2710 14TH ST BLDG A Geotech Report 2023-12-29

If water vapor migration through the slabs is objectionable, the capillary break gravel layer should be covered with heavy plastic sheeting at least 10-mil thick to act as a vapor retarder. This will be desirable where the slabs are in occupied spaces or will be surfaced with tile or will be carpeted. It may also be prudent to apply a sealer to the slab to further retard the migration of moisture through the floor. The contractor should be made responsible for maintainingthe integrity of the vapor barrier during construction. Additional water proofing measures that may be needed should be evaluated during design. 4.3.3. Underslab Drainage Groundwater could accumulate below the slab -on -grade for buildings that are cut into the hillside and that have below -grade walls or utility connections that tie into the buildings from the west side. To mitigate this condition, we recommend that the slabs of buildings that meet the criteria listed above be constructed with underslab drainage to collect and discharge groundwater from below the slabs. This can be accomplished by installinga 4-inch diameter, heavy -wall perforated collector pipe in a shallowtrench placed directly below the capillary break layer. The trench should measure about 18 inches wide by 12 inches deep and should be backfilled with the same drainage material as described in Section 4.2.4. The drainage material should be wrapped in a nonwoven geotextile, such as Mirafi 140N. We recommend installing a single underdrain collector pipe below the long axis of the buildings. The collector pipe should be sloped to drain and discharge into the storm water collection system to convey water off site. If connected to the footing drain pipe, the invert of the underslab drain pipe must be at a higher elevation than the footing drain pipe to prevent water from flowing under the buildings from the perimeter system. The pipe should also incorporate cleanouts, if possible. The cleanouts could be extended through the foundation walls to be accessible from the outside, or could be placed in flush mounted access boxes cast into the floor slabs. 4.4. Below -Grade Walls and Retaining Walls We understand that below -grade retaining walls may be necessary for construction of the buildings as well as other site improvements. The following recommendations should be used in design of below -grade walls that are intended to act as retaining walls and for other retaining structures that are used to achieve grade changes. 4.4.1. Design Parameters Lateral earth pressures for design of below -grade walls and retaining structures should be evaluated using an equivalent fluid density of 35 pcf provided that the walls will not be restrained against rotation when backfill is placed. If the walls will be restrained from rotation, we recommend using an equivalent fluid density of 55 pcf. Walls are assumed to be restrained if top movement during backfilling is less than H/1000, where H is the wall height. These lateral soil pressures assume that the ground surface behind the wall is horizontal. For unrestrained walls with backfill sloping up at 2H:1V, the design lateral earth pressure should be increased to 55 pcf, while restrained walls with a 2H:1V sloping backfill should be designed using an equivalent fluid density of 75 pcf. These lateral soil pressures do not include the effects of surcharges such as floor loads, traffic loads or other surface loading. Surcharge effects should be included as appropriate. Below -grade walls for buildings should also include seismic earth pressures. Seismic earth pressures should be determined using a rectangular distribution of 8H in psf, where H is the wall height. GEoENGINEERS� October3O,2O19 Page9 ' File No. 21288-002-00