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4. Non-structural fill, such as fill placed in landscape areas, should be compacted to at least 90 percent <br /> of the MDD. <br /> Weather Considerations <br /> Disturbance of near surface soils should be expected if earthwork is completed during periods of wet <br /> weather.During dry weather,the soils will:(1)be less susceptible to disturbance,(2)provide better support <br /> for construction equipment,and(3)be more likely to meet the required compaction criteria. <br /> The wet weather season generally begins in October and continues through May in western Washington; <br /> however, periods of wet weather may occur during any month of the year. For earthwork activities during <br /> wet weather,we recommend that the following steps be taken: <br /> ■ The ground surface in and around the work area should be sloped so that surface water is directed <br /> away from the work area.The ground surface should be graded so that areas of ponded water do not <br /> develop. Measures should be taken by the contractor to prevent surface water from collecting in <br /> excavations and trenches. Measures should be implemented to remove surface water from the work <br /> area. <br /> ■ Earthwork activities should not take place during periods of moderate to heavy precipitation. <br /> ■ Slopes with exposed soils should be covered with plastic sheeting. <br /> ■ The contractor should take necessary measures to prevent on-site soils and soils to be used as fill from <br /> becoming wet or unstable.These measures may include the use of plastic sheeting,sumps with pumps, <br /> and grading. The site soils should not be left uncompacted and exposed to moisture. Sealing the <br /> surficial soils by rolling with a smooth-drum roller prior to periods of precipitation will help reduce the <br /> extent that these soils become wet or unstable. <br /> ■ The contractor should cover all soil stockpiles that will be used as structural fill with plastic sheeting. <br /> ■ Construction traffic should be restricted to specific areas of the site,preferably areas that are surfaced <br /> with the existing asphalt or working pad materials not susceptible to wet weather disturbance. <br /> ■ Construction activities should be scheduled so that the length of time that soils are left exposed to <br /> moisture is reduced to the extent practical. <br /> Routing of equipment on the subgrade soils during the wet weather months should not be difficult in its <br /> current condition. <br /> Utility Trenches <br /> Trench excavation, pipe bedding,and trench backfilling should be completed using the general procedures <br /> described in the WSDOT Standard Specifications or other suitable procedures specified by the project civil <br /> engineer. The native and fill soils encountered at the site are generally of low corrosivity based on our <br /> experience in the Puget Sound area. <br /> Utility trench backfill should consist of structural fill and should be placed in lifts of 12 inches or less(loose <br /> thickness) when using heavy compaction equipment, and 6 inches or less when using hand compaction <br /> equipment, such that adequate compaction can be achieved throughout the lift. Each lift must be <br /> compacted prior to placing the subsequent lift. Prior to compaction, the backfill should be moisture <br /> conditioned to within 2 percent of the optimum moisture content, if necessary. The backfill should be <br /> GWENGINEERS� August3,2015 Page 15 <br /> File No.0643-015-00 <br />