11831 AIRPORT RD WHOLE SITE FILE Geotech Report 2022-10-25

Other Considerations Retaining systems used to achieve grade transitions or for landscaping can be constructed using traditional structural systems such as reinforced concrete and concrete masonry unit (CMU) blocks. Alternatively, 'retaining walls can consist of reinforced soil and block facing structures, which may be an economically reasonable alternative to more traditional retaining wall systems. Non-structural systems such as rockeries may also be used to achieve grade transitions, although these systems are more suitable against dense native soils rather than fill soils. We can provide additional design recommendations and wall details for retaining structures, if requested. The proposed wall for the station at north-bound Airport Road north of 100th Street SW will be situated upslope of an existing block wall constructed by Snohomish County. The existing block wall is about 8 to 10 feet in height.At this time,we do not know if the existing block wall can support the new fill. If the County determines that the existing wall cannot support the new fill, we recommend that light weight geofoam blocks be used behind the new retaining wall to establish the design grade. Wall 1 and 2 Cast-in-Place Walls Two conventional CIP walls are being considered for the improvements, one on the south side of 128th Street SE immediately west of the south-bound 1-5 on-ramp and north of Denny's (referred to as Wall 1),and the other on the north side of 128th Street SE immediately east of the north-bound 1-5 on-ramp (referred to as Wall 2). Wall 1 will range in height from about 4 feet to a maximum of 10 feet, Wall 2, depending on the final wall alignment, could vary in height from about 3 feet to a maximum of 15 feet. These walls will retain new fill placed as part of the roadway and sidewalk widening. Shallow foundations for CIP walls may be designed using AASHTO and WSDOT methods using the allowable bearing pressures presented in Table 4. The figure and table provide allowable bearing capacity for the combination of dead and long-term live loads. Bearing pressures may be increased by one-third to account for short term live loads such as those induced by wind or seismic forces. The foundation widths referenced in Table 4 are effective widths and should be corrected for eccentric loading conditions by the following equation: B' = B - 2ey Where: B' is the effective footing width; B is the actual footing width; and eY is the eccentricity along the footing width. The eccentricity is measured from the center of the footing to the location of the resultant reaction force along the length of the footing (x-axis) and the width of the footing (y-axis). Refer to Chapter 8.11 of the WSDOT GDM for additional information regarding eccentricity. GEOENGINEERS� April 14,2017 Page 14 File No.2095-032-00