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Home My WebLink About1.22 Geo Hazard Study North Hangs Mine February 2023 Appendix 3 – Geologic Hazard Analysis 6735 Kumar Heights Colorado Springs, CO 80918 phone: (719) 632-7009 fax: (719) 632-1049 email: kacolospgs@kumarusa.com www.kumarusa.com Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado GEOLOGIC HAZARDS STUDY PROPOSED NORTH HANGS MINE 6533 COUNTY ROAD 346 GARFIELD COUNTY, COLORADO PROJECT NO. 21-7-346 JUNE 3, 2021 REVISED JUNE 10, 2021 PREPARED FOR: WESTERN SLOPE MATERIALS LLC ATTN: SEAN MELLO P.O. BOX 1319 CARBONDALE, COLORADO 81623 smello@ws-materials.com Kumar & Associates, Inc. ® Project No. 21-7-346 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY ....................................................................................... - 2 - PROPOSED DEVELOPMENT.................................................................................................. - 2 - SITE CONDITIONS ................................................................................................................... - 2 - GEOLOGIC SETTING .............................................................................................................. - 3 - FIELD EXPLORATION ............................................................................................................ - 3 - SUBSURFACE CONDITIONS ................................................................................................. - 3 - GEOLOGIC SITE ASSESSMENT ............................................................................................ - 4 - POTENTIAL FLOODING ..................................................................................................... - 4 - SEASONALLY SHALLOW GROUNDWATER/SEASONAL SEEPAGE ......................... - 4 - SEISMICITY .......................................................................................................................... - 5 - RADIOACTIVE GASES........................................................................................................ - 5 - DEVELOPMENT CONSIDERATIONS ................................................................................... - 6 - SITE GRADING ..................................................................................................................... - 6 - LIMITATIONS ........................................................................................................................... - 6 - REFERENCES ........................................................................................................................... - 7 - FIGURE 1 – LOCATION OF EXPLORATORY BORINGS FIGURE 1A – GEOLOGY MAP FIGURE 2 – LOGS OF EXPLORATORY BORINGS FIGURE 3 – LEGEND AND NOTES FIGURES 4 and 5 – GRADATION TEST RESULTS TABLE 1 – SUMMARY OF LABORATORY TEST RESULTS - 2 - Kumar & Associates, Inc. ® Project No. 21-7-346 PURPOSE AND SCOPE OF STUDY This report presents the results of a Geologic Hazards Study for the proposed North Hangs Mine to be located at 6533 County Road 346, Garfield County, Colorado. The project site is shown on Figure 1. The purpose of the study was to evaluate the geologic conditions and their potential impact on the project. The study was conducted in accordance with our agreement for geological engineering services to Western Slope Materials, dated April 13, 2021. A reconnaissance of the project site was conducted on May 7 and 14, 2021 to obtain information on the geologic conditions. Aerial photographs and published regional geologic and engineering geology maps were also reviewed. This report summarizes the data obtained during this study and presents our conclusions, recommendations, and other geologic considerations based on the proposed construction and geologic conditions observed. PROPOSED DEVELOPMENT We understand the proposed construction will include development of approximately 72.8 acres of mostly vacant land. Site grading is proposed to have maximum cut depths up to around 26 feet. We understand that the geologic hazards study is required for the submittal for special use permit to Garfield County. If development plans change significantly from those described, we should be notified to re- evaluate the recommendations presented in this report. SITE CONDITIONS The proposed North Hangs Mine consists of about 72.8 acres located in parts of the NW portion of Section 9, T6S, R92W of the 6th Principal Meridian in Garfield County, Colorado. The project site is currently vacant. The property is bordered on the north by the Colorado River, the southeast by Interstate 70 and the west by privately owned ranchland. The proposed mine is located in Garfield County southwest of Silt, Colorado. The topography consists of valley bottom with nearly level slopes generally about ½ percent down to the northwest. An irrigation ditch runs from the Colorado River to the west through about the middle of the property. There are currently minor ditches and flood-irrigated fields throughout the property. Vegetation is mainly composed of grasses, with shrubs and some small trees, with larger cottonwood trees near the main ditch and Colorado River. Sand and gravel with cobbles and small boulders were exposed along the bank of the river and at the surface throughout the property. - 