400 RIVERSIDE RD SEPA 016 - 039 - RIVERSIDE BUSINESS PARK Geotech Report 2025-04-14

December 8,2016 Project No. T-7388-1 5.7 Drainage Surface Final exterior grades should promote free and positive drainage away from the building areas. 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 two percent should be provided, unless provisions are included for collection and disposal of surface water adjacent to the structure. ' Subsurface In our opinion, with the area immediately adjacent to the structure paved, and positive surface drainage maintained, perimeter foundation drains would not be necessary. If the grade is not positively drained away from ' the structure or is landscaped, perimeter foundation drains should be installed. To guard against hydrostatic pressure development, retaining wall drainage must be installed. We recommend ' that wall drainage consist of a minimum 12-inch thick layer of washed or pea gravel placed adjacent to the wall. Alternatively, a composite drainage panel such as Mirafi GI00N or equal can be used. A four-inch diameter perforated pipe should be placed on a bed of gravel along the base of the wall footing and directed to a suitable outlet. 5.8 Utilities Utility pipes should be bedded and backfilled in accordance with American Public Works Association (APWA), ' or City of Everett specifications. As a minimum, trench backfill should be placed and compacted as structural fill, as described in Section 5.2 of this report. Most inorganic native and existing fill soils excavated on the site should be suitable for use as backfill material during dry weather conditions. However, if utility construction takes place during the wet winter months, it may be necessary to import suitable wet weather fill for utility trench backfil ling. ' Depending on final utility grades, the utility contractor should also be prepared for encountering unstable soft peat soils below the pipe invert elevations. If not removed from below the pipe and replaced with crushed rock ' or additional bedding material, pipe deflections will occur as a result of the soil yielding and compressing in response to loading imposed by the trench backfill. Page No. 9