1001 N BROADWAY 2020 EVCC 2025-10-03

These recommendations are based on the assumption that all retaining walls will be provided with adequate drainage. The values for soil bearing, frictional resistance and passive resistance presented above for foundation design are applicable to retaining wall design. Walls located in level ground areas should be founded at a depth of 18 inches below the adjacent grade. 4.6.2.Wall Drainage To reduce the potential for hydrostatic water pressure buildup behind the retaining walls, we recommend that the walls be provided with adequate drainage, as shown in Figure 3. Wall drainage can be achieved by using free draining wail drainage material with perforated pipes to discharge the collected water. Wall drainage material may consist of Gravel Backfill for drains per WSDOT Standard Specification Section 9-03.12(4) surrounded with a nonwoven geotextile filter fabric such as Mirafi 140N (or approved equivalent), or imported Gravel Borrow if used in conjunction with a geocomposite wall drainage layer. The zone of wall drainage material should be 2 feet wide and should extend from the base of the wall to within 2 feet of the ground surface. The wall drainage material should be covered with 2 feet of less permeable material,such as the on-site silty sand that is properly moisture conditioned and compacted. A 4-inch-diameter perforated drain pipe should be installed within the free-draining material at the base of each wall. We recommend using either heavy-wall solid pipe (SDR-35 PVC) or rigid corrugated polyethylene pipe (ADS N-12, or equal). We "recommend against using flexible tubing for the wall drain pipe. The footing drain recommended above'`can be incorporated into the bottom of the drainage zone and used for this purpose. The pipes should be laid with minimum slopes of one-quarter percent and discharge into the storm water collection system to convey the water off site. The pipe installations should include a cleanout riser with cover located at the upper',end of each pipe run. The cleanouts could be placed in flush mounted access boxes. Collected downspout water should be routed to appropriate discharge points in separate pipe systems. 4.6. Ear :vvvork 4.6.1.Excavation Considerations Planned final site grades will likely be close to the existing grades. Based on the subsurface soil conditions encountered in the borings, we expect the soils at the site may be excavated using conventional heavy-duty construction equipment. The soils encountered in the upper portions of the borings consisted of medium dense to very dense sand with variable silt and gravel content, or sandy silt. Glacial deposits in the area commonly contain cobbles and boulders that may be encountered during excavation.Accordingly,the contractor should be prepared to deal with cobbles and boulders. The fill and native soils contains sufficient fines (material passing the U.S. Standard No. 200 sieve) to be highly moisture-sensitive and susceptible to disturbance, especially when wet. Ideally, earthwork should be undertaken during extended periods of dry weather when the surficial soils will be less susceptible to disturbance and provide better support for construction equipment. Dry weather construction will help reduce earthwork costs and increase the potential for using the native soils as structural fill. Trafficability on the site is not expected to be difficult during dry weather conditions. However, the native soils will be susceptible to disturbance from construction equipment during wet weather conditions and GMENGINEERS� tiny 22,2M Page 9 -•-- rile No.5916-011-00