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ENGEO <br /> INCORPORATED <br /> <br /> <br />5150.100.101 <br />May 2, 2008 Page 24 <br />9.2 RETAINING WALL DRAINAGE <br /> <br />Construct either graded rock drains or geosynthetic drainage composites behind the retaining <br />walls to reduce hydrostatic lateral forces. For rock drain construction, we recommend two types <br />of rock drain alternatives: <br /> <br />1. A minimum 12-inch-thick layer of Class 2 Permeable Filter Material (Caltrans Specification <br />68-1.025) placed directly behind the wall, or <br /> <br />2. A minimum 12-inch-thick layer of washed, crushed rock with 100 percent passing the ¾-inch <br />sieve and less than 5 percent passing the No. 4 sieve. Envelope rock in a non-woven <br />geotextile filter fabric such as Mirafi 140NC, or equivalent. <br /> <br />For both types of rock drains: <br /> <br />1. Place the rock drain directly behind the walls of the structure. <br /> <br />2. Extend rock drains from the wall base to within 12 inches of the top of the wall. <br /> <br />3. Place a minimum of 4-inch-diameter perforated pipe at the base of the wall, inside the rock <br />drain and fabric, with perforations placed down. <br /> <br />4. Place pipe at a gradient at least 1 percent to direct water away from the wall by gravity to a <br />drainage facility. <br /> <br />ENGEO should review and approve geosynthetic composite drainage systems prior to use. <br /> <br />9.3 BACKFILL <br /> <br />Backfill behind retaining walls should be placed and compacted in accordance with Section 4.5. <br />Use light compaction equipment within 5 feet of the wall face. If heavy compaction equipment <br />is used, the walls should be temporarily braced to avoid excessive wall movement. <br />