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Geotechnical Engineering Evaluation NGA File No. 961916 Mason Residence Addition and Deck August 24,2016 Everett,Washington Page 14 / 2.5 Site Drainage Surface Drainage: The finished ground surface should be graded such that runoff is directed to an appropriate stormwater collection system. Water should not be allowed to collect in any areas where footings, or slabs are to be constructed. Final site grades should allow for drainage away from the structure. We suggest that the finished ground be sloped at a minimum gradient of three percent, for a distance of at least 10 feet away from the structures. Surface water generated from paved areas and roof drains should be collected by permanent catch basins and drain lines, and be routed into an appropriate discharge system. No surface water should be allowed to flow towards the slope either during construction or on a permanent basis after the improvements are complete. Subsurface Drainage: If groundwater is encountered during construction, we recommend that the contractor slope the bottom of the excavation and collect the water into ditches and small sump pits where the water can be pumped from the excavation and routed to a suitable discharge point. We recommend the use of footing drains around the proposed structure. Footing drains should be installed at least one foot below planned finished floor elevation. The drains should consist of a minimum 4-inch-diameter, rigid, slotted or perforated, PVC pipe surrounded by free-draining material wrapped in a filter fabric. We recommend that the free-draining material consist of an 18-inch-wide zone of clean(less than three-percent fines), granular material placed along the back of walls. Washed rock is an acceptable drain material. The free-draining material should extend up the wall to one foot below the finished surface. The top foot of backfill should consist of low permeability soil placed over plastic sheeting or building paper to minimize the migration of surface water or silt into the footing drain. Footing drains should discharge into tightlines leading to an appropriate collection and discharge point with convenient cleanouts to prolong the useful life of the drains. Roof drains should not be connected to wall or footing drains. We recommend that all residence downspouts and yard drains be investigated to understand where they are directed. At a minimum, we recommend that all residence downspouts and yard drains be tightlined and directed to discharge at the bottom of the slope or to a suitable discharge location. If any irrigation systems are located along the steep slopes, we would recommend that they should also be abandoned and removed. CONSTRUCTION MONITORING We recommend that we be retained to provide construction monitoring services to evaluate conditions encountered in the field with respect to anticipated conditions, to provide recommendations for design NELSON GEOTECHNICAL ASSOCIATES, INC.