616 PECKS DR 2018-01-01 MF Import

�H H y � . r.��..., � � Everett School District W-6884 y � M 15 June 1990 Page 9 K �°a � °v N I-1 � � �` resist a lateral force equivalent to a fluid unit weight of 35 pcf (pounds per cubic foot). N � Walls that are structurally restrained from laterai movement at the top during backfili, H , � � „ such as by interior fioors, braces, or diaphragms, should be designed tc resist an H „ equivzlent fluid weight of 50 pcf. These recommended vaiues are based on a horizontal, � o y unweighted backfill surface. A minimum of 2 feet of clean granuiar backfill that can drain � � � freely to the wall footing drain should be placed immediately behind the wall, extending � y upward from the base of the wall to within � foot of the ground surface. This drainage backfill is intended to preclude the build-up of hydrostatic forces. Lateral forces on wall footings can be resisted by a combination of passive earth pressure and basal friction. An allowable passive earth pressure of 300 pcf, er.pressed as an equivalent fluid unit weight, may be assumed for each foot of penetration below the ground surface, neglecting the first foot. An allowable basal friction coefficient of 0.4 in recommended. These passive earth pressure and basal friction values incorporate a / � � factor-of-safery of 1.5 or more. We recommend that RZA be allowed to review these I �� design val�es if they should appiy to walls higher than about 5 feet. 1 '�� To prevent the build-up of hydrostatic pressures, the backfill immediately adjacent to the �� wall should consist of clean, free-draining sand a gravel having no more than 5 percent fines (based on the portion passing the U.S. No. 4 sieve. All backfili placed against the ` �-' wall should be compacted as per our recommendations previously given in the Structural Fill section of this report. To minimize lateral earth pressures, tt�e backfili should be compacted to approximately 90 pe�cent of the maximum Modified Procter dry I��j density (ASTM:D 1557). Additional compaction immediately adjacent to the wall wouid increase lateral pressures, whereas a lesser degreF of compaction may permit excessive , e�) 1 post-construction settiement in the backfill. � � , Drainage Considerations -'�.' Based on our borings, we do not anticipate sionificant seepage of groundwater into any • shallow site excavations during the dry summer months. However, localized "perched" groundwater zones should be expected during lhe wet seuson. Because the silty soils en�ountered at this site are highiy sensitive to moisture and are readily disturbed by traffic when �vet, we recommend that the contractor take precautions to m2intain a dry construction site and protect the subgrade from disturbance. The contractor should 'ue