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111 <br /> Administration Building Office Expansion Everett,Washington <br /> We recommend that all completed footing excavations be observed by a representative of our firm prior to <br /> placing mud mat, reinforcing steel, and structural concrete. Our representative will confirm that the bearing <br /> ' surface has been prepared in a manner consistent with our recommendations and that the subsurface <br /> conditions are as expected. <br /> Footing Drains <br /> We recommend that.perimeter footing drains be installed around the building addition. The perimeter drains <br /> should be installed at the base of the exterior footings as shown on Figure 3. The perimeter drains should be <br /> provided with cleanouts and should consist of at least 4-inch-diameter perforated pipe placed on a 3-inch bed <br /> of, and surrounded by, 6 inches of drainage material enclosed in a non-woven geotextile fabric such as <br /> Mirafi 140N (or approved equivalent) to prevent fine soil from migrating into the drain material. We <br /> recommend that the drainpipe consist of either heavy-wall solid pipe (SDR-35 PVC, or equal) or rigid <br /> corrugated smooth interior polyethylene pipe (ADS N-12, or equal). We recommend against using flexible <br /> tubing for footing drainpipes. The drainage material should consist of pea gravel or "Gravel Backfill for <br /> Drains" per Washington State Department of Transportation (WSDOT) standard specifications <br /> Section 9-03.12(4),see Figure 3. The perimeter drains should be sloped to drain by gravity, if practicable,to <br /> a suitable discharge point, preferably a storm drain. We recommend that the cleanouts be covered, and be <br /> placed in flush mounted utility boxes. Water collected in roof downspout lines must not be routed to the <br /> footing drain lines. <br /> Slab on Gracie Floor <br /> Subgrade Preparation <br /> We recommend concrete slabs-on-grade be constructed on a gravel layer to provide uniform support and <br /> drainage,and to act as a capillary break. We expect that slab-on-grade floors can be supported on the native <br /> very dense glacial till deposits provided that the capillary break subbase layers are constructed. Prior to <br /> placing the gravel layer, the subgrade should be evaluated and-proof rolled as described in the "Earthwork" <br /> section of this report. If necessary,the subgrade should be recompacted to a firm and unyielding condition. <br /> Design Parameters <br /> The gravel layer below slabs-on-grade should consist of at least 4 inches of clean crushed gravel with a <br /> maximum particle size of 11/2 inches and negligible sand or silt, such as WSDOT 9-03.1(4)C Grading No. 57. <br /> The gravel layer should be placed directly over native dense glacial soils. For slabs designed as a beam on an <br /> ' elastic foundation, a modulus of subgrade reaction of 100 pci may be used for subgrade soils prepared as <br /> recommended above. <br /> ' If water vapor migration through the slabs is objectionable,the gravel should be covered with a heavy plastic <br /> sheet, such as 10-mil plastic sheeting,to act as a vapor retarder. This will be desirable where the slabs will <br /> be surfaced with tile or will be carpeted. It may also be prudent to apply a sealer to the slab to further retard <br /> ' the migration of moisture through the floor. The contractor should be made responsible for maintaining the <br /> integrity of the vapor barrier during construction. <br /> i <br /> GEOENGINEERSI October 3O,2OO9 Page 5 <br /> ' File No.0482-031-02 <br />