1400 W MARINE VIEW DR WATERFRONT PLACE APARTMENTS Geotech Report 2022-10-25

• Drainage Positive drainage should be provided behind cast-in-place retaining walls by placing a minimum 2-foot-wide zone of Mineral Aggregate Type 17 (bank run gravel), City of Seattle Standard Specification 9-03.16, with the exception that the percent passing the U.S. No. 200 sieve is to be less than 3 percent. A perforated drainpipe should be placed near the base of the retaining wall to provide drainage. The drainpipe should be surrounded by a minimum of 6 inches of Mineral Aggregate Type 22 (3/4-inch crushed gravel) or Type 5 (1-inch washed gravel), City of Seattle Standard Specification 9-03.16, or an alternative approved by GeoEngineers.The Type 22 or Type 5 material should be wrapped with a geotextile filter fabric meeting the requirements of construction geotextile for underground drainage, WSDOT Standard Specification 9-33. The wall drainpipe should be connected to a header pipe and routed to a sump or gravity drain.Appropriate cleanouts for drainpipe maintenance should be installed. A larger-diameter pipe will allow for easier maintenance of drainage systems. If no drainage is used, the cast-in-place retaining walls shall be designed for hydrostatic pressures. For hydrostatic conditions,the wall should be designed using an equivalent fluid density of 90 pcf. These lateral soil pressures assume that the ground surface behind the wall is horizontal. Excavations Excavations are anticipated for construction of the building and for underground utilities.We anticipate that excavations can likely be made as temporary open cut slopes depending on site constraints. Where temporary cut slopes are not feasible, temporary shoring will be required. 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 workmen 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 techniques and available shoring systems,the design of temporary shoring is most appropriately left up to the contractor proposing to complete the installation.Temporary cut slopes and shoring must comply with the provisions of Title 296 Washington Administrative Code (WAC), Part N, "Excavation, Trenching and Shoring." The following sections summarize the general excavation recommendations. If conditions allow, excavations for the proposed building may be accomplished using temporary cut slopes. Temporary Cut Slopes For planning purposes, temporary unsupported cut slopes more than 4 feet high may be inclined at 1/H:1V(horizontal to vertical) maximum steepness within the fill. If significant seepage is present on the • cut face then the cut slopes may have to be flattened. However,temporary cuts should be discussed with GEOENGINEER.g May31,2017 Page 11 File No.22360-003-00