3121 TULALIP AVE 2026-02-05 - Laserfiche WebLink

Geotechnical Engineering Evaluation NGA File No. 959616 Watson Residence Addition July 6,2016 Everett,Washington Page 12 Slab-on-Grade The proposed addition slab-on-grade should be supported on subgrade soils prepared as described in the Site Preparation and Grading subsection of this report. If moisture control is important, we recommend that all floor slabs be underlain by at least six inches of free-draining gravel with less than three percent by weight of the material passing Sieve #200 for use as a capillary break. We recommend that the capillary break be hydraulically connected to the footing drain system to allow free drainage from under the slab. A suitable vapor barrier, such as heavy plastic sheeting(6-mil minimum), should be placed over the capillary break material. An additional 2-inch-thick moist sand layer may be used to cover the vapor barrier. This sand layer may be used to protect the vapor barrier membrane and to aid in curing the concrete. Site Drainage Surface Drainage: The finished ground surface should be graded such that runoff is directed away from the planned structures and the slope. Water should not be allowed to collect in any areas where footings, slabs, or pavements are to be constructed. Final site grades should allow for drainage away from the structures and steep slope. 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. 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. Water should not be allowed to flow over the slope. All roof downspouts, footing drains, and yard drains should be tightlined to an approved discharge system and away from the slope. We recommend the use of footing drains around the planned addition. Footing drains should be installed at least one-foot below planned finished floor elevation. The drains should consist of a minimum four- 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, or drainage composite may be used instead. The free-draining material or the drainage composite 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 NELSON GEOTECHNICAL ASSOCIATES, INC.