616 PECKS DR 2018-01-01 MF Import

� ~ � y -, �•-. p CC ° y Everett School District W-6884 � � � 15 June 1990 Page 6 H b N h0 N M � Z o � An imported, clean, granular soil free of organics and other deleterious material will � ;� � therefore likely be needed for use as structural fill during wet site conditions. "Clean" � y � soils are granular soils that have a fines content of less than 5 percent (by weight) based y y on the soil fraction passing the U.S. No. 4 Sieve. Material of this type may be � � N successfully placed and compacted under a wider variety of weather conditions. In ea r� � addition to their use as structural fills during wet site conditions, clean soils are required y for drainaye app lica tions as discusse d su bsequ e n t l y. Foundation Considerations In our opinion, isolated or continuous spread footings would provide suitable support for the proposed additions. Two footing design options are available, with each option offering a particular allowable soil bearing pressure for a particular footing depth. For seismic or transient loads this allowabie soil pressure may be increased by up to one- third. � `� The first design option would be to extend the footings downward to bear directly on the native dense to very dense, silty sands (Weathered Till or Glaciai Tili). This would involve 1 �� removing the loose, root-laden surficial soils or uncontrolled fill which, based on our i �� explorations, extended to a depth of roughly 5 feet in the courtyard and up to 2 feet in the proposed building area southaast of the school. Any deeper, locai zones of soft, � 1�� loose, organic, or uncontrolled fili soil shouid be removed and replaced with structural fill compacted to at least 90 percent of the modified Proctor maximum dry density. The � exposed footing subgrade should be compacted, if warranted by soil conditions, to a 1� � firm and non-yieiding state. If required, the footing subgrade couid then be raised to the desired elevation by replacing the excavated soils with properly compacted structural fill, •• ��-' ' with the footings bearing directly onto this prism of structural fill. We recommend that the '� structural fill piaced within building areas be compacted to at least 90 percent of the I modified Proctor maximum dry density. Also, the prism of structural fill should extend � I�� laterally beyond the edges of the footing a distance equal to the thickness of the fill prism � below the footing. A maximum allowable soil bearing pressure of 2500 psf (pounds per square foot) would be appropriate for this t�oting design option. - We anticipate that t�tal settlements of the foundations should not exceed 1 inch, with differentiai settlements on the order of one-half of the total.