The URL can be used to link to this page

Home My WebLink About1.20 Soils/Geohazards Evaluation 266 Red Cliff Circle, Glenwood Springs, CO 81601, (970) 230-9208 October 17, 2018 Project No. 18-050G-C1 Mr. Andy Bruno 5670 Brentwood Drive Hoffman Estates, IL 60192 andy@nutrientfarm.com Subject: Soils and Geohazard Evaluation, Riverbend PUD in Garfield County, Colorado Dear Mr. Bruno, RJ Engineering & Consulting, Inc. was requested to provide a soil and geohazard evaluation for the proposed Riverbend PUD site in Garfield County, Colorado. The evaluation was conducted to address requirements of the Garfield County LUDC impact analysis as outlined in Section 4- 203 G, items 3, 4 and a portion of 5. Figure 1 presents the approximate site location that we visited on October 10, 2018. SITE GEOLOGY We reviewed the “Geologic Map of the Storm King Mountain Quadrangle, Garfield County, Colorado by Bruce Bryan, Ralph R. Shroda, Anne E. Harding, and Kyle E. Murray, USGS Miscellaneous Field Studies Map MF-2389, 2002.” A section of the map is shown on Figure 2. The mapping indicates the flatter areas of the site consist of alluvial deposits generally comprised of sands, silts, clays and gravel of river or stream origin (map symbol Qfy). Two older landslides are mapped near the base of the slopes along the south side of the property (map symbol Qls). Colluvial deposits (map symbol Qc) are mapped in the central and east portions of the site. These materials typically heterogenous and consist of a mixture of sand, clay and gravel depending on source materials. The steeper slopes in the central portion, south and east sides of the side consist of varying bedrock formations. Bedrock below the west and south portions of the site generally consist of the Mancos Formation (map symbols Kmu, Kmn and Kml). The Mancos Formation generally consists of fine-grained shale and claystone bedrock with varying amounts of silt and sand. Bedrock below the east portion of the site appears to consist of Jurassic age Morrison Formation (map symbol Jm), and the Triassic age Chinle (map symbol Tc) and State Bridge (map symbol TPs) Formations. These formations generally consist of fine-grained shale and claystone bedrock with interbedded sandstone bedrock. The Dakota Formation (map symbol Kd) is mapped in the north-central section of the site. The Dakota Formation consists of sandstone bedrock and varies from weathered to very hard and typically forms ridge tops. Soils and Geohazard Evaluation Riverbend PUD, Garfield County Page | 2 GEOHAZARDS We reviewed the “Surficial Geology, Geomorphology, and General Engineering Geology of Parts of the Colorado River Valley, Roaring Fork River Valley, and Adjacent Areas, Garfield County, Colorado by B.K. Stover and J.M. Soule, Colorado Geologic Survey Open File Report 85-1, 1985.” The geohazard map for the site is shown on Figure 3. Based on the mapping and our observations, we believe the following geohazards may be present at the site: • Collapsible/Compressible Soils • Expansive/Swelling Soils • Debris Flows • Landslides and Potentially Unstable Slopes • Erosive Soils • Previous Mining Operations The lower sections of the site underlain by the alluvial soils may consist of potentially collapsible soils (map symbol CS). Our experience suggests that low density alluvial deposits can exhibit collapse or compression during and after development of a site. The west and south sections of the site generally consist of the Mancos Formation (map symbols Kmu, Kmn and Kml). Mancos can exhibit very high swell potential depending on the location. Bedrock below the east section of the site appears to consist of Jurassic age Morrison Formation (map symbol Jm) and the Triassic age Chinle (map symbol Tc) Formations. Our experience suggests that shale and claystone within all these formations can exhibit moderate to very high swell potential. The southern section of the site along the base of the steeper slopes is mapped as a debris/mud flow hazard (map symbol DMA). During moderate to heavy precipitation events, debris and/or mud flows can originate and mobilize down the drainages. These flows generally deposit materials in areas where the topography flattens. Field observation indicated that the southern section of the site is an area where debris flow deposition occurs. Two older landslides are mapped near the base of the slopes along the south side of the property (See Figure 2, map symbol Qls). Site observations