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9 4.6 Below Grade Walls We recommend general waterproofing of the below grade walls. We recommend either placing drainage composite against the backs of the exterior walls or backfilling adjacent to the walls with sand having less than 50 percent of the particles by weight passing the #40 sieve and less than 5 percent of the particles by weight passing the #200 sieve. The sand backfill should be placed within 2 feet horizontally of the wall. We recommend the balance of the backfill for the walls consist of sand however the sand may contain up to 20 percent of the particles by weight passing the #200 sieve. We recommend installing drain tile behind the below grade walls, adjacent to the wall footing and below the slab elevation. Preferably the drain tile should consist of perforated pipe embedded in gravel. A geotextile filter fabric should encase the pipe and gravel. The drain tile should be routed to a storm sewer, sump pump or other suitable disposal site. Foundation walls or below grade (basement) walls will have lateral loads from the surrounding soil transmitted to them. Active earth pressures can be used to design the below grade walls if the walls are allowed to rotate slightly. If wall rotation cannot be tolerated, then below grade wall design should be based on at-rest earth pressures. It is our opinion that the estimated soil parameters presented in Table 4 can be used for below grade wall design. These estimated parameters are based on the assumptions that the walls are drained, there are no surcharge loads within a horizontal distance equal to the height of the wall and the backfill is level. Table 4. Estimated Soil Parameters Soil Type Estimated Unit Weight (pcf) Estimated Friction Angle (degrees) At-Rest Pressure (pcf) Active Soil Pressure (pcf) Passive Soil Pressure (pcf) Sand (SP & SP-SM) 120 32 55 35 390 Other Soils (SM, SC, ML, CL) 135 28 70 50 375 Resistance to lateral earth pressures will be provided by passive resistance against the wall footings and by sliding resistance along the bottom of the wall footings. We recommend a sliding coefficient of 0.35. This value does not include a factor of safety. 4.7 Retaining Walls We are not aware of any retaining walls proposed for this project and were not provided any information regarding any proposed retaining walls. Retaining wall designers/installers should be aware that soil borings for any retaining walls were not completed as part of this evaluation. Because of that, additional geotechnical explorations (soil borings) could be required to determine and evaluate the suitability and/or stability of site soil conditions to support their design(s). Retaining wall designers and/or installers will be solely responsible to conduct additional geotechnical evaluation(s) as needed.