1815 13TH ST SEATTLE CHILDRENS NORTH CLINIC Geotech Report 2022-10-25

distribution), while non-yielding walls supporting horizontal backfill be designed using an equivalent fluid density of 55 pcf(triangular distribution).For seismic loading conditions,a rectangular earth pressure equal to 7H pounds per square foot (psf) (where H is the height of the wall in feet) should be added to the active/at-rest pressures. If cast-in-place walls will be subjected to the influence of surcharge loading from traffic or nearby retaining walls within a horizontal distance equal to the height of the retaining wall, the walls should be designed for the additional horizontal pressure using an appropriate design method. A common practice is to assume a surcharge loading equivalent to 2 feet of additional fill to simulate traffic loading; we consider this method appropriate for typical situations. Other surcharge loading should be applied as appropriate. Lateral resistance for conventional cast-in-place walls can be provided by frictional resistance along the base of the wall and passive resistance in front of the wall. For walls founded on native soils,the allowable frictional resistance may be computed using a coefficient of friction of 0.4 applied to vertical dead-load forces. The allowable passive resistance may be computed using an equivalent fluid density of 400 pcf (triangular distribution). The above coefficient of friction and passive equivalent fluid density values incorporate a factor of safety of about 1.5. The above soil pressures assume that wall drains will be installed to prevent the buildup of hydrostatic pressure behind the walls,as discussed below. 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 or the detention vault shall be designed for hydrostatic pressures. For hydrostatic conditions, an equivalent fluid density of 62 pcf (triangular distribution)should be added to the active/at-rest earth pressures. Earthwork and Structural Fill Excavation Considerations Fill and glacially consolidated soils were observed in the explorations. We anticipate that these soils may be excavated with conventional excavation equipment,such as trackhoes or dozers.Cobbles and boulders were observed in the excavations and the contractor should be prepared to deal with them during construction. GEOENGINEERSI April 22,2016 1 Page 8 File No.10738.010.01