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October 19, 2021 <br />Page 5 of 12 <br />Geotechnical Evaluation <br />pounds per square foot (psf) may be used for design. Detention vaults set at least 5 feet below <br />grade maybe designed with a bearing pressure of 5,000 psL <br />A 1/3 increase in the above value may be used for short duration loads, such as those imposed by <br />wind and seismic events. Structural fill placed on bearing, native subgrade should be compacted <br />to at least 95 percent of the maximum dry density based on ASTM Test Method D1557. Footing <br />excavations should be inspected to verify that the foundations will bear on suitable material. <br />Exterior footings should have a minimum depth of 18 inches below pad subgrade (soil grade) or <br />adjacent exterior grade, whichever is lower. Interior footings should have a minimum depth of 12 <br />inches below pad subgrade (soil grade) or adjacent exterior grade, whichever is lower. <br />If constructed as recommended, the total foundation settlement is not expected to exceed 1 inch. <br />Differential settlement, along a 25-foot exterior wall footing, or between adjoining column <br />footings, should be less than 1/2 inch. This translates to an angular distortion of 0.002. Most <br />settlement is expected to occur during construction, as the loads are applied. However, additional <br />post -construction settlement may occur if the foundation soils are flooded or saturated. All <br />footing excavations should be observed by a qualified geotechnical consultant. <br />Resistance to lateral footing displacement can be determined using an allowable friction factor of <br />0.4o acting between the base of foundations and the supporting subgrades. Lateral resistance for <br />footings can also be developed using an allowable equivalent fluid passive pressure of 225 pounds <br />per cubic foot (pcf) acting against the appropriate vertical footing faces (neglect the upper 12 <br />inches below grade in exterior areas). The frictional and passive resistance of the soil may be <br />combined without reduction in determining the total lateral resistance. <br />Care should be taken to prevent wetting or drying of the bearing materials during construction. <br />Any extremely wet or dry materials, or any loose or disturbed materials at the bottom of the <br />footing excavations, should be removed prior to placing concrete. The potential for wetting or <br />drying of the bearing materials can be reduced by pouring concrete as soon as possible after <br />completing the footing excavation and evaluating the bearing surface by the geotechnical engineer <br />or his representative. <br />Concrete Retaining Walls <br />The following table, titled Wall Design Criteria, presents the recommended soil related design <br />parameters for retaining walls with a level backslope. Contact Cobalt if an alternate retaining wall <br />system is used. This has been included for new cast in place walls. <br />Wall Design Criteria <br />"At -rest" Conditions (Lateral Earth Pressure — EFD*) <br />55 pcf (Equivalent Fluid Density) <br />"Active" Conditions (Lateral Earth Pressure <br />— EFD*) <br />35 pcf (Equivalent Fluid Density) <br />Seismic Increase for "At-resf' <br />(Lateral Earth Pressure) <br />Conditions <br />21H* (Uniform Distribution) 1 in 2,500 year <br />event <br />Seismic Increase for "At-resf' <br />(Lateral Earth Pressure) <br />Conditions <br />14H* (Uniform Distribution) 1 in 500 year event <br />Seismic Increase for "Active" <br />(Lateral Earth Pressure) <br />Conditions <br />9H* (Uniform Distribution) <br />www.cobaltgeo.com (206) 331-1097 <br />