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330 Land Company, LLC Arroyo Lago 9785.004.001 Design-Level Geotechnical Report Page | 24 April 16, 2024 Latest Revision October 31, 2025 additional equivalent fluid pressure of 40 pcf be added to the values recommended above for both restrained and unrestrained walls. Damp-proofing of the walls should be included in areas where wall moisture would be problematic. 7.2 RETAINING WALL FOOTINGS Site retaining walls can be supported on footings or drilled piers. For footings, passive resistance acting on foundations and shear keys may be assumed as 300 pcf, provided that the area in front of the retaining wall is level for a distance of at least 10 feet or three times the depth of foundation and shear key, whichever is greater. The upper 1 foot of soil should be excluded from passive resistance computations unless it is confined by pavement or a concrete slab. The friction factor for sliding resistance may be assumed as 0.30. It is recommended that retaining wall footings be designed using an allowable bearing pressure of 2,500 psf in firm native materials or engineered fill. The footings should be at least 18 inches below lowest adjacent grades. 7.3 RETAINING WALL DRAINAGE Either graded rock drains or geosynthetic drainage composites should be constructed behind the retaining walls to reduce hydrostatic lateral forces. For rock drain construction, we recommend two types of rock drain alternatives. 1. A minimum 12-inch-thick layer of Class 2 Permeable Filter Material (Caltrans Specification 68-2.02F) placed directly behind the wall, or 2. A minimum 12-inch-thick layer of washed, crushed rock with 100 percent passing the ¾-inch sieve and less than 5 percent passing the No. 4 sieve. Envelop rock in a minimum 6 -ounce, non-woven geotextile filter fabric. For both types of rock drains: 1. The rock drain should be placed directly behind the walls of the structure. 2. The rock drains should extend from the wall base to roughly 12 inches below soil grade at the top of the wall. 3. A minimum 4-inch-diameter perforated pipe (SDR 35 or Schedule 40) with glued joints and end caps should be placed at the base of the wall, inside the rock drain, with perforations placed down. 4. The pipe should be placed at a gradient of at least 1 percent if placed behind the footing to direct water away from the wall by gravity to a drainage facility. Alternatively, the pipe can be placed directly on top of the footing, if pre-approved by ENGEO. We should review and approve geosynthetic composite drainage systems prior to use. 7.4 WALL BACKFILL Backfill behind the retaining walls should be placed and compacted in accordance with Section 5.6. Use light compaction equipment within 5 feet of the wall face. If heavy compaction equipment is used, the walls should be temporarily braced to avoid excessive wall movement.