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Home My WebLink AboutGeologic Hazards Report 04.02.2020lffiÍtffi** An Hnployaa ûræd CoflÞonf 5020 County Road 154 Glenwood Springs, CO 81601 phone: (970) 945-7988 fax: (970) 945-8454 email : kaglenwood@kumarusa.com www.kumarusa.com Ofüce l¡cations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit Counff, Colorado April2,202A Michaleen and Michael Jeronimus l0l2 l3ù Street Golderu CO 80401 maj eronimus@ gnail. com Project No. 20-7-147 Subject Geologic Hazards Review, Proposed Ræidence, Bam/ADU, and Pond, Burry Ranch,9155 Highway 82, Garfield County, Colorado Ladies and Gentlemen: As requested, Kumar & Associates, Inc. has reviewed the potential geologig hazards that could impact the project site. Our findings are presentd in this report. The sen¡ices were perforured in accordance with our prolasal for professional engineering services to Michaleen and Michael Jeronimus dated February 12,2A2A, Pmpsal No. P7-20-159. PROPOSED CONSTRUCTION The proposed reside,nce will be a hilo-story wood frame stn¡cft¡re with attached garage. The proposed bam is assumed to be a two-story structure with accessory dwelling unit on the upper level. Ground floon are assumed to be structural over qrawlspace 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 gades 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 Yz foot. The Roaring Fork River is to the wæt and south more than l0 feet below the pmposed house elevation. Tlrere is a 30 to 50 percent slope down to the west that is the edge of a higher terrace located about I 50 feet to the east of the proposed development. Vegetation at the site consists of native grass and wecds 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 Riverbottoms due to a reduction in vertical sFess 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 April 2,2AZA Page 2 (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 ofthe 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 stoppd. If still active, present deformations may be occurring at rates similar to past long-term rates of between 0.5 and 1.6 inches per 100 years. Thse 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 hydrccompræsive soils. These conditions, theirpotential risks, and mitigations to reduce the potential risks are discussed below. Potential hydrocompressive soil haza¡ds and mitigation options are discussed in our subsoil study report dated March 18,2020, ProjectNo. 20-7-147. Potentially Unstable Slopes: The stæp 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 ofbulgrng, ground cracks, or water seepsge were observed on the steep sþe. In our opinion, the steep hillside is currently stable and the pmposed development will not adversely impact the existing stability of the steep slope. Subsidence and Sinkhole Potenti¡h Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the subject site" These rocks are a sequence of gpsiferous shale, ñne-graind 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 ofthe gypsum under certain conditions can cause sin}¡holes to develop and can produce areas of localized subsidence. During previous work in the area" several sinkholes were observed scattered throughout the Carùondale 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 v..^-- g Â-^^^i-1¡¡ t-¡ Á)E!.^:--ó tl^ qã a A ta Michaleen and Michael Jeronimus April2,2020 Page 3 proposed residence, in our opiniorç is low; howevero 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 dqlression was obserr¡ed to the north of the proposed ba¡n about 200 feet north of the proposed driveway. The depression was about 45 feet across in the north-south directior¡ 30 feet across in the east-west direction and about 4 feet deep. The depression had rounded granitic and sandstone derivd 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. Vfithout additional subsurface exploration, the overall extent of the potential sinkhole cannot be determined. Development is not recommended \A.ithin 100 feet of the edges of the depression. The current development is more than 100 feet from the edges of the depression. Potenti¡l Flooding: According to the "Flood Insurance Rate Map'n, ffiâp number 080205 1465 B by the Federal Emergency Management Agency (FEMA, 1986); the proposed development area is in Zone C (unshaded - area of minimal flooding). Radiation Potentiel: The project site is not located on geologic deposits that would be expected to have high concenfation of radioactive minerals. However, there is a potential that radon gas could be