3 - Kumar & Associates, Inc. ® Project No. 21-7-346 GEOLOGIC SETTING The main geologic features in the project area are shown on Figure 1A. This map is based on the published regional map by Shroba and Scott (2001) and on our field mapping performed on May 7 and 14, 2021. The project site lies to the north of the northwest-southeast trending axis of the Rifle Syncline and is underlain by the Eocene-age Shire Member of the Wasatch Formation. The Shire Member consists of interbedded claystone and siltstone with minor sandstone beds ranging in color from purple to brown to yellowish gray and has a bedding dip of less than about 5 degrees to the west- northwest in the area of the project site. Surficial deposits at the project site predominately consist of Holocene and late Pleistocene-age floodplain and stream-channel deposits (Qfp). These deposits consist of silty sand with gravel and gravel and cobbles with scattered small boulders in a silty sand matrix. The deposits range from about 16 to 33 feet thick along the Colorado River (Shroba and Scott, 2001). FIELD EXPLORATION The field exploration for the project was conducted on May 14, 2021. Six exploratory borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions. The borings were advanced with 4 inch diameter continuous flight augers powered by a truck- mounted CME-45B drill rig. The borings were logged by a representative of Kumar & Associates, Inc. Samples of the subsoils were taken with 1⅜ inch and 2 inch I.D. spoon samplers. The samplers were driven into the subsoils at various depths with blows from a 140 pound hammer falling 30 inches. This test is similar to the standard penetration test described by ASTM Method D-1586. The penetration resistance values are an indication of the relative density or consistency of the subsoils. Depths at which the samples were taken and the penetration resistance values are shown on the Logs of Exploratory Borings, Figure 2. The samples were returned to our laboratory for review by the project engineer and testing. SUBSURFACE CONDITIONS Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The subsoils consist of nil to about 3½ feet of topsoil overlying medium dense to very dense silty sand and gravel with cobbles and probable boulders. About 2 feet of previously placed fill material consisting of very sandy gravel and clay was encountered at the surface in Boring 5. - 4 - Kumar & Associates, Inc. ® Project No. 21-7-346 About 1½ to 6½ feet of sandy to very sandy clay was encountered below the topsoil/fill in Borings 4 and 5, respectively. Drilling in the dense granular soils with auger equipment was difficult due to the cobbles and boulders and drilling refusal was encountered in the deposit in Borings 3 and 6. Laboratory testing performed on samples obtained from the borings included natural moisture content and density and gradation analyses. Results of gradation analyses performed on small diameter drive samples (minus 1½-inch fraction) of the coarse granular subsoils are shown on Figures 4 and 5. The laboratory testing is summarized in Table 1. Free water was encountered in the borings at the time of drilling at depths between 3 and 8½ feet in Borings 1, 2, 4, 5, and 6 and when checked 11 days later at depths between 2 and 6 feet in Borings 2, 4, 5, and 6 and the subsoils were moist to wet with depth. GEOLOGIC SITE ASSESSMENT The project site geology should not present major constraints or unusually high risks to the proposed development. There are, however, several conditions of a geologic nature that should be considered. Geologic conditions that should be considered, their potential risks, and mitigations to reduce the potential risks are discussed below. The site could experience moderate levels of earthquake related ground shaking. POTENTIAL FLOODING According to the “Flood Insurance Rate Map”, map number 0802051091C by the Federal Emergency Management Agency (FEMA, 2006); the site is in Zone A (areas subject to flooding by the 1 percent annual chance (100-year) flood). Flood modelling is beyond the scope of this study. We understand a flood hazard assessment is being performed by others. SEASONALLY SHALLOW GROUNDWATER/SEASONAL SEEPAGE Indications of possible seasonally shallow groundwater were observed along the existing irrigation ditches in the area of Boring 1. The shallow groundwater and seepage is likely due to ongoing flood irrigation of the fields currently in the area and should not be a concern once irrigation is stopped and development of the gravel pit begins. If seepage is encountered during development, it may need to be collected and diverted away from structures and roadways. Significant dewatering is expected to be needed for the gravel pit development below groundwater level. - 5 - Kumar & Associates, Inc. ® Project No. 21-7-346 SEISMICITY Historic earthquakes within 150 miles of the project site have typically been moderately strong with magnitudes less than 5.5 and maximum Modified Mercalli Intensities less than VI (Widmann and Others, 1998). The largest historic earthquake in the project region occurred in 1882. It was located in the northern Front Range and had an estimated magnitude of about M6.4 ± 0.2 and a maximum intensity of VII. Historic ground shaking at the project site associated