The Mark at Eugene January 6, 2026 <br />Geotechnical Investigation and Seismic Hazard Study 11 Project No.: 2251077 <br />Eugene, Oregon Landmark Construction, LLC <br />Retaining Walls <br />We understand relatively short retaining walls may be used in some locations. The <br />retaining walls may be either cantilevered or restrained and are expected to <br />predominately retain landscaping; however, due to the proximity of the new structure <br />may be required to support surcharging as a result of the building loads. Therefore, <br />we recommend using at-rest earth pressures for all retaining walls. <br />Based on our explorations, we anticipate the retaining walls will be supported on stiff <br />sandy silt with some clay or crushed gravel fill. For our analysis, we assumed the <br />retained material will consist of crushed gravel fill and the foundation material <br />consists of fine-grained alluvium. The recommended soil properties for retaining wall <br />design are summarized in Table 1. <br />Table 1. Recommended Soil Properties for Retaining Wall Design <br />Zone Soil Type Unit Weight <br />(pcf) <br />Friction Angle <br />(φ) <br />(deg) <br />Cohesion <br />(c) <br />(psf) <br />Retained Soil Crushed Gravel 130 34 - <br />Foundation Soil Sandy SILT, some clay 110 30 - <br /> <br />Wall Drainage. A wall drain should be installed behind the wall backfill with the <br />flowline set at the base of the wall foundation. The wall drain should consist of 3- <br />inch or 4-inch diameter, perforated or slotted, HDPE or PVC pipe. The pipe should be <br />backfilled with Granular Drain Backfill, as defined in the Recommendations section <br />of this report, extending at least 8 inches beyond the top and sides of the pipe. The <br />entire mass of the Granular Drain Backfill should be wrapped in a Subsurface Drainage <br />Geotextile that laps a minimum of 12 inches at the top. The drainpipe should include <br />cleanouts located at regular intervals and should discharge the water away from the <br />wall. <br />If wall drainage is not practical, the retaining wall should include water proofing and <br />be designed for hydrostatic pressures as well as earth pressures. Temporary drainage <br />of these areas may be required for construction of the walls. <br />Lateral Earth Pressure. An equivalent fluid density of 55 pcf may be used to represent <br />the lateral earth pressure against walls with drains. This value assumes an at-rest <br />condition (i.e., the wall is relatively rigid against rotation) and level backfill. <br />Walls that do not include drainage should be designed using an equivalent fluid <br />density of 90 pcf to represent the hydrostatic pressure and the undrained lateral <br />earth pressure on the wall.