indicate that the landslides appear to be inactive at this time. However, excavation at the base of the slopes and changes in groundwater conditions can act to reactivate these slide areas or initiate a new slides. Cut slopes, excavations, and surface water management should be properly designed to address global slope stability. A large portion of the site is mapped as moderate to high erosion potential (map symbols MEP and HEP). Erosion potential is defined as areas undergoing rapid headward erosion, gullying, and/or sheet erosion and exhibit moderate to high potential for continued erosion. Previous coal mining activity has occurred in areas south and west of the site. A coal mine is mapped about 1000 feet west of the site (See Figure 3). Our review of available mapping suggests that mining did not occur on or below this site. Mapping did suggest several locations where mine dumping occurred (See Figure 3). These materials consist of waste-rock debris from mining operations. In addition, observations made during our site visit indicated that previous grading activity has occurred in the southwest portion of the site. The approximate extents of this area are shown on Figure 3 and in Photo 1 below. Poorly compacted/low density Soils and Geohazard Evaluation Riverbend PUD, Garfield County Page | 3 fill may be present in this area. Low density fill material could consolidate under structural loading from buildings or other improvements. Photo 1 – Southwest portion of site looking south CONCLUSIONS Several potential geohazards were identified that could impact the development of this site. We believe these potential hazards can be mitigated with proper engineering design and planning. For example, collapsible and expansive soils can be mitigated using deep foundation systems for structures or engineered ground improvement techniques. Debris flow hazards can be mitigated through channeling of flows, avoidance of high-risk areas or through the use of fences or barriers. The risk of landslides and unstable slopes can be reduced through engineered cut slopes and surface water management that address global stability. Erosive soils can be mitigated with conventional erosion control techniques and proper drainage design. A site-specific geotechnical investigation should be performed to determine which geohazards and to what extent they are present at this site. The investigation should consist of test hole drilling and subsoil sampling. LIMITATIONS This study has been conducted in accordance with generally accepted geological and geotechnical engineering practices in this area for use by the client. The suggestions submitted in this report are based upon the data obtained from field reconnaissance and available documentation. Geologic conditions will also change over time periods, so that results of the analyses and recommendations will also change over time. Changes in vegetation due to flooding, fires, or regrading will change the analyses and recommendations presented herein. Approximate Area of Previous Site Grading Soils and Geohazard Evaluation Riverbend PUD, Garfield County Page | 4 The report was prepared in substantial accordance with the generally accepted standards of practice for geotechnical engineering as exist in the site area at the time of our investigation. No warranties, express or implied, are intended or made. If you have questions or need additional information, please call us at 970-230-9208. Sincerely, RJ Engineering & Consulting, Inc. Richard D. Johnson, P.E. Principal Approximate Site Location 0 500'1000' Figure 1Approximate SCALE: 1" = 1000' N Riverbend PUD PROJECT NO:PROJECT: 18-050G-C1 Geologic Map 0 500 1000 Figure 2Approximate SCALE: 1" = 1000' N Riverbend PUD PROJECT NO:PROJECT: 18-050G-C1 Note: Base map from "Geologic Map of the Storm King Mountain Quadrangle, Garfield County, Colorado by Bruce Bryant, Ralph R. Shroda, Anne E. Harding, and Kyle E. Murray, USGS Miscellaneous Field Studies Map MF-2389, 2002." Possible Mine Dump Areas Approximate Location of Previous Site Grading Geohazards Map 0 500 1000 Figure 3Approximate SCALE: 1" = 1000' N Riverbend PUD PROJECT NO:PROJECT: 18-050G-C1 Note: Base map from "Surficial Geology, Geomorphology, and General Engineering Geology of Parts of the Colorado River Valley, Roaring Fork River Valley, and Adjacent Areas, Garfield County, Colorado by B.K. Stover and J.M. Soule, Colorado Geologic Survey Open-file Report 85-1, 1985."