3005 PANAVIEW BLVD 2016-01-01 MF Import

� 3 973-1164 J� io,i�� 3. PIELD INVESTiGATION — Our field investigation was mmpleted on June 17,1997. The smpe of work included field mapping the 9utface�a�8 of�e mud/debris flow,the slide scarps,geologic confficts,and the location of your house in relation to the mud/debris flow. Several exploration stations wem located in the main body of the slide and at selected areas of your property to assess the aubsurface mnditions and are shown on Figure 2. At these etatione we used either a post-hole digger or hand auger to explore the subsurface soil ^ mnditions. The locations of the exploration stations are shown on Figure 2,Site Plan. The subaurface soils were examined and classified,and a draft cross-section was prepared ttu'ough the body of the mud/debris flow,using a tape and clinauteter. The '" site plan and crosa sections presented in Figures 2 through 4 wem completed using field �eaeuring equipment such as a measuring tape,hand level and clinometer. They should be considered appro��• 4. RESULTS UP THE PIELD INVES'I'IGATION , 4.1 Geologic Conditione —� The geologic deposita forming the uPland ridge whern your house is located wern deposited by the advance and mtreat of the Vashon stade glaciera about 12 to 15 thousand yeaxs ago. The geologic mnditions as observed in the ravine are rnmposed of � dense glacial sediments consisting of advance outwash sand and till. These soils were consolidated by the vast weight of the overlying glacier ice. Over time the natural pmcess of weathering and erosion has sculpted the deposits fomung river valleys and ' ravinee. On the slopes botderin8 these features,the exposed sedimenb weather and forcn a loose soil layer which is called mlluvium Colluviiun deposib am found on most slopes overlying t1►e native geologic unib and are generally thickest near the toe of the slope. This loose mlluvium soil layer is susceptible to failure particularly during periods _, of heavy rainfall H'hen it may become saturated. ' We undetatand from interviews of several homeowners in your area that the east-facing i ateep slopes in your neighborhood have generally been stable for the past iwenty years or more. Colluvium laye=s on the slope wern thick and in many areas fill was also placed on the slope during house mnstruction. However,during the snow and rain storm of December,1996 and January,1997 there were numerous slope failures(slumPs and mud/debris flows)on the east-facing slopes. The failures were pmdominately mud/debris flows indicating intense saturation of the mlluvium layer on the slope. The unuaval climate rnnditiona mntributed to these failures when over a foot of snow was immediately followed by warming canditions and heavy rainfall causing npid melting of the snow and ground saturation of the near surface soils. Based upon the geologic mapping and subsurface explorations, the stratigrapluc sequence of soils exposed in the area of the slide appears to be as follows (from oldest to y ouungest): Golder Associates