with the 1882 earthquake and the other larger historic earthquakes in the region does not appear to have exceeded Modified Mercalli Intensity VI (Kirkham and Rogers, 2000). Modified Mercalli Intensity VI ground shaking should be expected during a reasonable exposure time for the mine, but the probability of stronger ground shaking is low. Intensity VI ground shaking is felt by most people and causes general alarm, but results in negligible damage to structures of good design and construction. Using estimated shear wave velocities for the subgrade materials encountered based on standard penetration testing, calculations indicate that the seismic soil profile at the project site should be considered as Class C, very dense soil and soft rock, as described in the 2018 International Building Code, unless site specific shear wave velocity studies show otherwise. Based on the subsurface profile and the anticipated ground conditions, liquefaction is not a design consideration. Seismicity at the subject site is similar to that of the surrounding area. Using the USGS National Earthquake Hazard Reduction Program online database, the following probabilistic ground motion values are reported for the project site. Intensity Measure Type Intensity Measure Level 2 percent in 50 Years 0.2 Sec. Spectral Acceleration Ss 0.347 1.0 Sec. Spectral Acceleration S1 0.078 The USGS National Earthquake Hazard Reduction Program online database also indicates a peak ground acceleration (PGA) of 0.212g at the subject site. The PGA is the lower of either the deterministic or probabilistic value with a 2% exceedance probability for a 50-year exposure time at the project site (statistical recurrence interval of 2,500 years). RADIOACTIVE GASES The project site is not located on geologic deposits that would be expected to have high concentration of radioactive minerals. However, there is a potential that radon gas could be present in the area. It is difficult to assess future radon gas concentrations in buildings before the buildings are constructed. Testing for radon gas levels could be done when occupied structures - 6 - Kumar & Associates, Inc. ® Project No. 21-7-346 have been completed. New buildings are often designed with provisions for ventilation of lower enclosed areas should post construction testing show unacceptable radon gas concentration. No permanent structures are proposed as part of the gravel pit development. DEVELOPMENT CONSIDERATIONS Presented below is a discussion of geologic and geotechnical engineering related development considerations, including identified geologic hazards. SITE GRADING The topsoil, existing fill, silty sand and clay soils should be stripped down to the gravel deposit. Mining cut slopes should not be steeper than 1½:1 (horizontal to vertical) and reclaimed cut and fill slopes should not be steeper than 2:1 (horizontal to vertical) for dry granular soil conditions. The risk of slope instability will be significantly increased if seepage develops in cuts and fills. If seepage is encountered in permanent cuts or fills, an investigation should be conducted to determine if the seepage will adversely affect the slope stability. We assume dewatering will be performed as mine development progresses and should be designed to prevent seepage in cut slopes. Positive surface drainage should be provided for all permanent cuts and fills to direct the surface runoff away from the slope faces. Cut and fill slopes and other stripped areas should be protected against erosion by revegetation or other means. Fills should be benched into slopes exceeding 3 horizontal to 1 vertical. Site grading should be planned to provide positive surface drainage away from all building and roadway areas. Formal stability analyses were not performed to evaluate the slopes recommended above. Published literature and our experience with similar cuts and fills indicate the recommended slopes should have adequate factors of safety. If a detailed stability analysis is required, we should be notified. LIMITATIONS This study has been conducted in accordance with generally accepted geological and geotechnical engineering principles and practices in this area at this time. We make no warranty either express or implied. The conclusions and preliminary recommendations submitted in this report are based upon our field observations, aerial photograph interpretations, published regional geology information, the proposed type of construction, and our experience in the area. This report has been prepared for the exclusive use by our client for planning and preliminary design purposes. We are not responsible for technical interpretations by others of our Kumar & Associates Kumar & Associates Kumar & Associates Kumar & Associates Kumar & Associates TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Project No. 21-7-346 SAMPLE LOCATION NATURAL MOISTURE CONTENT NATURAL DRY DENSITY GRADATION PERCENT PASSING NO. 200 SIEVE ATTERBERG LIMITS UNCONFINED COMPRESSIVE STRENGTH SOIL TYPE BORING DEPTH GRAVEL SAND LIQUID LIMIT PLASTIC INDEX (%) (%) (ft) (%) (pcf) (%) (%) (psf) 1 3 and 5 65 29 6 Slightly Silty Sandy Gravel 2 5 and 10 62 32 6 Slightly Silty Sandy Gravel 4 5 and 10 59 31 10 Slightly Silty Sandy Gravel 6 5 6.5 131 56 37 7 Slightly Silty Very Sandy Gravel