The Mark at Eugene January 6, 2026 <br />Geotechnical Investigation and Seismic Hazard Study 6 Project No.: 2251077 <br />Eugene, Oregon Landmark Construction, LLC <br />The Site Class used in calculating ground motion parameters is based on definitions <br />provided in Chapter 20 of ASCE 7-16, where the subsurface is characterized based <br />on the average shear wave velocity or soil stiffness/density within the upper <br />100 feet. The shear wave velocity was estimated using ReMi test methods as <br />documented in the report prepared by Earth Dynamics, LLC (Appendix E). Their <br />analysis indicates an average shear wave velocity (Vs30) of 974 ft/s, corresponding <br />to a Site Class D profile. The seismic design parameters and 2022 OSSC response <br />spectrum are shown in Figure 3A. <br />When developing the design response spectrum for a Site Class D, the 2022 OSSC <br />requires the criteria of ASCE 7-16 Section 11.4.8 to be considered. ASCE 7-16 <br />Section 11.4.8 requires a ground motion hazard analysis to be performed in <br />accordance with ASCE 7-16 Section 21.2 at sites where the 1.0 second spectral <br />acceleration on rock (S1) is greater than or equal to 0.2g. However, there is an <br />exception in ASCE 7-16 Section 11.4.8. It stipulates a ground motion hazard <br />analysis is not required when the seismic response coefficient (Cs) is calculated based <br />on Eq. 12.8-2 for values of T ≤ 1.5TS and taken as equal to 1.5 times the value <br />computed using either Eq. 12.8-3 for T > 1.5TS or Eq. 12.8-4 for T ≥ 1.5TL. This <br />applies where TS = SD1/SDS and TL is the long-period transition period shown in Figure <br />22-14 in ASCE 7-16 Chapter 22. The TL value for Oregon is 16 seconds. <br />The adjustment in the Cs value is intended to better model long-period spectral <br />accelerations for softer soils coupled with strong ground motions. However, the <br />adjustment applies only to the design of long-period structures (i.e., typically <br />structures with a height of five stories or greater). For the current project, we assume <br />the period of interest for the structure will be less than 1.3 seconds (i.e., 1.5TS). <br />Consequently, there is no Cs adjustment necessary when using the exception in <br />Section 11.4.8. Therefore, we developed the site response spectrum shown in <br />Figure 3A using the mapped risk-targeted maximum considered earthquake (MCER) <br />ground motions and the general procedure in ASCE 7-16 Section 11.4.6 with Fa <br />selected based on Table 11.4-1 and Fv selected based on Table 11.4-2. <br />Liquefaction Hazard <br />Liquefaction is a phenomenon that occurs when dynamic loading causes contraction <br />of the soil structure, leading to an increase in pore water pressure and loss of soil <br />shear strength. Soils subject to liquefaction typically consist of saturated, loose to <br />medium dense sand and non-plastic or soft to medium stiff non-plastic to low <br />plasticity silt (i.e., a plasticity index (PI) less than 8). These soils were not <br />encountered in our explorations to a maximum depth of ±81.5 feet. Therefore, we <br />have concluded the liquefaction risk is low. <br />DISCUSSION OF GEOTECHNICAL CONSIDERATIONS <br />The following sections provide a discussion of the site soils as they relate to <br />considerations for foundation design and construction. Additional details are provided <br />in the Recommendations section of this report.