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HomeMy WebLinkAboutGeological Hazards Review 08.27.2018I r Lt Geotechnical Engineering 1 Engineering Geology Materials Testing 1 Environmental 5020 County Road 154 Glenwood Springs, CO 81601 Phone: (970) 945-7988 Fax: (970) 945-8454 Email: hpkglenwood@kumarusa.com Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, Summit County, Colorado August 27, 2018 Tito Montes c/o Palomino Design Build Attn: Jack Palomino 919 Palmer Avenue Glenwood Springs, CO 81601 )ackpaloinino55 0.0g ail. com RECEIVED twirl i GARFIELD COUNTY COMMUNITY DEVELOPMENT Project No. 18-7-532 Subject: Geologic Hazards Review, Proposed Residence, TBA Highway 6, Garfield County, Colorado Gentlemen: As requested, HP/Kumar 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 Tito Montes dated August 15, 2018. PROPOSED CONSTRUCTION We understand that the proposed residence will be located in the general vicinity of the center of the lot, to the west of the existing overhead powerline. The Onsite Wastewater Treatment System (OWTS) soil treatment area is proposed to be located to the southwest of the proposed residence. The proposed residence will be a 1,900 square foot, one-story wood -frame structure with an attached garage. The floors will be slab -on -grade. SITE CONDITIONS The site is currently vacant. It is bordered on the south (downhill) by U. S. Highway 6. Topography at the site is valley bottom with terrain strongly sloping down to the south-southwest at around 10 to 15 percent. Slopes to the north of the lot are steep down to the southwest at variable grades but generally greater than 30 percent. Vegetation at the site consists of native grass and weeds with brush growing along the southern edge of the lot and landscaped fruit trees growing along the north side of the lot. There are two, moderately sized southerly -trending drainages coming down from the steep slopes to the north that outlet above the subject site. Maroon Formation is exposed in these drainages and in outcrops along the northern Colorado River Valley. There is a ditch along the western edge of the property coming down from the adjacent property to the north. There is a small ditch along the southern edge of the property bordering U. S. Highway 6. The Colorado River is just to the south of the site, on the southern side of Interstate 70. Tito Montes c/o Palomino Design Build August 27, 2018 Page 2 GEOLOGIC SETTING The project area is near the axis of an east- to west- trending syncline. This regional geologic structure was formed due to salt tectonism in the late Cenozoic, consisting of the migration of Eagle Valley Evaporite into areas of lower vertical stress, i.e. river valley bottoms (Kirkham and Rogers, 1985). Near, surface formation rock below the surficial deposits in the immediate area is the Permian and Pennsylvanian -age Maroon Formation. The steep Colorado River valley side is to the north, and south across the river of the project site. The Colorado River valley sides are made up of the Maroon Formation. The Grand Hogback Monocline is to the south of the subject site (Bryant and Others, 2002). The Maroon Formation is mainly sandstone, conglomerate, and mudstone and with occasional grey limestone. The color can range from maroon to greyish -red (Bryant and Others, 2002 and Tweto and Others, 1978). The surficial soil deposits in the project area are mostly the result of mass wasting and erosion of the steep valley side to the north. Holocene and latest Pleistocene -age younger fan alluvium and debris flow deposits underlie the project site (Bryant and Others, 2002). GEOLOGIC HAZARDS REVIEW Potential major geologic hazards that could impact the site consist of debris flows and debris floods (hyperconcentrated flows). These conditions, their potential risks, and mitigations to reduce the potential risks are discussed below. Potential Debris Flow/Flood (Hyperconcentrated Flows): Debris flows occur from hillside and mountainside drainages due to heavy rainfall sometimes associated with extreme thunderstorms and snowmelt. Debris flows are common in the Canyon Creek area along the steep sides of the Colorado River valley west of Glenwood Springs. Without long term observation or detailed fan specific stratigraphic studies it is not possible to evaluate the statistical recurrence probability of major hyperconcentrated flows at the project site with a high level of confidence. In our opinion, the statistical recurrence probability of major hyperconcentrated flows at the project site is likely long and may exceed 100 years. However, a major hyperconcentrated flow event has the potential to damage structures in the project site and deposition of mud and debris should be expected in the project site. The debris flow hazard at the subject site is due to the two natural drainage channels which outlet above the site. No existing natural channels were observed on the subject lot. It appears that the channels that outlet above the project site have been diverted into a ditch that runs along the western edge of the property. At the time of our visit to the site, water was flowing in this ditch. Water was spilling out of the ditch and into the road at the southwest corner of the lot. There