1809 HOLBROOK AVE 2018-01-02 MF Import

Terhune Homes, Inc. JN 99028 February 25, 1999 Page 5 Heavy construction equipment should not oP operated behind retaining and foundation walls within a distance equal to the height of a wall, unless the walls are designed for the additional lateral pressures resulting from the equipment. The wall design criteria assume that the backfill will be well -compacted in lifts no thicker than 12 inches. The compaction of backfill near the walls should be accomplished with hand -operated equipment to prevent the walls from being overloaded by the higher soil forces that occur during compaction. Retaining Wall Backfill Backfill placed behind retaining or foundation walls should be coarse, free -draining, structural fill cnntain.ng no organics. This backfill should contain no more than 5 percent silt or clay particles and have n -) gravel greater than 4 inches in diameter. The percentage of particles passing the No. 4 sieve should be between 25 and 70 percent. If the native sand is used as backfill, a drainage composite similar to Miradrain 6u00 should be placed against the backfilled retaining walls. The drainage composites should be hydraulically connected to the foundation drain system. For increased protection, drainage composites should be placed along cut slope faces, and the walls should be backfilled with pervious soil. 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 surface 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 moist conditions or some seepage through the walls are not acceptable, waterproofing should be provided. This typically includes limiting cold -joints and Nall penetrations, and using bentonite panels or membranes on the outside of the walls. Applying a thin coat of asphalt emulsion is not considered waterproofing, but will only help to prevent moisture, generated from water vapor or capillary action, from seeping thro!igh the concrete. Rockeries We anticipate that rockeries will be used on the western portion of the site. A rockery is not intended to function as an engineered structure to resist lateral earth pressures, as a retaining wall would do. The primary function of a rockery is to cover the exposed, excavated surface and thereby retard the erosion process. We recommend limiting rockeries to a height of 8 feet and placing them against only dense, competent, native soil. The construction of rockeries is, to a large extent, an art not entirely controllable by engineering methods and standards. It is imperative that rockeries, if used, are constructed with care and in a proper manner by an experienced contractor with proven ability in rockery construction. The rockeries should be constructed with hard, sound, durable rock in accordance with accepted local practice and Snohomish County standards. Soft rock, or rock with c significant number of fractures or inclusions, should not be used, in order to limit the amount of maintenance and repair needed GEOTECH CONSULTANTS, INC. &