6307 36TH AVE W Geotech Report 2025-07-29

August 13,2008 - Project No.T-6241 5.6 Stormwater Facilities Based on site topography and soil conditions, infiltration of stormwater is not advised. We recommend the use of stormwater detention ponds or vaults. Detention Ponds There is currently one stormwater pond planned in the eastern portion of the site. The pond is situated immediately west of the eastern descending slope. Test pits TP-5 through TP-7 were advanced within or in the vicinity of the planned pond. At each test pit we observed 3 to 5 feet of loose to medium dense weathered silty sand. Below the upper weathered soils at Test Pit TP-5, we observed glacial till to its termination depth of ten feet. At Test Pits TP-6 and TP-7, we observed silty sand, and sand with varying amounts of silt to their ' termination. No pond grading plans were available for our review at the time of our investigation. For stormwater detention ponds,we recommend interior pond slopes below the maximum stored water level be graded at a 3:1 inclination. Slope cuts above the stored water level can be graded at 2:1. If a fill berm is required, all fill material placed for formation of the containment berm must be placed and compacted structurally as recommended in Section 5.2 of this report. The berm fill should be keyed into the underlying competent native soil a minimum depth of two feet, The width of the berm at the pond crest should be equal to the height of the berm fill, but not less than 5 feet nor greater than 15 feet. The exterior slope of the berm fill should be graded no steeper than 2:1. ' To prevent potential seepage impacts to the existing slope located immediately west of the stormwater detention pond, lining the pond may be necessary. Once the pond has been sited and proposed design grading established, we should review the plans in order to provide more detailed recommendations for the facility. Additional exploration including additional test pits or soil borings may be required dependent on proposed pond grading. ' 5.7 Drainage Surface Final exterior grades should promote free and positive drainage away from the building areas. Water must not be allowed to pond or collect adjacent foundations or within the immediate building area. We recommend providing a gradient of at least three percent for a minimum distance of ten feet from the building perimeter, except in paved locations. In paved locations, a minimum gradient of one percent should be provided unless provisions are included for collection and disposal of surface water adjacent the structure. 5.8 Utilities Utility pipes should be bedded and backfilled in accordance with American Public Works Association (APWA) ' or local jurisdictional requirements. At minimum, trench backfill should be placed and compacted as structural fill, as described in Section 5.2 of this report. As noted, the on-site soils should be suitable for use as backfill material. However, at the time of our study, the upper soil's moisture content appeared to be above optimum. ' Therefore, drying back or other means to condition the material will likely be necessary to facilitate proper compaction. If utility construction takes place during the winter, it may be necessary to import suitable wet weather fill for utility trench backfilling. 1 Page No. 10 1