5209 GLENWOOD AVE 2017-04-07 - Laserfiche WebLink

Revised Redi-Rock Retaining Waif Design Western Geotechnical Consuitants,Inc. 52XX Glenwood Ave,Property 1 14 91 2 Everett WA April 17,2015 As the results show, the site is stable from a slope stability perspective. The graphical results of the analyses are included in Appendix A. . to this report. Geologic Hazards and Mitigation Erosion Hazard The USDA Soil Conservation Service(SCS,)"Soil Survey of Snohomish County Area, Washington"has classified site soils as Alderwood-Urban Land Complex. The.Alderwood soil unit is located on till plains and it is derived from the glacial till. The permeability of the Alderwood soils is moderately rapid above the till but very slow through the till. Therefore, a seasonal perched water table should be anticipated in the less steep areas. Runoff is slow and the hazard of water erosion is slight. The glacial till and weathered till are NOT of the high erosion potential and we did not observe areas of erosion. Disturbance of the weathered topsoil zone could result in a short term erosion/siltation hazard. Therefore during construction any areas that are disturbed should be revegetated as soon as possible. All stormwater collected from impervious surfaces, such as roofs,paved areas and footing drains should be collected and transmitted in a tightline pipe to the stormwater disposal system for the site along Glenwood Ave. Landslide Hazard The site is underlain by hard glacial till at approximately 2 to 3 feet below grade. Because of the relatively high shear strength of the till soils,the potential for deep-seated failure to occur on the site is highly unlikely. To verify landslide hazard, we performed stability analyses which are detailed in an early section to this report. The analyses demonstrated that deep seated failure is highly unlikely at the site. Seismic Hazard Puget Sound region is an active seismic zone. The site is underlain at shallow depth by hard glacial till; therefore, the potential for seismic hazards such as landslides and liquefaction is minimal. We analyzed slope stability under seismic conditions in an earlier section of this report. The proposed block wall was designed to be stable under seismic loading conditions. Redi-Rock Wall Design We analyzed the wall both as a gravity wall and a mechanically stabilized earth (MSE)wall because our analysis revealed that a gravity wall cannot exceed 15 feet total height(10 blocks). The section of wall that is greater than 13.5 feet (up to 23 feet total) will require the use of geogrid reinforcement(MSE wall)to provide a stable design. The design includes construction of a gravity wall up to 9 blocks high and then transition to an MSE wall for the higher portion of the wall. The wall is highest where the driveway enters the property off of Greenwood Avenue. At the highest point the wall will be 15 blocks high, up to 23 feet. As the drive goes up the slope to the top of the property, the wall becomes shorter. When the wall is greater than 9 blocks high, geogrid is required between each of the blocks,from the top of the base block to the base of the 3