The Mark at Eugene January 6, 2026 <br />Seismic Hazard Study 3 Project No.: 2251077 <br />Eugene, Oregon Landmark Construction, LLC <br />The older flood deposits, typically found within the Portland Basin, usually consist of <br />layers of cobbles/boulders, gravel, and sand during a period of time when the river(s) <br />had sufficiently high flow to move large boulders (i.e., erratics) and coarser material. <br />In the Southern Willamette Valley, turbid floodwater eventually settled, depositing a <br />relatively thick layer (50 to 100 feet) of silt and clay (i.e., Willamette Silt) (Orr and Orr, <br />1999). <br />Local Geology <br />The project site is located within downtown/campus area of Eugene, south of the <br />Willamette River. Local geologic mapping indicates the site is underlain by alluvial fan- <br />delta deposits that transition to the Eugene Formation (Yeats et al., 1996; Madin and <br />Murray, 2006; McClaughry et al., 2010). The fan-delta alluvium consist of silt to <br />boulder-size material that was primarily deposited by the McKenzie and Willamette <br />Rivers (Yeats et al., 1996; Orr and Orr, 1999; O'Connor et al., 2001; Madin and <br />Murray, 2006). However, no particles larger than coarse gravel-size (±3 inches or <br />less) were observed in the alluvium during our explorations. The Eugene Formation <br />consists of tuffaceous sandstone and siltstone deposited in a shallow marine <br />environment (Yeats et al., 1996; Orr and Orr, 1999; O'Connor et al., 2001; Madin and <br />Murray, 2006). <br />A boring (ES5) and accompanying shear wave velocity log were completed at <br />Roosevelt Middle School (±1 mile south-southwest of the project site) in 1996 by <br />DOGAMI. This boring was part of a shear wave velocity study of the <br />Eugene-Springfield area (Wang et al., 1998). Subsurface conditions at ES5 included <br />clay and sand with trace rounded gravel underlain by sandstone (Eugene Formation). <br />The recorded shear wave velocities at ES5 were 325 to 748 feet/second (ft/s) in the <br />soil extending to ±16 feet and 1,703 to 5,758 ft/s in the underlying sandstone <br />extending to ±32.5 feet (Wang et al., 1998). The ES5 site is ±1 mile south- <br />southwest of the project site. <br />The ReMi testing was conducted at the site, and the shear wave velocities at the site <br />ranged from 176 to 784 ft/s in the upper 51 feet and 1,051 to 1,970 ft/s from ±51 <br />to 100 feet. Silt and sand residual soil (i.e., bedrock that has decomposed in place to <br />the consistency of soil) was encountered at the project site at a depth ranging from <br />±20 to 35.4 feet and extending to a maximum depth of 81.5 feet in the current <br />borings. Possible siltstone was encountered on site in the previous borings at ±50 feet <br />in BH-2 and ±43 feet in BH-4. <br />The shear wave velocities at ES5 and the project site are comparable to the subsurface <br />conditions encountered at each location. More information on the on-site ReMi study <br />can be found in Appendix E. <br />The subsurface conditions encountered in our exploratory borings are consistent with <br />the mapped local geology. Residual soil was encountered in each of the borings. Details <br />are provided in the Subsurface Conditions section of the main report and on the <br />exploration logs in Appendix B.