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, ' ' ' ' ' 1 ' ' ' � 1 ' , 1 ' ' ' Geotechnical Engi�leei-iilg Letter Alllussaini Slope Stabilization and Retaining Walls Everett, Washington NGA File No. 867612 December 10, 2012 Page 8 used to control erosion. Areas disturbed during construction should be protected from erosion. Measures taken nzay include divertinQ surface water away fi-om the stripped areas. Silt feuces or straw bales should be erected to prevent inuddy water from flowing over the site slopcs or into the existing storm system. Disturbed areas should be replanted with ve�etation at the end of construction as part of� an ap�roved erosioil control atid slope re-vegetation plan. "I'he vegetalion should be maintaiiled until established. Final grading should inco�porate appropriate erosion control measui-es to 1-outc stor�nwater runoi�f away fi-oiu the top of slopE aild to appropi-iate ciiscliarge locations. Runoff generated within the site, including roof downspouts, yard areas, and any other hard surfaces should be i�ivestigated atld collected into catcl� basins and yard drains and tightliiled into an approved stoi-mwater managenlent system. Under no cu-cumstances should runoff be allowed to flow over the slope either during construction or on a permanent basis after construction has been completed. Vegetation on the slope should consist of decp-i-ooted, drought-resisting vegetation. All irrigation systems on o�� near the slope should be avoided. Future yard waste, gass clippiilgs, or any othcr waste material or household trasll sho�ild never be cast over the slope or piled tiear the walls. Sucl� activity could lead to futw�e slope instability. Additionally, drainpipes associated with Che ilew Keystone walls, sl�ould all be tightliiled to tlie bottrnn of the slope to an appi-oved stormwater managen7ent system. Site Preparation and Grading After erosion control ineasures are inlplemented, site preparation should consist of removinb the entire recently placed fill nzaterials and vegetation. Our explorations generally encountered approximately 4.0 to 5.0 feet of silty fine to medium sand with gravel that we interpreted as the i-ecently placed fill ulatei•ial. However, additioiial stripping may be required if ulore �mdocumentecl tlll, loose soil, or organic materials are eilcountered in unexplored at•eas of the site. The organio-rich materials should be removed from tl�e site. Sonie of tlle granular undocu�llented fill soils could be re-used for wall backflll bchind tlle proposed walls depending on the moistui-e co�ltent at the time oi� placement. We should be retained to evaluate proposed structw-al fill NELSON GEOTECHNICAL ASSOCIATES, INC.