8925 AIRPORT RD MUKILTEO SCHOOL DISTRICT 2022-01-19

GeoTest Services, Inc. Mukilteo SD —Wall Design, Everett, Washington February 26, 2018 Job No. 18-0068 • GeoTest recommends a 4 inch diameter perforated PVC pipe be installed at the base of the retaining wall, with perforations oriented downward as shown in Figures 3 and 4. The PVC pipe should outlet to a suitable drainage receptacle. Backfill placed behind the wall should be placed on a horizontal surface in uniform lifts and be compacted to a dense and non -yielding condition. The fill should be placed in 8 to 10 inch, loose horizontal lifts and thoroughly compacted. Within the active zone behind the wall, the fill should be compacted to 95% of the maximum dry density as determined by ASTM D1557 test procedure except within 2 feet immediately behind the wall where 90% is recommended to reduce lateral stresses on the wall during construction processes. • Every effort should be made to minimize surface water infiltration during and after construction. Temporary excavations in excess of 4 ft should be shored or sloped in accordance with Safety Standards for Construction Work Part N, WAC 296-155- 66403. Temporary unsupported excavations in medium dense to dense, native soils (Type C soil) may be sloped as steep as 1'/2H:1V. All soils units with seepage are Type C soils and should be at 1'hH:1V or flatter. Flatter slopes or temporary shoring may be required in areas where groundwater flow is present and unstable conditions develop. The contractor is responsible for monitoring and maintaining safe excavation conditions. Due to the site geometry/topography and the presence of other features such as walls and utilities, the proposed wall alignment, base preparation and block sections may need to be field fit and/or modified slightly to account for site conditions during construction. Therefore, we recommend that GeoTest assist with consultation and design modifications during the construction of the wall in order to verify appropriate construction methods and wall geometries, in accordance with our wall design detail. Stability Analysis Stability analysis was performed for the wall using limit equilibrium techniques to analyze the margin of safety against sliding, overturning, and global stability. The methods of analyses compute the ratio of forces (or moments) resisting movement, as compared to forces (or moments) tending to cause movement. We determined that the safety factors for sliding and overturning were in excess of 1.5 and 2.0, respectively, which represents typical minimum safety factors normally used for stability design. Bearing capacity for the wall section was also analyzed and found to be sufficient for wall support using assumed competent native soil or compacted structural. Using UltraBlocke version 4.0.16287.0 (Build date: 10-13-2016) software, the resulting stable wall configuration is presented in the attached Figures 3 and 4. Global stability analyses were also performed on the proposed wall cross section based on the topography. The same software referenced above was used to determine factors of safety for the global stability of the proposed wall sections under both static and seismic conditions. The computer stability program was used to randomly generate and evaluate circular and block failures within the area of interest using the simplified Bishop's method of slices. The potential effect of seismic loading on the global stability Page 5 of 8