1208 50TH ST SW 2017-01-27 - Laserfiche WebLink

' 1 ' Geotechnical Lngineeriilg Letter Alllussaini Slope Stabilizatiou and Retaining Walls Everett, Washington NGA File No. 867612 Dece�nber 10, 2012 I'abe 5 Hazards associated wit11 seismic activity include lic�uefaction potential and a�liplification of � ground motion by soft deposits. Liquefaction is caused by a rise in pore pressures in a loose, iine sand deposit beneath the groundwater table. It is our opinion that tlie glacial soils iilterpreted to � underlie tlle site at deptll ]lave a low potential for liquefaction or amplification of ground motiou duc to their generally mediu�l� dellsc or better conditioii. ' 1 ' ' ' i� 11 � , 1 "1'l�e dense glacial till soils in�eipreted to Iorm lhe core of the site slope at depth are coilsidered st�ble with respect to decp-seated slope failures. However, tl�c loose surficial materials and undocunieuted t7ll on the slope have the potential for shallow slougl�ing failures during seisnlic events. Sucli events should not affect lhe planned structures provided the cetaining wa11s al�d along with all erosion controL and drainage systems are designed and constructed as recom�nended in tlus letter. Erosion Hazard TIIe ci•iteria used for determination of erosion hazard areas include soil type, slope gradient, ve�etation covcr, and groundwater conditions. The erosion sensitivity is related to vegetative cover aud the speeific suriace soil types, which ai-e related to the underlyiug geologic soil uilits. The Soil Survev of Snohomisl� County, Washin rt� by the Soil Conservation Service (SCS) was reviewed to deteculine the erosion hazard of the on-site soils. The site stu-face soils were classiiied usiilg the SCS classitication systelil as Alder�vood-Everett gravelly sandy loa�ns, 25 to 70 percent slopes. "I'hese soits are listed as having a moderate hazard of water ei-osion. Tl1e native soils should have a low to nzoder�te hazard for erosion ii� areas tl�at are not disturbed �nd where the vegetation cover is not removed. The undocunlented fill found on the slope has a high potential for erosion in its current condition. Landslide Hazard/Slope Stability The criteria used for evaluation of landslide hazards include soil type, slope gradient, and � grouiidwater conditions. Portions of the slope have an inclination of �pproxi�nately 29 degrees (55 percent). We observed iildicatio�is of signiticant slope il�ovemeiit such as large tensioil cracks ' 1 ' along tl7e top of the slope aucl recent soil erosion and movement within the southei-�I portion of the NELSON GEOTECHNICAL ASSOCIATES, INC.