2414 BAKER AVE 2017-01-13 - Laserfiche WebLink

Tom Eiden JN 10114 June 21, 2010 Page 5 The design values given above do not include the effects of any hydrostatic pressures behind the walls and assume that no surcharges, such as those caused by slopes, vehicles, or adjacent foundations will be exerted on the walls. If these conditions exist, those pressures should be added to the above lateral soil pressures. Where sloping backfill is desired behind the walls, we will need to be given the wall dimensions and the slope of the backfill in order to provide the appropriate design earth pressures. Heavy construction equipment should not be operated behind retaining and foundation walls within a distance equal to the height of a wall, unless the walis are designed for the additional lateral pressures resulting from the equipment. The wall design criteria assume that the backfill wiil be well-compacted in lifts no thicker than 12 inches. The compaction of backfill near the walis should be accomplished with hand-operated equipment to prevent the walls from being overloaded by the higher soil forces that occur during compaction. Refainin_q WaI/ 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. If the native silty sand is used as backfill, a drainage composite similar to Miradrain 6000 should be placed against the backfilled retaining walls. The sandy silt soils should not be used for wall backfill due to their poor drainage characteristics. The drainage composites should be hydraulically connected to the foundation drain system. Free-draining backfill or gravel should be used for the entire width of the backfill where seepage is encountered. For increased protection, drainage composites should be placed along cut siope faces, and the walls should be backfilled entirely with free-draining soii. The tater section entitled Drainage Considerations should also be reviewed for recommendations related to subsurface drainage behind foundation and retaining walls. 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 irr�permeable soil or topsoil, or the surface should be paved. The ground surface must also slope away from backfilled walis to reduce the potential for surface waier to percolate into the backfill. The section entitled Genera/ Earthwork and Structura/ Fill contains recommendations regarding the placement and compaction of structural fill behind retaining and foundation wails. The above recomrnendations are not intended to waterproof below-grade walls, or to prevent the formation of mold, mildew or fungi in interior spaces. Over time, the performance of subsurface drainage systems can degrade, subsurface groundwater flow patterns can change, and utilities can break or develop leaks. Therefore, waterproofing should be provided where future seepage through the walls is not acceptable. This typically includes limiting cold-joints and wall penetrations, and using bentonite products, spray-on liners, or membranes on the outside of the walls. Applying a thin coat of asphalt emulsion to the outside face of a wall is not considered waterproofing, and will only help to reduce moisture generated from water vapor or capillary action from seeping through the concrete. As with any project, adequate ventilation of basement and crawl space areas is important to prevent a build up of water vapor that is commonly transmitted through concrete walls from the surrounding soil, even when seepage is not present. This is appropriate even when GEOTECH CONSULTANTS. INC.