2710 14TH ST BLDG A Geotech Report 2023-12-29

' ■ The contractor should cover all soil stockpiles that will be used as structural fill with plastic sheeting. ■ Construction and foot traffic should be restricted to specific areas of the site, preferably areas that are surfaced with materials not susceptible to wet weather disturbance. ■ Construction activities should be scheduled so that the length of time that soils are left exposed to ' moisture is reduced to the extent practicable. 4.5.8. Utility Trenches ' Trench excavation, pipe bedding, and trench backfilling should be completed using the general procedures described in the 2018 WSDOT Standard Specifications or other suitable procedures specified by the project ' civil engineer. The glacial deposits, and fill soils encountered at the site are generally of low corrosivity based on our experience in the Puget Sound area. ' Utility trench backfill should consist of structural fill and should be placed in lifts of 12 inches or less (loose thickness) when using heavy compaction equipment or 6 inches or less when using hand -operated equipment such that adequate compaction can be achieved throughout the lift. Each lift must be ' compacted prior to placing the subsequent lift. Prior to compaction, the backfill should be moisture conditioned to within 2 percent of the optimum moisture content, if necessary. The backfill should be compacted in accordance with the criteria discussed above. 4.5.9. Pavement Subgrade Preparation We recommend that the subgrade soils in new pavement areas be prepared and evaluated as described ' in Sections 4.5.4 and 4.6. In cut areas in medium dense to very dense glacial till, we recommend that the exposed subgrade be proofrolled. Where existing fill or loose to medium dense native soils exist, we recommend that the upper 12 inches of the existing site soils be compacted to at least 95 percent of the ' MDD per ASTM D1557 and then proofrolled prior to placing pavement section materials. If the subgrade soils are loose or soft, it may be necessary to excavate the soils and replace them with structural fill, gravel borrow, or gravel base material. Based on our explorations, the majority of the subgrade soils are expected to consist of fill, weathered native soils, and relatively unweathered glacial till. Pavement subgrade conditions should be observed and proofrolled during construction to evaluate the presence of unsuitable ' subgrade soils and the need for over excavation. 4.5.10. Excavations ' Temporary open cut slopes will likely be used to complete excavations for the project. Excavations are also required for underground utilities. The stability of open cut slopes is a function of soil type, groundwater seepage, slope inclination, slope height and nearby surface loads. The use of inadequately designed open cuts could impact the stability of adjacent work areas, existing utilities, and endanger personnel. The contractor performing the work has the primary responsibility for protection of workers and adjacent ' improvements. In our opinion, the contractor will be in the best position to observe subsurface conditions continuously throughout the construction process and to respond to variable soil and groundwater conditions. Therefore, the contractor should have the primary responsibility for deciding whether or not to ' use open cut slopes for much of the excavations rather than some form of temporary excavation support, and for establishing the safe inclination of the cut slope. Acceptable slope inclinations for utilities and ancillary excavations should be determined during construction. Because of the diversity of construction ' GMENGINEERS� October30,2019 Page16 File No. 21288-002 00