1205 50TH ST SW 2020-07-02 - Laserfiche WebLink

Geotechnical Engineering Evaluation NGA File No. 1033518 Larsen Property Garage May B,2018 Everett,Washington Page 5 is no significant potential for deep-seated slope failures under current site conditions. Proper site grading and drainage as well as adequate setback distances and foundation placement as recommended in this report should help maintain current stability conditions. CONCLUSIONS AND RECOMMENDATIONS General It is our opinion that the planned garage construction within this property is feasible from a geotechnical standpoint. It is also our opinion that the soils that underlie the site and form the core of the site slopes within the vicinity of the proposed development area should be stable with respect to deep-seated earth movements,due to their inherent strength and slope geometry. However,shallow failures could occur on the slopes in the loose surficial soil, especially during adverse weather or a significant seismic event. Proper erosion and drainage control measures as recommended in this report should reduce the frequency of such events. We:understand the proposed garage is to be located along the toe of the moderately sloping area below an existing concrete rubble retaining wall associated with the neighboring property above and to the east of the subject property. We did not specifically evaluate the conditions of the concrete rubble retaining wall as it is located on the neighboring property to the east. Based on our visual observations of the concrete rubble wall it appears that the retaining wall is in disrepair,likely not engineered, and if it were to fail could potentially impact the proposed garage structure below. As a result, we recommend that the temporary excavation cuts associated with the garage construction maintain a minimum 1.5 Horizontal to 1 Vertical(1.5H:1 V)gradient between the toe of the excavation and the base of the concrete rubble wall. The garage and associated excavation should be appropriately"located to maintain this setback. In addition to the proposed excavation setback from the upper concrete rubble wall,we recommend that the entire eastern wall of the garage be designed as a concrete retaining wall and extend the full height of the eastern garage wall. This eastern retaining wall would also be designed as a debris catchment wall to adequately protect the garage structure against potential failures on the slopes or the concrete rubble wall above. Extreme care should be taken during the excavation of the garage as to not adversely impact the existing slopes and/or the existing concrete rubble retaining wall above. The existing rubble wall should be continuously monitored during construction for any indications of instability. After the garage foundation has been completed, we recommend that the sloping area between the garage and the existing concrete rubble wall be restored and the resulting ground surface have an inclination no greater than 2H:1 V. Deep-rooting vegetation should be planted within the sloping area and maintained until established. NELSON GEOTECHNICAL ASSOCIATES,INC.