1707 SCENIC DR SNO CO PUD Geotech Report 2022-10-25

• if wet weather construction is planned, we recommend that all footing subgrades be protected using a lean concrete mud mat or a clean crushed gravel layer. The mud mat should be placed the same day that the footing subgrade is excavated and approved for foundation support. We recommend that all completed foundation excavations be observed by a representative of our firm prior to placing mud mat, reinforcing steel, or structural concrete. Our representative will confirm that the bearing surFace has been prepared in a manner consistent with our recommendations and that the subsurface conditions are as expected. Slab-on-Grade — Control Building Subgrade Preparation The control building floor slab can be founded on imported structural fill, which is recommended to support the building foundation. The exposed soil should be firm and unyielding. Disturbed areas should be re-compacted if possible or removed and replaced with compacted structural fill. Design Parameters The slab-on-grade floor should be supported on a minimum 4-inch-thick layer of 11/z-inch minus clean crushed gravel with negligible sand or silt, to provide uniform support and form a capillary break beneath the floor. For slabs designed as a beam on an elastic foundation, a modulus of subgrade reaction of 100 pci may be used for design. If water vapor migration through the floor slab is objectionable, the gravel should be covered with a heavy plastic sheet, such as 10-mil plastic sheeting, to act as a vapor retarder. This will be desirable where the floors will be surfaced with tile or will be carpeted. It may also be prudent to apply a sealer to the floor to further retard the migration of moisture through the floor. The contractor should be made responsible for maintaining the integrity of the vapor barrier during construction. Mat Foundations General We recommend that the transformers be supported on mat foundations founded on structural fill or control density fill (CDF) extending down to medium dense to dense native soils as described in the Subgrade Preparation section of the report. Existing fill should be removed from below the transformer foundations. Other structures associated with the switch yard equipment may be supported on mat foundations as described below. If foundations are supported on structural fill then the zone of structural fill should extend beyond the edge of the foundation a distance at least equal to the thickness of the structural fill. CDF having a minimum design strength of 200 psi should be beyond the edges of the foundations by one-half the excavation depth required to remove unsuitable soils. We understand that light-weight equipment located in the 115kV yard may also be designed to be supported on shallow foundations, including mat foundations. For light-weight equipment we recommend that the foundations be supported on at least 2 feet on structural fill, placed and compacted on a prepared subgrade. The subgrade should be compacted to at least 95 percent MDD or to the extent practical prior to placing the structural fill layer. The structural fill should extend beyond the edges of the foundations by at least 2 feet. GEOENGINEER� October24,2014 Page 14 File No.0482-027-02