1915 W MUKILTEO BLVD 2018-01-02 MF Import

Geotechnical Engineering Evaluation and Slide Repairs 1915 Mukilteo Boulevard July 26, 2006 NGA File No. 743206 Page 8 the slope should be evaluated for compatibility with desired slope stability conditions, and a vegetation management plan be devised to enhance slope stability. The wall and slope should be monitored on an ongoing 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 To lessen the potential impacts of construction on the slope and to reduce cost overruns and delays, we -ecomntend that construction take place during the drier summer months. If construction takes place wring the rainy months, additional expenses and delays should he expected. Additional expenses could include the need for placing erosion control and temporary drainage measures to protect the slope, the need for placing a blanket of rock spalls on exposed subgrades, construction traffic , aved areas. Also, improvements recommended on the slope below the wall will be very diffiL _-.: ,,cement during the wet season. Erosion Control The on -site soils can have a high erosion potential, 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. Measures taken may include diverting surface water away from the stripped areas. Silt fences or straw bales should be erected to prevent muddy water from flowing over the site slopes or down towards the tracks or Possession Sound. Disturbed areas should be replanted with vegetation at the end of construction. The vegetation should be maintained until established. Final grading should incorporate appropriate erosion control measures to route stonnwater ninoff away from the top of slopes and to appropriate discharge locations. Retaining Wall Design and Construction The wall proposed for protection of the top of the severely affected portion of the slope should consist of 4-inch diameter driven steel pipe piles bridged using treated timber lagging and anchored back into the slope using grouted anchors. This wall system is suitable for the intended purpose of protecting the top of the slope from erosion and sloughing, but should not be considered a permanent slope stabilization method. The final extent and heights of the wall should be determined in the field based on site conditions. NELSON GEOTECHNICAL ASSOCIATES, INC. i'