2400 33RD ST GEOTECHNICAL INVESTIGATION 2018-01-02 MF Import

SC 130 Milbor-Pita &S, , ac equipment, building materials, and surcharge loads such as excavated soil should not be allowed within 1/3 of the slope height from the top of any excavation. For temporary excavations exceeding 4 feet in height, excavations must be sloped in accordance with part N of the Washington Administrative Code (WAC) 296-155, or be shored. The existing fill or disturbed natural s)il classifies as soil Type C and may be inclined no steeper than 1 'h H to 1 V. The glacial silt/clay soil clas.,ifies as Type A soil and may be inclined no steeper than 3/4 H to I V according to the WAC (Figure N-14). Specific design for temporary slopes is not included herein, since the contractor has control over factors during construction that are critical to the stability of the slope. Such factors include the amount of slope opened at one time, the length of time the slope is left open, and to some extent the time of exposure of a soil to wet weather. Thus, maintaining safe and stable temporary excavations is the responsibility of the contractor. With time and the presence of seepage and/or precipitation, the stability of temporary unsupported cut slopes can be significantly reduced. Therefore, temporary slopes should be protected from erosion by installation of a surface water diversion ditch or berm at the top of the slope and by covering the cut face with well -anchored plastic sheets. In addition, the contractor should monitor the stability of the temporary cut slopes and adjust the construction schedule and slope inclination accordingly. 5.5 Utility Installation Considerations Both in -place natural soil and properly placed 'structural fill' will result in suitable bearing for utilities. The disturbed bottom of the trench should be recompacted prior to the placement of pipe bedding. Since nearly all the site is to be used to support structures and pavement, it is recommended that the all the utility trenches be compacted to the same standard as the adjacent soil. The objective is to restore the area to a uniform hearing surface. It is recommended that a geutechnician be present during the compaction so testing can occur as the work progresses. 5.6 Wall Lateral Loads & Drainage 5.6.1 Wall Loadines Since the currently known underground structures are to be held rigid at the top, it is recommended that the wall loading be an 'at -rest' pressure. We recommend an equivalent fluid pressure of 60 pounds per cubic foot (pcf) be applied to the walls. We have recommended this upper level of loading because the area just outside these walls will be surcharged with traffic and the backfill soils will probably be compacted to a dense condition. 5.6.2 Wall Drainage Considerations Typically a wall is drained to both reduce the design lateral loading and to keep the interior of the structure dry. However, in the case of the subsurface water retention basin, Geolechnical, Tunnel and Llvironmental Fnginoers 3080 125°i Ave NE %one (425) 969-5778 Bellevue, WA 98005 Page 16 Fm (425)861-0677