7800 EVERGREEN WAY HYUNDAI Wetland Report 2025-08-05

4.2. Shallow Foundations The planned building addition is underlain by 2 to 8 feet of compressible loose to medium dense fill soils. Unsuitable fill soils and highly weathered glacial till deposits will vary across the planned building addition. We anticipate that competent dense glacial till deposits will be encountered about 2 feet below existing site grades (boring B-3) at the south end of the planned addition and about 8 feet below existing grade (boring B-1) at the north end of the planned addition. We recommend that foundations for the building addition be supported on: (1) undisturbed dense to very dense glacial till, (2) structural fill extending to dense glacial till deposits, or (3) at least 2 feet of structural fill. Exposed subgrade soils consisting of fill or weathered glacial till should be compacted prior to placing the 2-foot-thick structural fill layer below footings. The structural fill zone should extend at least 2 feet beyond the edges of the foundations. Footings supported on undisturbed dense to very dense glacial till deposits may be designed using a maximum allowable bearing pressure of 5,000 psf. Footings supported on structural fill that extends down to undisturbed glacial deposits or is supported on at least 2 feet of compacted structural fill may be designed using a maximum allowable bearing pressure of 3,000 psf. The allowable bearing pressures may be increased by one-third for short duration loads such as wind or seismic events. The design frost depth for the Puget Sound area is 12 inches, therefore, we recommend that all exterior footings be founded at least 18 inches below lowest adjacent finished grade. Interior footings should be founded at least 12 inches below bottom of slab or adjacent finished grade. For shallow foundation support, we recommend widths of at least 18 and 24 inches, respectively, for continuous wall and isolated column footings supporting the proposed building addition. 4.2.1. Foundation Settlement We estimate that the post -construction settlement of footings founded on the undisturbed dense glacial till deposits or on at least 2 feet of structural fill as recommended above, will be less than 1 inch. Differential settlement between comparably loaded column footings or along a 25-foot section of continuous wall footing should be less than 1/rinch over a distance of 25 feet. We expect most of the footing settlements will occur as loads are applied. Loose or disturbed soils not removed from footing excavations prior to placing concrete will result in additional settlement. 4.2.2. Lateral Resistance Lateral loads can be resisted by passive resistance on the sides of the footings and by friction on the base of the footings. Passive resistance should be evaluated using an equivalent fluid density of 350 pounds per cubic foot (pcf) where footings are poured neat against native soil or are surrounded by structural fill compacted to at least 95 percent of MDD, as recommended. Resistance to passive pressure should be calculated from the bottom of adjacent floor slabs and paving or below a depth of 1 footwhere the adjacent area is unpaved, as appropriate. Frictional resistance can be evaluated using 0.35 for the coefficient of base friction against footings. The above values incorporate 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. GEOENGINEER� December6,2021 Page4 File No. 24 59-003-00