1901 W MUKILTEO BLVD Geotech Report 2022-10-25

Scriven Residence Improvements-REVISED NGA File No 974316 1901 West Mukilteo Boulevard November 29,2016 Everett,Washington Page 8 subsection of this report. It is also our opinion that the proposed setback of approximately 45 feet from the top of the steep slope should be suitable for the proposed addition provided that the proposed addition and new retaining wall are designed and constructed in accordance with the recommendations presented in this report. The surficial soils encountered on this site are considered moisture-sensitive and may disturb easily when wet. To lessen the potential impacts of construction and to reduce cost overruns and delays, we recommend that construction take place during the drier summer months if possible. If construction takes place during the rainy months, additional expenses and delays should be expected. These extra expenses could include additional erosion control and temporary drainage measures to protect the proposed development area including placement of a blanket of rock spalls to protect exposed subgrades, and the need for importing all-weather materials for structural fill. All construction operations and drainage improvements planned as part of this project should be planned and completed in a manner that enhances the stability of the steep slope, not reduces it. Any excavation spoils generated during site improvements should not be stockpiled on site but rather promptly hauled away. Also, all current and future runoff generated within the site should be collected and routed to a permanent discharge location at the bottom of the slope, or to an approved drainage system. Under no circumstances should water be allowed to concentrate or flow uncontrollably over the slope. The vegetation cover on the slope should be evaluated for compatibility with desired slope stability conditions,and a vegetation management plan should be devised to enhance slope stability. The recommended retaining wall and the slope should be monitored on an on-going basis after construction especially during the wet season for any signs of instability, and corrective actions promptly taken should any signs of instability be observed. Lawn clippings and any other household trash or debris should never be cast over the slope. Any future plans for structures near the slope or modifications to the slope should be specifically evaluated by NGA and approved by the City of Everett. Erosion Control and Slope Protection The on-site soils are listed as having a moderate potential for erosion when cleared of vegetation, but the actual erosion potential could be severe depending on how the site is graded and how water is allowed to concentrate. Best Management Practices (BMPs) should be used to control erosion. Areas disturbed during construction should be protected from erosion. Erosion control measures may include diverting surface water away from the stripped areas. Silt fences or straw bales could be erected to prevent muddy NELSON GEOTECHNICAL ASSOCIATES, INC.