2320 CALIFORNIA ST Geotech Report 2025-07-25

i Allowable Bearing Pressure For foundations bearing on competent undisturbed dense to very dense glacial soils,the foundations may be designed using an allowable soil bearing pressure of 8,000 psf for isolated spread footings and continuous footings. The allowable soil bearing value applies to the total of dead and long-term live loads and may be ' increased by up to one-third for wind or seismic loads. If existing fill or unsuitable native soil is observed at the foundation bearing elevation, we recommend that the contractor remove the material and backfill with lean concrete or control density fill (CDF) having a design strength of at least 200 pounds per square inch (psi). Settlement Estimates Provided all loose soil is removed and the subgrade is prepared as recommended under the Construction Considerations section below, we estimate the total settlement of shallow foundations will be less than 3/4-inch. The settlements will occur rapidly, essentially as loads are applied. Differential settlement between comparably loaded column footings or along a 25-foot section of continuous wall footing should be less than 1/cinch. Foundation Design We recommend widths of at least 18 and 24 inches, respectively, for continuous wall and isolated column footings supporting the proposed addition. The design frost depth for the Puget Sound area is 12 inches; therefore, we recommend that the footings for the addition be founded at least 18 inches below lowest adjacent finished grade. We recommend that all utility trenches crossing under building foundations be back filled with CDF within a 1H:1V(horizontal:vertical)influence line projected down from the edges of the foundations. Lateral Resistance Lateral loads can be resisted by a combination of friction between the footing and the supporting soil,and by the passive lateral resistance of the soil surrounding the embedded portions of the footings. A coefficient of friction between concrete and soil of 0.4 and a passive lateral resistance corresponding to an equivalent fluid density of 350 pounds per cubic foot(pcf) may be used for design. The friction coefficient and passive lateral resistance are allowable values and include a factor of safety of about 1.5. If soils adjacent to footings are disturbed during construction, the disturbed soils must be recompacted; ' otherwise the lateral passive resistance value must be reduced. Construction Considerations Immediately prior to placing concrete, all debris and loose soils that accumulated in the footing excavations during forming and steel placement must be removed. Debris or loose soils not removed from the footing excavations will result in increased settlement. If wet weather construction is planned, we recommend that all footing subgrades be protected using a lean ' concrete mud mat. The mud mat should be placed the same day that the footing subgrade is excavated and approved for foundation support. 111 1 Page 4