2025-06-23_REVISION - M1980244 (31)

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Cripple Creek&Victor Gold Mine Valley Leach Facility Expansions ■ Detailed Design for Permitting NewFields Project No.475.0106.064 • March 2024 o For open excavation where the depth to bedrock is greater than 7 feet below the final SLF subgrade surface: Backfill the open shaft with coarse shaft backfill to 3 feet below the soil/bedrock interface. Then place a concrete plug, cemented rockfill, and structural fill to achieve SLF subgrade surface. o For collapsed excavations where the depth to bedrock is less than 7 feet below the final SLF subgrade surface: Excavate collapsed material from shafts to a depth of 25 feet below the SLF subgrade surface, then backfill with coarse shaft backfill to approximately 10 feet below the SLF subgrade surface and then place a concrete plug, cemented rock fill, and structural fill to achieve SLF subgrade surface. o For collapsed excavations where the depth to bedrock is greater than 7 feet below the final SLF subgrade surface: Excavate collapsed materials from shafts to a depth of 25 feet below the SLF subgrade surface, then backfill with coarse shaft backfill to approximately 3 feet below the soil/bedrock interface, and then place a concrete plug, cemented rock fill, and structural fill to achieve SLF subgrade surface. Excavate shallow surface pits to rock or to 25 feet below the SLF subgrade surface • (whichever is less), then backfill with compacted select structural fill and/or structural fill to the final SLF subgrade elevation. For shallow mine adits and tunnels: Collapse the crown pillar by drilling and blasting, excavate collapsed crown pillar, backfill with coarse shaft backfill, then install a geogrid cap. 2.4. Seismicity The site-specific seismic hazard assessment (SHA) was reviewed and updated from previous reporting. The "Valley Leach Facility 2 Phase 3 Detailed Design for Permitting" (NewFields, 2020) applied a peak ground acceleration of 0.14g for a 1 in 2,475 years annual exceedance value (AEP) seismic event. NewFields re-evaluated the SHA using the United States Geologic Service (USGS) 2018 National Seismic Hazard Maps (NSHM) to identify background seismicity and local fault sources. The foundation of the VLF Expansions can be classified as a Site Class B/C based on the shear wave velocity recorded during the geophysical survey completed by Intermountain GeoEnvironmental Services (IGES). A Site Class B/C classifies as rock or very dense soil and soft rock in the upper 100-feet of the subsurface profile with an average velocity between 1,200 and 5,000 feet per second (ASCE 7-16) seismic design criteria, as defined by the NSHM for a Site Class B/C foundation condition is presented in Table 2.1.-This design criteria was applied in the current evaluation. • Page 4