4704 HARBOR LN 2018-01-02 MF Import

Geotechnical Engineering Evaluation Anderson Landslide Repairs 4704 Harbor Lane Everett, Washington September 15, 2009 NGA File No. 816909 Summary — page 2 slide are roughly in line. It seems the leaking water travelled through the upper sandy material found in the vicinity of the residence and eventually made its way onto the slope below the residence, saturating the upper portion of the slope. The over -saturation of the upper soils and subsequent loss of their shear strength was likely the direct cause of the slide. Other contributing factors may include the presence of undocumented fill and debris near the top of the slope from past grading operations and human activity. In our opinion, the recent tree removal front the slope was not the cause of this slide, although this action has rendered the slope more prone to more frequent and serious activity if conditions are not improved. 'free removal on steel) slopes call lend the slope more prone to shallow sloughing and erosion -type lailures. 'these events usually take place during the rainy season after the trees have been removed, as the surficial soils are exposed to the elements. The trees were removed in early June at the beginning of a long period of chy weather. The slide took pla-. in late June. No significant precipitation was experienced leading up to the slide. On the other hand, large-scale slides, such as the one that took place on this property, are usually caused by hydrostatic pressure build-up and soil saturation stemming front a subsurface water flow. hl this case, the substantial water leak that saturated the slope likely resulted in the loss of the soil shear strength and the slide, rather than the earlier removal of the trees. Eased on our explorations and site observations, it is our opinion that permanent stabilization of the site slope and prevention of further slope movement will require retaining walls and re -grading. We recommend a majority of the slope be stabilized using a soldier pile wall with tic -backs. To maintain impacts to the slope, the beams for the wall should be drilled using an open -hole method. If caving is encountered, the hole should be cased. After the installation of the soldier pile wall, unstable soil would be removed in front of the wall for lagging and tie -back installation. The tie -backs should extend past a no-load zone that is 20 feet wide. Below the wall, slope restoration and re -vegetation should continence to improve surficial stability of the slope. Groundwater was not encountered while drilling the explorations, but groundwater was observed emanating front the slope r,t several locations. In July, temporary water seepage was observed emanating from the slope within the slide scarp approximately 15 Icct below the top of slope. This seepage was attributed to the water leakage that we concluded as the cause of the slide. Pennanent water seepage was observed approximately 50 to 60 feet below the top of slope. We do not believe that the installation of the recommended wall will change the hydrologic make-up of the area since the majority of the water currently emanating from the slope is well below the bottom of the wall, and therefore the wall should not change the overall drainage picture of the slope. Also, vegetation will be restored on the slope which should improve the cvaportranspiruion capabilities of the slope. In the attached report, we have included specific recommendations for designing and installing the recommended shoring system. We have also provided recommendations for improving the erosion and drainage conditions of the site and slope. We should emphasize that maintaining the correct construction sequence and employing low -impact methods and techniques are of great importance to maintaining stable site conditions. Close cooperation between the contractor and all of the design teann members will be needed during the final design stage and during construction ofthis project. 3