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95 percent of the MDD beyond 5 feet of the walls. The upper 2 feet of fill below floor slab subgrade <br />should also be compacted to at least 95 percent of the MDD. The contractor should keep all heavy <br />construction equipment away from the top of retaining walls a distance equal to half the height of the ' <br />wall, or at least 5 feet, whichever is greater. <br />3. Structural fill in new pavement and hardscape areas, including utility trench backfill, should be <br />compacted to at least 90 percent of the MDD, except that the upper 2 feet of fill below final subgrade <br />should be compacted to at least 95 percent of the MDD, see Figure 5, Compaction Criteria for Trench <br />Backfill. ' <br />4. Structural fill placed as crushed rock base course below pavements should be compacted to <br />95 percent of the MDD. <br />5. Non-structural fill, such as fill placed in landscape areas, should be compacted to at least 90 percent , <br />of the MDD. <br />4.5.7. Weather Considerations , <br />The on -site soils and common borrow contain a sufficient percentage of fines (silt and clay) to be highly <br />moisture sensitive. When the moisture content of these soils is more than a few percent above the optimum ' <br />moisture content, these soils become muddy and unstable, operation of equipment on these soils will be <br />difficult and it will be difficult or impossible to meet the required compaction criteria. Additionally, <br />disturbance of near -surface soils should be expected if earthwork is completed during periods of wet ' <br />weather. It will be preferable to schedule site preparation and earthwork activities during periods of dry <br />weather when the soils will: (1) be less susceptible to disturbance and (2) provide better support for ' <br />construction equipment. <br />The wet weather season in the Puget Sound region generally begins in October and continues through May; <br />however, periods of wet weather may occur during any month of the year. The optimum earthwork period <br />' <br />for these types of soils is typically June through September. If wet weather earthwork is unavoidable, we <br />recommend the following: <br />, <br />■ Structural fill placed during the wet season or during periods of wet weather should consist of imported <br />gravel borrow with less than 5 percent fines (material passing the U.S. No. 200 sieve). <br />is directed <br />' <br />■ The ground surface in and around the work area should be sloped so that surface water <br />away from the work area. <br />■ The ground surface should be graded such that areas of ponded water do not develop. <br />' <br />■ The contractor should take measures to prevent surface water from collecting in excavations and <br />trenches. <br />' <br />■ Earthwork activities should not take place during periods of heavy precipitation. <br />■ Slopes with exposed soils should be covered with plastic sheeting or similar means. <br />, <br />■ Measures should be taken to prevent on -site soils and soils to be used as fill from becoming wet or <br />unstable. These measures may include the use of plastic sheeting, sumps with pumps, and grading. <br />The site soils should not be left uncompacted and exposed to moisture. Sealing the surficial soils by <br />, <br />rolling with a smooth -drum roller prior to periods of precipitation will reduce the extent to which these <br />soils become wet or unstable. <br />' <br />GEoENGINEERS� October30, 2019 Page 15 <br />Flee No 21288 002 00 <br />