2514 RAINIER AVE Geotech Report 2022-10-25

1 • amec foster wheeler bearing pad at least 1 foot thick that extends 1 foot beyond the edges of the footing. The pad should consist of compacted gravel such as: t ► `Ballast" per WSDOT Standard Specification 9-03.9(1); or ► "Crushed Surfacing Top and Base Course" per WSDOT Standard Specification 9-03.9(3). If the subgrade, directly below the bearing pad, consists of lacustrine silts, then a geotextile separator should be placed between the lacustrine silt subgrade and the gravel bearing pad. • Static Bearing Pressure: Conventional spread footings supported on lacustrine silt and gravel bearing pads can be designed for an allowable static bearing pressure of 2,500 pounds per square foot (psf). Lightly Loaded Foundations: For small outbuildings, short (less than 4 feet high) retaining walls, and other small structures, we expect the foundation loads will be much lighter. If an allowable static bearing pressure of 1,500 psf is acceptable and the foundation will bear on stiff undisturbed lacustrine silt, than the gravel bearing pad and geotextile separator will not be necessary. Seismic Bearing Pressure: For seismic design, we coordinated with the structural engineer for the design of the brace frame footings. During a design earthquake event,the brace frame footings tend to rock, so a higher contact pressure is applied to the outer portion of the footing. Please refer to the I diagram in Appendix C for a typical brace-frame footing contact pressure. We analyzed this typical brace frame footing and determined that a short-term seismic bearing pressure of 3,600 psf could resist the outer portion of the footing with less than inch of additional settlement. We also checked Ibearing capacity (shear failure) of the soil and determined there was adequate capacity to withstand an extreme contact pressure at the end of the footing of 6,000 psf. FootingSettlements: We estimate that settlement due to static loadingof properly designed footings p p Y 9 9 will be less than 1 inch. Differential settlement between the new structure and the old structure could approach the total settlement of 1 inch, while differential settlement between two new footings could approach '/2 inch. An additional '/ inch of footing settlement is possible following a design-level ' earthquake. Subqrade Verification: We recommend footing subgrades be verified by an Amec Foster Wheeler ' representative before any concrete is placed. Footings should never be cast on loose, soft, or frozen soil; slough; debris; existing uncontrolled or loose fill; or surfaces covered by standing water. Amec Foster Wheeler Project No.6-917-18116-0 11 \\Sea-fs1\WordProc\_Projects\18000s\18116 Everett School District\Geotech Report\Revised\2017_10-19_Everett Middle School Geotech_Sx.docx