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HomeMy WebLinkAboutGeological Hazards Report 04.02.2020l(t ffiffiffii'fi*"RECEIVED ,iir'¡i t ,1 "il;,':t GARFIELD COUNTY rn ¡mprofS5rt$flIffi uêf'l{ffdåy 5020 County Road 154 Glenwood Springs, CO 81601 phone: (970) 945-7988 fax: (970) 94s-8454 email : kaglenwood@kumarusa.corn www.kumarusa.col¡ Office L¡cations: Denver (HQ), Parke¡ Colorado Springs, Forl Collir'¡s, Glenwood Springs, and Summit County, Colorado Apnl2,2020 Michaleen and Michael Jeronimus 1012 13th Street Golden, CO 80401 rnaj eronimus@ grnail. com Project No.20-7-147 Subject: Geologic Hazarðs Review, Proposed Residence, Barn/ADU, and Pond, Burry Ranch, 9155 Highway 82, Garfield County, Colorado Ladies and Gentlemen: As requested, Kumar & Associates, Inc. has reviewed the potential geologic hazards that could impact the project site. Our findings are presented in this report. The services were performed in accordance with our proposal for professional engineering services to Michaleen and Michael Jeronimus dated February 12,2020, Proposal No. P7-20-159. PROPOSED CONSTRUCTION The proposed residence will be a two-story wood frame structure with attached garage. The proposed barn is assumed to be a two-story structure with accessory dwelling unit on the upper level. Ground floors are assumed to be structural over crawlspace for the residence and slab-on- grade for the garage and barn. Grading for the structure is assumed to be relatively minor with cut depths between about 2 to 5 feet. SITE CONDITIONS The subject site was vacant at the time of our field exploration. The ground surface is sloping gently down to the west at grades between 2 and 4 percent in the development area. Elevation difference across the proposed house location is about 3 feet and across the proposed barn location is about lz foot. The Roaring Fork River is to the west and south more than 10 feet below the proposed house elevation. There is a 30 to 50 percent slope down to the west that is the edge of a higher terrace located about 150 feet to the east of the proposed development. Vegetation at the site consists of native grass and weeds in the development area with sage and other brush along the steeper slope areas and along the river. GEOLOGIC SETTING The project site is located in the Carbondale Collapse Center. The Carbondale Collapse Center formed in the late Cenozoic due to evaporite tectonism. The Eagle Valley Evaporite migrated plastically upwards and laterally toward the Colorado River and Roaring Fork River bottoms due to a reduction in vertical stress caused by the erosion of overburden material by the rivers. Subsidence occurred in areas of the thinned evaporite and beneath the rivers due to dissolution Michaleen and Michael Jeronimus Apnl2,2020 Page2 (Kirkham, and Others, 2003). Much of this subsidence appears to have occurred within the past 3 million years which also corresponds to high incision rates of the Roaring Fork and Crystal Rivers (Kunk and Others, 2002). It is uncertain if the regional subsidence is still an active geomorphic process or if evaporite subsidence has stopped. If still active, present deformations may be occurring at rates similar to past long-term rates of between 0.5 and l.ó inches per 100 years. These slow deformation rates should not present a potential risk to the proposed development. The project site lies on the axis of the Cattle Creek Anticline. The site also lies on the axis of a younger (Pleistocene- to Holocene-age) anticline related to evaporite diapirism that essentially follows the alignment of the Roaring Fork River. The site is underlain by late Pleistocene-age terrace deposits. The terraces have been back-tilted due to upwelling of the Eagle Valley Evaporite along the Roaring Fork River (Kirkham and Others, 2014). A third anticline bisects the site trending to the northeast. GEOLOGIC HAZARDS REVIEW Potential major geologic hazards that could impact the site consist of potentially unstable slopes, potential sinkholes, flooding, and hydrocompressive soils. These conditions, their potential risks, and mitigations to reduce the potential risks are discussed below. Potential hydrocompressive soil hazards and mitigation options are discussed in our subsoil study report dated March 18,2020, Project No.20-7-147 . Potentially Unstable Slopes: The steep hillside to the east of the development area is sloping at greater than 30 percent down to the west. No development is planned on the steep slope. No signs of bulging, ground cracks, or water seepage were observed on the steep slope. In our opinion, the steep hillside is currently stable and the proposed development will not adversely impact the existing stability of the steep slope. Subsidence and Sinkhole Potential: Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the subject site. These rocks are a sequence ofgypsiferous shale, fine-grained sandstone and siltstone with some massive beds of gypsum and limestone. There is a possibility that massive gypsum deposits associated with the Eagle Valley Evaporite underlie portions of the lot. Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can produce areas of localized subsidence. During previous work in the area, several sinkholes were observed scattered throughout the Carbondale and Aspen Glen areas. These sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the Eagle Valley. No evidence of cavities was encountered in the subsurface materials; however, the exploratory borings were relatively shallow, for foundation design only. Based on our present knowledge of the subsurface conditions at the site, it cannot be said for certain that sinkholes will not develop. The risk of future ground subsidence on the subject site throughout the service life of the Kumar & Associates, lnc. ô Project No, 20-7-147 Michaleen and Michael Jeronimus Apnl2,2020 Page 3 proposed residence, in our opinion, is low; however, the owner should be made aware of the potential for sinkhole development. If further investigation of possible cavities in the bedrock below the site is desired, we should be contacted. At the time of our visit to the site, a small depression was observed to the north of the proposed bam about 200 feet north of the proposed driveway. The depression was about 45 feet across in the north-south direction, 30 feet across in the east-west direction and about 4 feet deep. The depression had rounded granitic and sandstone derived cobbles and small boulders exposed in the bottom and was vegetated with willows and native grass and weeds. The small depression is possibly a sinkhole. Without additional subsurface exploration, the overall extent of the potential sinkhole cannot be determined. Development is not recommended within 100 feet of the edges of the depression. The current development is more than 100 feet from the edges of the depression. Potential Flooding: According to the "Flood Insurance Rate Map", map number 080205 1465 B by the Federal Emergency Management Agency (FEMA, 1986); the proposed development area is inZone C (unshaded - area of minimal flooding). Radiation Potential: 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. lt is difficult to assess future radon gas concentrations in buildings before the buildings are constructed. Testing for radon gas levels could be done when the residence and other occupied structures 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. Earthquake Considerations: 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. 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 M6.2 + 0.3 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, 1985). Modified Mercalli Intensity VI ground shaking should be expected during a reasonable exposure time for the residence, 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. The U. S. Geological Survey 2014 National Seismic Hazard Maps indicates that a peak ground acceleration of 0.109 has a 10% exceedance probabilify for a 50-year exposure time and a peak ground acceleration of 0.309 has a2o/o exceedance probability for a 50-year exposure time at the Kumar & Associates, lnc.0 Projec{ No. 20-7-147 Michaleen and Michael Jeronimus Ãpnl2,2020 Page 4 project site (Peterson and Others,2014). This corresponds to a statistical recurrence time of about 500 years and2,500 years, respectively. These accelerations are forfirm rock sites with shear wave velocities of 2,500 þs and higher in the upper 100 feet and should be modified for soil profile amplification at the project site. The seismic soil profile at the project site should be considered as Class D, stiffsoil sites as described in the 2015 International Building Code unless site specific shear wave velocity studies show otherwise. LIMITATIONS This study was conducted according to generally accepted engineering geology principles and practices in this area at this time. We make no warranty either express or implied. The conclusions and recommendations submitted in this report are based on our field observations, aerial photograph interpretations, published regional geology information, the current proposed development plan, and our experience in the area. This report has been prepared exclusively for our client and is an evaluation of the geologic constraints and their potential influence on the proposed development. We are not responsible for technical interpretations by others of our information. If you have any questions or need fuither assistance, please call our office. Sincerely, Respectfully Submitted, '/U,tl/Atr' Robert L. Duran, P.E. Reviewed by: RLDlkac REFERENCES Federal Emergency Management Agency (FEMA), January 3,1986, Flood Insurance Rate Map (FIRM): Map No. 080205 1465 B. Kirkham, R., M., Streufert, R.K., Hemborg, T., and Stelling, P.L.,2004, Geologic Map of the Cattle Creek Quadrangle, Garfield County, Colorado: Colorado Geological Survey Open-File Report OF 14-14. E Kumar & Associates, lnc. @ Project No. 20-7-147 Michaleen and Michael Jeronimus April2,2020 Page 5 Kirkham, R.M., White, J. L., Sares, M. A.,Mock, R. G., and Lidke D. J.,2003, Engineering and Environmental Aspects of Evaporite Karst in West-Central Colorado in Johns, K. S., and Neal, J. T. (eds), Evaporite Karst and Engineering/Environmental Problems in the United States: Oklahoma Geological Survey Circular 109, p. 279-292. Kunk, M., J., and Others,2002,40Ar/39Ar Ages of Late Cenozoic Volcanic Roclæ within and Around the Carbondale and Eagle Collapse Centers, Colorado: Constraints on the Timing of Evaporite-Related Colløpse and Incisíon of the Colorado Ríver, in Kirkham R. M., Scott, R. 8., and Judkins, T. W. eds., Løte Cenozoic Evaporite Tectonism and Volcanism in West-Central Colorado: Geological Society of America Special Paper 336, Boulder, Colorado. Kirkham, R. M. and Rogers, W.P.o 1985, Colorado Earthquake Data and Interpretations 1867 to 1985: Colorado Geological Survey Bulletin 46. Peterson, M. D. and Others, 2014, Documentationfor the 2014 Update of the National Seismic Hazard Maps: U. S. Geological Survey Open-File Report 2014-109I. Widmann B. L. and Others, 1998, Preliminary Quaternary Fault and Fold Map and Data Base of Colorado: Colorado Geological Survey Open-File Report 98-8. Kumar & Associates, lnc. @ Project No, 20-7.147