2312 W CASINO RD COMMUNITY TRANSIT Geotech Report 2022-10-25

4.7. Retaining Walls 4.7.1.Lateral Soil Pressures The lateral soil pressures acting on retaining walls will depend on the nature and density of the soil behind the wall, the amount of lateral wall movement that occurs as backfill is placed, and the inclination of the backfill surface. For walls that are free to yield at the top at least two-thousandths of the height of the wall (i.e., wall height times 0.002), soil pressures will be less than if movement is restrained. We recommend that"free" walls supporting horizontal backfill be designed using an equivalent fluid density of 35 pcf, and restrained walls be designed using an equivalent fluid density of 55 pcf. The above-recommended lateral soil pressures do not include the effects of sloping backfill surfaces or surcharges such as traffic loads or other surface loading.The equivalent fluid density should be increased to 50 pcf fora 2H:1V sloping backfill for unrestrained walls. We recommend a rectangular seismic surcharge;of 8H psf and 15H psf for the yielding and non-yielding wall conditions, respectively,where H is the height of the wall. 4.7.2.Wall Backfill All backfill for retaining walls should consist of structural fill as described previously with the exception that care should be taken to prevent the buildup of excessive lateral soil pressures due to overcompaction of the backfill behind the wall.This can be accomplished by placing the backfill located within about 5 feet of the wall in lifts not exceeding 6 inches in loose thickness and compacting this zone with hand-operated compaction equipment. 4.7.3.Wall Drainage The recommended equivalent fluid density assumes a free-draining condition behind the wall. Permanent drainage systems should be provided to collect water and prevent the buildup of hydrostatic pressure against the wall.We recommend that the drainage system consist of(1)a 2-foot-wide zone of free-draining backfill against the back of the wall or(2)a prefabricated geocomposite wall drain.The free-draining backfill should consist of Gravel Backfill for Drains as shown in Figure 3.A perforated drainpipe having a minimum diameter of 4 inches should be placed in the granular fill or in the bottom of the prefabricated drain at the base of walls for the entire length. 4.8. Pavement Considerations The pavement subgrade area should be proofrolled and evaluated by the geotechnical engineer prior to placing base course. It is critical that all construction traffic be kept off the silty subgrade soils during wet weather to prevent disturbance(rutting and weaving)from occurring.We recommend the minimum 6-inch- thick gravel base contain less than 5 percent passing the U.S. No.200 sieve to perform as a drainage layer between the silty soils and the pavement section. The minimum thickness is not intended to serve as a working surface for construction traffic during wet weather. Additional subbase will likely be required if earthwork and site grading is not completed during the dry season. GEOENGINEERS� November 14,2019 Page 10 File No.2095-019-07