was apool of standing water in the southwestern corner of the lot. H-P�KUMAR Project No. 18-7-532 Tito Montes c/o Palomino Design Build August 27, 2018 Page 3 Risk mitigation would likely be direct protection of the structures by wall reinforcement or the construction of deflection berms and diversion channels down through the development. The residence is proposed to be located away from the mouths of the debris flow channels and less extensive mitigation may be acceptable, with some risk of damage due to debris flow. Positive drainage away from the proposed residence and OWTS soil treatment area is especially important. Material traveling down the two debris flow channels that outlet above the subject site will likely lose energy and spread out over the fan at the mouth of the channels. Construction of a small berm (3 feet high), on the uphill side of the residence and OWTS soil treatment area to divert surface water runoff could reduce the risk of damage due to debris flow/flood (hyperconcentrated flow). Improving the existing ditch along the west side of the property could also help with surface water ponding in the southwestern corner of the lot and could reduce the risk of damage due to hyperconcentrated flow. 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 iri 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 the residences 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, see Figure 4. 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 caused 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 0.07g has a 10% exceedance probability for a 50 -year exposure time and a peak ground acceleration of 0.20g has a 2% exceedance probability for a 50 -year exposure time at the project site (Peterson and Others, 2014). This corresponds to a statistical recurrence time of about 500 years and 2,500 years, respectively. These accelerations are for firm rock sites with shear wave velocities of 2,500 fps and higher in the upper 100 feet and should be modified for H-P�KUMAR Project No. 18-7-532 Tito Montes c/o Palomino Design Build August 27, 2018 Page 4 soil profile amplification at the project site. The seismic soil profile at the project site should be considered as Class D, stiff soil sites as described in the 2015 International Building Code unless site specific shear wave velocity or subsurface exploration 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 further assistance, please call our office. Sincerely, Respectfully Submitted, Ver#4 Robert L. Duran, E. I. Reviewed by: DO f .4yhy`;� "i.,tP Daniel E. Hardin, r` rE; 24443 -0% $1,6// RLD/kac • • ti attachments Figure 1'i- Figure 2 — Geologically Young Faults and Historic Earthquakes H-P-WIJMAR Project No. 18-7-532 Tito Montes c/o Palomino Design Build August 27, 2018 Page 5 REFERENCES Bryant, Bruce, Shroba, R. R., Harding, A. E., and Murray, K. E., 2002, Geologic Map of the Storm King Mountain Quadrangle, Garfield County, Colorado: United States Geological Survey Miscellaneous Field Studies Map MF -2389. Kirkham, R. M. and Rogers, W. P., 1985, Colorado Earthquake Data and Interpretations 1867 to 1985: Colorado Geological Survey Bulletin 46. Peterson, M. D. and Others, 2014, Documentation for the 2014 Update of the National Seismic Hazard Maps: U. S. Geological Survey Open -File Report 2014-1091. Tweto, Ogden, Moench, R. H., and Reed, J. C., 1978, Geologic Map of the Leadville 1 degree x 2 degrees Quadrangle, Northwestern Colorado: U. S. Geological Survey Miscellaneous Investigations Series Map I-999. 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. H-Pk'KUMAR Project No. 18-7-532 O Project Site Location • . Roti >._ Google 18-7-532 H -P KUMAR Proposed Residence, TBD U.S. HWY 6 and 24, Garfield County - Project Site Location Figure 1 Intermountain l_____ Seismic Belt WY. Mid 1977 M 5.0 termountaln Seismic Belt Moab Uf. Rocky CO. �Itr�S ,'Vyoi ing 150 miles c WY. NB. O angeiy Lily Park 1871 Axial Basin 1891 VI Cr Meeker Rio Bianco (Explosion) 977 M 5.7 0 Rifle Rullsan (Explosion) 1969 M 5.3 Grand Junction .off Plate NM Delta S. Grand 19 gbadc 44 VI GA/ Montrose Cortez Ridgeway VI 1913 w 0 tD Glenwood Spriry}s Walden 0 Steamboat Springs N. Front 1882 M 6.2 VII Boulder Fon Collins Greeley Golden Denver 0 . kir ❑ ❑ Nowa CasVe Rock Project Site ❑ Aspen O 7 - Cimarron Ridge Gunnison 1960 L7 M 5.5 Lake City 1955 VI Durango Dulce 1966 M 5.1 VII iQ56b Salida Q57\ o- Q69a agoSprings Q69c Alamosa Chama 069d C uredo Sp. Pueblo 0 Walsenburg Trinidad 2011 M 5.3 VII °Tdnidad co. Fort Morgan Great Limon Plains ❑ Raton Explanation: Post -Glacial Faults: Fault younger than about 15,000 years. Larger Historic Earthquakes: Earthquakes with maximum intensity greater than VI or magnitude greater than M 5.0 from 1867 to present. Nuclear Explosion: Large underground nuclear explosion for natural gas reservoir enhancement. Historic Seismic Zones: Areas with historically high seismic activity. M Local, surface wave or body wave magnitude VI Modified Mercalll intensity References: Widmann and Others (1998) U. S. Geological Survey Earthquake Catalogs 0 50 mi. [ I Scale: 1 in. = 50 mi. 18-7-532 I H^PtiKUMAR Proposed Residence, TBD U.S. HWY 6 and 24, Garfield County Geologically Young Faults and Larger Historic Earthquakes Fig. 2