1010 NORTH PARK DR 2016-01-01 MF Import

Churchill Residence 1-I501 1010 Nonh P�rk Drivc Apnl 10, 2003 Everett, Washington Page 2 conditions. Some of these issues �vill not become apparent until construction u�ilcss fiirthcr evaluation of the subsurface conditions is completed adjacent to the house. Specific recommendations tbr project desien and construction considerations are presented in the following sections. Slope Stability Analysis A slope stability analysis was performed for the site usine the `CSTr1BL�.2 computer program. The stability analysis �vas based on the generalized subsurface soil profile presented on Figure 2 of our 6 December 2002 geotechnical report and the proposed plan prepared by Chope(as and Associates lnc. The profile consists of glacial Lacustine deposits over Whidbey Pormation deposits with a 4%x-foot thick mantle of culluvium located on the slope below the proposed concrete retaining wall. Tlie proposed retaining wall and wall backfill �vere modeled using surcharge loads of I,500 psf and 770 psf, respectively. r1 friction angle of 30 deerees and a cohesion value of �0 psf �vere used for the colluvia] soils. A friction angle of zero degrees and a cohesion value of 1;000 psf were used for the elacial lacustrine deposits. A friction angle of 40 degrees and a cohesion value of zero were used for the Whidbey Formation deposits. Based on the relative density, grain size distriburion, depositional t�istory, and tl�e site specific boring data, it is our opinion that the referenced friction anele and cohesion values arc reasonable estimates of the site soil strength parameters. The above values also fall within the rangc of strength parameters listed in the Washingron State Department of Na[ural Resources Bulletin 73 Geotechnical Properties of Geoi��gic iLfaterials, 1959. Our analysis evaluated both static and d�mamic (seismic) conditions. The USGS Scismic Hazard Iviapping Project earthquake hazard map for the area indicated a peak horizontal ground acceleration (am�) of 0.27g for a ground motion with a 10 percent exceedance in 50 years. In this area, the 10 percent exceedance in 50 year design seismic event is approximately equivalent ro a Richter magnitude 7.0 earthquake. Our analysis used a pseudostatic coefficient (kh) of O.ldg based on dynamic embankment studies conducted and presenred by the Army Corps of Engineers in the publication Ratiaia(i>ing the Seisn:ic Cos�cient �Lletlrod, 1984. In our opinion, a minimum factor of safety of 1.5 under static cond tions and a minimum factor of safery of l. i under dynamic conditions would be acceptable for ttus project and site. Based on our analysis, the surficial colluvium layer on the slope located belo�v the existing timber pile wall is marginaliy stable relative to shallow failures. Our analysis indicated a static factor of safety of approximately 1.1 and a pseudostadc factor of safety of 0.9 relative to shallow su�cial slides within the colluvial soils that mantle the ravine slope and extend to a depth of about 3'/ to 4'/: feet below the surface. Given the relatively low safery factors associated with the colluvium, we recommend that the upper 5 feet of colluvial soils located do�vnslope from die wall be neglecred when calculating passive resistance. Zipocr Zcman Associates. Inc. I&905 33" Avenuc Wcst, Suile I l7 Lynnu�oW,lVashinynan 9&036 (425) 771 —7304 J-I501 I\� j/� �� \__. -