6300 MERRILL CREEK PKY BASE FILE 2018-01-01 MF Import

�uadrant JN 97092 August 13, 1997 Page 6 Retaining Wall Backfill Backfill placed behind retaining or foundation walls should be coarse, free-draining, structural fill containing no organics. This backfill should contain no more than 5 percent silt or clay particles and have no gravel greater than 4 inches in diameter. The percentage of particles passing the No. 4 sieve should be between 25 and 70 percent. The purpose of these backfill requirements is to ensure that the design criteria for a retaining wall are not exceeded because of a build-up of hydrostatic pressure behind the wall. The top 12 to 18 inches of the backfill should consist of a compacted, relatively impermeable soil or topsoil, or the surface should be paved. The ground surface must also slope away from backfilled walls to reduce the potential for surtace water to percolate into the backfill. The sub-section entitled General Earthwork and Structural Fill contains recommendations regarding the placement and compaction of structural fill behind retaining and foundation walls. The above recommendations are not intended to waterproof the below-grade walls. If some seepage through the walls or moist conditions are not acceptable, damp-proofing or waterproofing should be provided. This couid include Iimiting cold-joints and wall penetrations, and possibly using bentonite panels or membranes on the outside of the walls. Applying a thin coat of asphalt emulsion is not considered waterproofing, but it will only help to prevent moisture, generated from water vapor or capillary action, from seeping through the concrete. Excavations and Slopes Excavation slopes should not exceed the limits specified in local, state, and national government safety regulations. Temporary cuts to a depth of about 4 feet may be attempted vertically in unsaturated soil, if there are no indications of slope instability. Based upon Washington Administrative Code (VVAC) 296, Part N, the soil type at the subject site would be classified as Type A. Therefore, temporary cut slopes greater than 4 feet in height cannot be excavated at an inclination steeper than 1:1 (Horizontal:Vertical), extending continuously between the top and the bottom of a cut. The above-recommended temporary slope inclination is based on what has been successful at other sites with similar soil conditions. Temporary cuts are those that will remain unsupported for a relativeiy short duration to allow for the construction of foundations, retaining walls, or utilities. Temporary cut slopes should be protected with plastic sheeting during wet weather. The cut slopes should also be backfilled or retaired as soon as possible to reduce the potential for instability. All permanent cuts into native soil should be inclined no steeper than 2:1 (H:V). Fill slopes should also not be constructed with an inclination greater than 2:1 (H:�. To reduce the potentiai for shalloH� sloughing, fill must be compacted to the face of these slopes. This could be accomFlished by overbuilding the compacted fill and then trimming it back to its final inclination. Water should not be allowed to flow uncontrolled over the top of any temporary or permanent slope. Also, all permanently exposed slopes should be seeded with an appr�priate species of vegetation to reduce erosion and improve the stability of the su�cial layer of soil. GEOTECIi CONSULTANTS,INC.