present in the a¡ea. It is difñcult to assess fi¡ture radon gas concentrations in buildings before the buildings are constn¡cted. Testing for radon gas levels could be done when the residence and other occupied structures have been completed. New buildings are ofren designed with provisions for ventilation of lower enclosed areas should post conshuction testing show unaccçtable radon gas concentration. Earthquake Consider¡tions: Historic earthquakes within 150 miles of the project site have typically been moderateþ shong 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. lt 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 theproject site associated with the 1882 earttrquake and the other larger historic earthquakes in the region does not appear to have exceeded Modified Mercalli Intensþ 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 shonger ground shaking is low. Intensþ VI ground shaking is felt by most people and causes general alarm, but results in negligible damage to struchrres of good design and construction. The U. S. Geological Survey 2014 National Seismic Hazard Maps indicates that a peak ground acceleration of 0.10g has a l0o/o exceedance probability for a S0-year exposure time and a peak ground acceleration of 0.309 has a 2% exceedance probability for a S0-year exposure time at the V,.6^- O ^-ã^âi^a^â l-^ Gi D-¡:-i lt- 1^ r ¡ ¡t Michaleen and Mictrael Jeronimus April2,2020 Page 4 project site (Peterson and Others, 2014). This corresponds to a statistical recu¡rmce time of about 500 years and 2,500 y@rs, respectively. These accelerations are forfizn rocÈsdfes with shear wave velocities of 2,500 þs and highq in the upper 100 feet and should be modified for soil profile anrplification at the project site. The seisrrric soil profile at the project site should be considered as Class D, stiffsoílsr'fas as described in the 2015 Intemational Building Code rmless site specific shem wave velocity studies show otherwise. LIMTTATIONS This study was conducted according to generally accepted engine€ring geology principles and practices in this area at this time. 'We make no warranty either express or implid. The conclusions and recommendations submitted in this report are based on our field observations, aerial photograph interpretations, published regional geology informatior¡ the curre, rt proposed development plan, and our experience in the area. This re,port has been prepared exclusively for our client and is an evaluation of the geologic consEaints and theirpote,ntial influence on the prcposed development. We are not responsible for technical interpretations by others of our information. If you have any questions or ned furttrer assistance, please call our office. Sincerelyo Respectfully Submitted,,,{)d/t/AU-'- Robert L. Duran, P.E. Reviewedby: E. Hardin, RLD/kac REFERENCES Federal Emerge,ncy Managerrent Agency (FENÍA), January 3, 1986, Flood Insurance Rate Map (FIRM): Map No. 080205 1465 B. Kirlr:ham, R., M., SfreufErt, R. K., Hernborg, T., and Stelling P.L.,2004, Geolagic Map of the Cattle Creek Quadrangle, Garfeld County, Colorado: Colorado Geological Survey Open-File Report OF 14-14. 4{43 {" [,o E.i E L{ L I I V'.-^- O ^--^^Í-¡^Ã l-- fi'h--:--. a)- 4^ , a at Michaleen and Michael Jeronimus April2,2020 Page 5 Kirkham, R.M., Whitq J. L., Sares, M. 4., Mocþ R.G., and Lidke D. J.,2003, Engineering and Environmental Aspects of Evaparìte Karst in West-Central Colorado in Johnsn K. S., and Neal, J. T. (eds), Evaporite Karst and EngineeringÆnvironmental P¡oblems in the United States: Oklatroma Geological Survey Circular 109, p. 279-292. Kunk, M., J., and Others,20A2,4A.4r/39.4rAges of Late Cenozoic YolcanicR¿c,ts wìthin and Around the Carbondale and Eagle Collapse Centers, Colorado: Constraints on the Timing af Evaporite-Related Callapse and Incision of the Colorado River,in Kirkham R. M., Scott, R. 8., and Judkins, T. W. eds.,Iaúe Cenozoic Evaporite Teetonism and Itolcanism in West-Central Calorado: Geological Society of America Special Paper 336, Boulder, Colorado. Kirkham, R. M. and Rogers, W. P., 1985, Colorado Earthquake Data and Intetpretations 1867 to 1985: Colorado Geological Survey Bulletin 46. Peterson, M. D. and Others,2014, Documentationþr the 2AI4 Update of the Natianal,Selsn¡c Hazard Maps: U. S. Geological Survey Open-File Report 2014-1091. Widmann B. L. and Others, 1998, Preliminary Quaternary Fault and Fold Map and Data Base of Colorada: Colorado Geological Suruey Open-File Report 98-8. V,'^^.9 ¡l-â^^iña^- l-^ d)Þ.^¡^^r ll^ t^ a I ta