of recent slide activity near approximate STA 24+50 (below a�out 3409 Shore Avenue) . Within this area� we believe come younger Vashon age soils occur near the top-of-slope. �hese soils are more permeable and looser than the surrounding slope area, and considerable debris placement along the top-of-slope has occurred. It appears the Iooser upper ele- vation aoils and debria have slid down the slope above the more dense underlying soils. This soil change area, coupled with obaerved ground water outcropping, debris placement and under- lying dense soil have created a particularly active area for slope instability. POTENTIAL INSTABILITIES The area along Shore Avenue between Powder Mill Gulch and Edge- water Creek is basically totally developed in single family resi- dences. Review of 194� serial photograghs of the area indicated that a substantial portion of the development had occurred at ', that time. ThereSore instabilities commonly associat�� with development of an area from the natti^al setting are expected to � be o£ a low level of the incidences. Moat oP the instabilities of the slope are expected to be associated with the natural back- ; wasting of the slopes and local man induced local accelerations oP the backwasting. Backwasting of the slope occurs as the natural, dense soils � deteriorate with time reducing their strength pruperties, and then slide down the slope to establish a flatter slope in line with the reduced strength p.roperties. Some of the factors which reduce the strength are freeze-thaw cracking, weather5.ng and i ground water seepage. Once the soils are weakened, they are ! suaceptible to overloading by additional weight being placed on the slope and surface water erosion, particularly in areas of poor vegetative cover. -8-