1343 MADRONA AVE Geotech Report 2022-10-25

• Geotechnical Engineering Report Stark Residence Addition 1343 Madrona Avenue Everett, Washington December 8, 2014 RN File No. 2902-001A Page 5 The sections were analyzed using the Simplified Bishops method of slices. Slide generates random potential failure surfaces and determines their corresponding factors of safety with respect to failure. The factor of safety is defined as the ratio of the internal soil strength divided by the gravity driving forces that cause failure. By generating a large number of random surfaces, the factor of safety can be obtained as the lowest number calculated. Proposed conditions were used to calculate static slope stability in Cross-Section A-A'. We used the water levels observed in the explorations as the water surface in our analysis. The factor of safety results are shown in Figure 7. Factors of safety against deep-seated failures under static conditions were above 1.5 for the proposed conditions. We evaluated the slope stability for seismic conditions by using the peak ground acceleration (PGAm) of 0.596g obtained from ASCE Standard 7. Table 10.1 of Soil Strength and Slope Stability by J. Michael Duncan and Stephan G. Wright references published information from Makdisi and Seed (1978) indicating that an acceleration multiplier of 0.2 is applicable for a magnitude 8.25 earthquake. The horizontal acceleration of 0.596g(.2) = 0.119g is less than the common value of 0.15g. Therefore, we used the more conservative 0.15g in our design and obtained a factor of safety greater than 1.15. The factor of safety results are shown in Figure 8. Site Preparation and Grading The first step of site preparation should be to strip the vegetation, topsoil, loose soils and pavement to expose medium dense or firmer native soils in building areas. The excavated material should be removed from the site, or stockpiled for later use as landscaping fill. The resulting subgrade should be compacted to a firm, non-yielding condition. Areas observed to pump or yield should be repaired prior to placing hard surfaces. The on-site transitional beds likely to be exposed during construction are considered moisture sensitive, and the surface will disturb easily when wet. We expect these soils would be difficult, if not impossible, to compact to structural fill specifications in wet weather. We recommend that earthwork be conducted during the drier months. Additional expenses of wet weather or winter construction could include extra excavation and use of imported fill or rock spalls. During wet weather, alternative site preparation methods may be necessary. These methods may include utilizing a smooth-bucket trackhoe to complete site stripping and diverting construction traffic around prepared subgrades. Disturbance to the prepared subgrade may be minimized by placing a blanket of rock spells or imported sand and gravel in traffic and roadway areas. Cutoff drains or ditches can also be helpful in reducing grading costs during the wet season. These methods can be evaluated at the time of construction. Structural Fill General: All fill placed beneath the addition or other settlement sensitive features should be placed as structural fill. Structural fill, by definition, is placed in accordance with prescribed methods and standards, and is observed by an experienced geotechnical professional or soils technician. Field observation procedures would include the performance of a representative • Robinson Noble, Inc