1215 MADRONA AVE 2016-08-24 - Laserfiche WebLink

� 1 ' ' ' ' ' ' ' ' ' ' ' ' ' �I ' 1 1 Gustafson Residence Retaining Wall NGA File No 924115 1215 Madrona Avenue June 12, 2015 Everett, Washington Page 6 93 percent) are located within the northern portion of the property immediately below the existing concrete rubble walls. The core of tl�e slope is inferred to consist primarily of inedium dense/stiff or better native sand and silt deposits. Inclinations of up to 43 degrees on the slope indicate higl� strength and internal friction angle within the underlying soils. Periodic sloughing failures as well as shallow landslides are natural processes and should be expected on this slope during extreme weather conditions as has been experienced recently and in the past. It is our opinion that tl�ere is significant potential far erosion, soil creep, and shallow failures to continue to occur within tl�e loose surficial soils on the slope. Proper slope repairs and regular maintenance, along with vegetation improvements as recommended in this letter should iinprove the site stability conditions. CONCLUSIONS AND RECOMMENDATIONS General Based on our explorations and observations of the current conditions in the vicinity of the top of slope area, it is our opinion tl�at the proposed installation of a pin pile cantilever retaining wall along the top of the steep slope is feasible from a geotechnical standpoint. Due to the presence of loose, undocumented fill soils within the yard and steep slope witl�in the vicinity of the top of the steep slope, it is also our opinion that these soils may experience settlement and failures potentially impacting the back yard area, especially during a seismic event if the proposed retaining wall is not constructed. In our opinion, construction of the proposed retaining wall should aid in stabilizing the top of slope area along the yard area and liinit potential impacts associated with potential slope movements on the yard area and residence. Due to site constraints, and based on our explorations and site observations, we recommend the use of a "light" retaining wall to protect the top of the slope along the entire extent of the yard area, along with erosion control and drainage systems to be utilized below the new retaining wall. We have provided recommendations for design and installation of a new cantilever pipe pile wall with timber lagging along the top of the steep slope. Depending on the exposed height of the proposed retaining wall, tieback anchors will likely be needed to support lateral pressures associated with the retaining wall. We recommend that these anchors consist of grouted or mechanical anchors. We have also provided recommendations for iinproveinents on tl�e slope below the wall to lessen the potential for future sliding on the slope within the vicinity of the retaining wall. The approximate location of the proposed retaining NELSON GEOTECHNICAL ASSOCIATES, INC.