1707 SCENIC DR SNO CO PUD Geotech Report 2022-10-25

� Wall Drainage To reduce the potential for hydrostatic water pressure buildup behind the retaining walis, we recommend that the walls be provided with adequate drainage, as shown in Figure 5. Wall drainage can be achieved by using free draining wall drainage material with perforated pipes to discharge the collected water. 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 glacial soils that are 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 pipes should be laid with minimum slopes of one-quarter percent and discharge into the storm water collection system or to the adjacent greenbelt. The pipe installations should include a cleanout riser with cover located at the upper end of each pipe run. Drainage Considerations We recommend that the ground surface be sloped to drain away from the proposed structures and the 115kV and 2301�,yards, such that surFace water runoff is collected and routed to suitable discharge points. We recommend that all retaining walls be designed and constructed with drainage measures behind the walls. For reinforced concrete cantilever walls and fill section soldier pile walls, drainage measures should consist of gravel backfill and weep holes. For MSE walls, drainage measures should consist of gravel borrow as backfill with a perForated drain pipe at the back of the reinforced zone. For cut section soldier pile walls, drainage measures should consist of prefabricated geocomposite drainage mats that are connected to a drain pipe. For gravity walls, drainage measures should consist of a zone of washed drain rock between the native slope and the backfill, with a perforated drain pipe at the base of the drainage zone. All drain pipes should be tight lined either to the storm drain system or to the base of the slope. � Infilfiration Co�isiderations i - We understand that storm water infiltration is being considered at the site. Where infiltration is being t ' considered, impacts to the existing siopes, proposed slopes, and existing fill soils should be evaluated. _.,_. _ _ _ __ __ - - _ �__ PITs were completed to determine design infiltration rates in the 115kV and 230kV yards where � �" irifiltration trenches are planned. Sieve analyses were also performed on soil samples collected from the fill, weathered glacial till, and unweathered till soils to supplement the PIT results. Pilot Infiltration Tests Two large scale PITs were completed within the weathered glacial till at the site in test pits TP-12 and TP-17. The PITs were completed following the methodology described in, "Stormwater Management Manual for Western Washington, Volume III, Hydrologic Analysis and Flow Control BMPs" (Washington State DOE, 2012). The locations of the PITs are shown on the Site Plan, Figure 2 and a description of the PIT procedure is included in Appendix A. The PIT results are presented graphically in Figures 7 and 8 for TP-12 and TP-17, respectively. Figures 7 and 8 show plots of the initial hydraulic conductivity rate (KSat-;n;t;ai) during an approximate 24 saturation hour period. The PIT conducted in TP-12 (Figure 7) GEoENGINEeRS� October 24,2014 Page 19 Fle No.0482-027-02