2400 33RD ST GEOTECHNICAL INVESTIGATION 2018-01-02 MF Import

Over -excavation and Subgrade Preparation Recommendations As state(] in the MPA 2001 report, the site is underlain by medium dense, crushed granular soil over undocumented fill and medium stiff to very stiff glacially deposited silts and clays. Based on the results of the proof -rolling investigation, it is our opinion that the sandy gravel will provide suitable bearing for the pavement. As discussed above, there is a 1.2 to 1.5 foot layer of material, which overlies the site that is suitable for pavement subgrade. However, ti:>. site has not yet been contoured to the final subgrade elevations. There may be areas of the site where portions of this layer are excavated to improve site drainage. The depth and extent of these excavations are not known at this time. It is important that the cut areas have a minimum of 1_5 feet of competent subgrade soils beneath the final subgrade elevations prior to pavement placement. This is to minimize the potential for settlements and provide uniform subgrade support. In areas where the granular layer is cut into or excavated completely, additional unsuitable soil should be over -excavated and backfilled with compacted structural fill, so there is always a 1.5 foot layer of competent soil beneath the final pavement subgrade elevations. This over - excavation and backfilling should be done at the direction of the Engineer. An alternate plan to over -excavating to provide a 1.5 foot layer of competent subgrade beneath the final pavement subgrade elevations is to over -excavate such that there is a 12-inch layer of competent subgrade and use a geotcxtile. The cut areas would be over - excavated, a geotextile placed at the bottom of the excavation, and then the excavation backfilled with compacted structural fill. A woven, polypropylene, permeable geotextile, such as Mirafi Geolon IIP-370 or equivalent could be used at the base of the excavation. The structural fill and compaction- methods should comply with the recommendations outlined in the MPA 2001 report. Detention Vaults In addition to the recommendations provided in the MPA 2001 report, we recommend that there is 18 inches of compacted structural fill underneath the vaults to minimize s--ttlement. Also, ditching around the vault foundation pad may be required so that both rain, surface and groundwater is conveyed to the outside. The water can then be pumped out as required. Recommendations for Pond Filling There is in existing drainage detention pond on site. We understand that the pond is approximately four feet deep and will be filled as part of the site preparation. Our recommendations for filling the pond are as follows: • Drain the pond; • Place onsite material as fill up to the level of 24 inches below the surface of subgrade. Geolechnical and Tunnel Consultants 17270 Woodinville -Redmond Rd, Ste 703 Phone (425) 486-6561 Woodinville. WA 98072 Page 5 Fax (425)488-2660