9001 AIRPORT RD SNO ISLE SKILL CTR EXPANSION 2009 Geotech Report 2018-01-02 MF Import

i I Subsurface£xplora[ion, Geologic Hazard, and Sno-lsle Ski(Is Center Addition Preliminary Geotechnica!Engineering Repon Everett, Washington Geologic Hazards and Mitigmions Iturbidiry reading is 25 NTU or lower. If the monitored turbidity exceeds 250 NTU, the resulu must be reported to Ecology within 24 hours and corrective action taken. Daily turbidity Imonitoring is continued undl the corrective action lowers the turbidiry to below 25 NTU. I In order to meet U►e current Ecology requiremenu, a properly developed, constructed, and maintained erosion control plan consistent with the City of Everett standazds and best management erosion conuol practices will be required for this project. AESI is available to I assist the project civil engineer in developing site-specific erosion conVol plans. Based on past experience, it will be necessary to make adjustments and provide additional measures to the TESC plan in order to optimize its effectiveness. Ultimately> the success of the TESC plan � depends on a proactive approach to project planning and contractor implementation and maintenance. I The erosion hazard of the site soils is low. The most effective erosion conuol measure is the maintenance of adequate ground cover. Maintaining cover measures atop disturbed ground provides the greatest reduction to the potential generation of turbid runoff and sediment I transport. During the local wet season (October 1° tluough Mazch 31°), exposed soil should not remain uncovered for more than 2 days unless it is actively being worked. Ground-cover I measures can include erosion control matting, plastic sheeting> straw mulch, crushed rock or recycled concrete, or mature hydroseed. � Flow-convol measures aze also essential for collecting and controlling the site runoff. Flow paths across slopes shouid be kept to less than 50 feet in order to reduce the erosion and sediment vansport potential of concentrated flow. Ditclilswale spacing will need to be I shortened with increasing slope gradient. Ditches and swales that exceed a gradient of about 7 to 10 percent, depending on theit flow length, should have properly constructed check dams installed to reduce the flow velociry of the runoff and reduce the erosion potential within the � ditch. Flow paths that aze required to be constructed on gradients between 10 to 15 percent should be placed in a riprap-lined swale with the riprap properly sized for the flow conditions. Flow paths constructed on slope gradients steeper than 15 percent should be placed in a pipe � slope drain. AESI is available to assist the project civil engineer in developing a suitable erosion conttol plan with proper flow control. � Some fine-graioed surface soils aze the result of natural weathering processes that have broken down pazent materials into their mineral componenu. These mineral components can have an inherent electrical chazge. Electrically chazged mineral fines will attract oppositely chazged particles and can combine (flocculate) co form lazger particles that will settle out of suspension. The sediments produced during the recent glaciation of Puget Sound aze, however, most commonly the suspended ��sls that are carried by site storm water. The fine-grained fraction of the glacially derived soil is referred to as "rock flour," which is primarily a silt-sized particle with no electrical chazge. These particles, once suspended in water, may have settiing i times in periods of months, not hours. �une�1 z008 ASSOCIATED EARTN SCIENCES,INC. � ( EGad-F�'OB0113A5-Prolen11I0W02731FPIWP Page 7 I