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HomeMy WebLinkAboutGeological Site Assessment 03.31.2008h HEPWORTH - PAWLAK GEOTECHNICAL March 31, 2008 Scott Steele 4985 County Road 113 Carbondale, Colorado 81623 ■ RECEIVED 1 011-41.‘ '11111-, '4- ' MAY 2 2 2008 j GARFIELD COUNTY BUILDING & PLANNING Job No. 107 0861 Subject: Geologic Site Assessment, Proposed Garage , 4985 County Road 113, Lot 12, Cottonwood Hollow, Garfield County, Colorado Dear Mr. Steele: As requested, we performed a geologic site assessment for a proposed garage to be Iocated on Lot 12 of the Cottonwood Hollow Subdivision. The project site and vicinity are shown on Figure 1. The purpose of the assessment was to evaluate if there are geologic conditions that could present unusually high risks or major constraints to the garage. A Feld reconnaissance of the project vicinity was made on March 7, 2008 to observe the geology and site conditions. In addition, we have reviewed published regional geology studies, looked at aerial photographs and reviewed our previous work in the vicinity. Hepworth-Pawlak Geotechnical observed the garage excavation on November 16, 2007 and developed geotechnical recommendations for the foundation and cut slope stabilization (Hepworth-Pawlak Geotechnical, 2008). This report summarizes the fundings of our geologic site assessment and presents our conclusions and recommendations. SUMMARY OF FINDINGS This study shows that the geologic conditions at the project site should not present unusually high risks or major design constraints for the proposed garage. There are several conditions of a geologic nature that should be considered in the garage design as discussed in the Geologic Site Assessment section of this report. PROPOSED CONSTUCTION It is our understanding that you plan to build a garage in the southeastern part of Lot 12, see Figure 1. The garage site is located near the existing driveway entrance off of County Road 113 to the southeast of the existing residence. The garage will be a single story, wood frame building with a thickened -edge slab foundation system. Our January 29, Scott Steele March 31, 2008 Page 2 2008 excavation observation report may be referred to for geotechnical engineering reconunendations for the garage (Hepworth-Pawlak Geotechnical, 2008). SITE CONDITIONS Lot 12 is located on the north side of Cattle Creek about five and one-half miles upstream from the confluence with the Roaring Fork River. The topography in the area is shown by the contour lines on Figure 1. At the project site, Cattle Creek has eroded a narrow, steep sided valley about 800 feet below the rolling uplands of Missouri Heights. The garage site is at the transition between the moderately sloping valley floor and the steep valley side. The ground slope over most of the garage site is less thanl 5 percent. The valley side along the north side of the garage site is steep, about 40 percent. At the time of our field reconnaissance on March 7, 2008 the garage foundation excavation had been completed for some time. The cut slope along the north side of the garage site is about 20 feet high and has a slope of around 1:1 (horizontal to vertical). Basalt cobbles and boulders were loosely stacked along the lower two-thirds of the cut slope. Vegetation on the valley side is mostly juniper trees and brush . GEOLOGIC CONDITIONS Geologic conditions that pertain to the proposed garage are described below. Regional Geology: The project site is in the Southern Rocky Mountains to the west of the Rio Grande rift and to the east of the Colorado Plateau, see Figure 2. The site is in the western Colorado evaporite region and is in the Carbondale collapse center, see Figure 3. The Carbondale collapse center has experienced as much as 4,000 feet of evaporite related regional subsidence during the past 10 million years (Kirkham and Others 2002). The long term subsidence rates have been very slow, between 0.5 and 1.6 inches per 100 years. It is uncertain if regional evaporite subsidence is still occurring, of if it is currently inactive. If still active, these regional deformations because of their very slow rates should not have a significant impact on the proposed garage. Geologically Young Faults: Geologically young faults less than 15,000 years old and considered capable of generating large earthquakes are not located close to the project site. The closets geologically young faults considered capable of generating large earthquakes are over 60 miles from the project site, see Figure 2. At this distance large Job No 107 086I Scott Steele March 31, 2008 Page 3 earthquakes on the fault should not produce strong ground shaking at the project site that is greater than the ground shaking shown on the U. S. Geological Survey 2002 National Seismic Hazards Maps (Frankel and Others, 2002). Surf tial Soil Deposits: Soil deposits in the project area consist mostly of colluvium and collapse debris formed by subsidence deformation in the Carbondale collapse center. Colluvium consisting of basalt rocks from gravel- to boulder -size in a sandy clay soil matrix is present in the proposed garage excavation. Formation Rock: Rock outcrops are not present in the project vicinity. As discussed above, the formation rock is covered by thick colluvium and collapse debris. Regional geologic mapping shows that the near surface formation rock at the project site is most likely the middle Pennsylvanian -age, Eagle Valley Evaporite (Kirkham and Widmann, 1997). It is a sequence of evaporite rock consisting of massive to laminated gypsum, anhydrite and halite interbedded with light-colored mudstone, fine-grained sandstone, thin limestone and dolomite beds and black shale (Kirkham and Widmann, 1997). The evaporite minerals are relatively soluble in circulating groundwater and subsurface solution voids and related surface sinkholes are locally present in these rocks throughout the western Colorado evaporitc region including the project site. The closest known sinkhole to the project site is located about one and one-quarter miles to the south (Kirkham and Widmann, 1997). Landslides: Regional geologic mapping shows that large landslide complexes are present on the Cattle Creek valley in the vicinity of the project site (Kirkham and Widmann, 1997). The locations of these landslides are shown on Figure 1. The landslide boundaries shown are modifications of the regional mapping by Kirkham and Widmann (1997) based on our field observations. The proposed garage site is located about 1,000 feet to the west of the closest landslide. If this landslide were to reactivate in the future it should not present a risk to the proposed garage. GEOLOGIC SITE ASSESSMENT Geologic conditions that could present an unusually high risk to the proposed garage were not identified by this study. There are, however, geologic conditions that should be considered in the garage design. These conditions, their potential risks and possible Job No 107 0861 Gggtech Scott Steele March 31, 2008 Page 4 mitigations to reduce the risks are discussed below. The potential risks of geologic hazards impacting the garage site are similar to the nearby existing house site. Sinkholes: Geologically young sinkholes are present in the western Colorado evaporite region mostly in areas where the Eagle Valley Formation and Eagle Valley Evaporite are shallow, sec Figure 3. In this region a few sinkholes have collapsed at the ground surface with little or no warning during historic times. Although geologically active in the region , the likelihood that a sinkhole will development during a reasonable exposure time at the garage is considered to be low. This inference is based on the large extent of sinkhole prone areas in the region in comparison to the small number of sinkholes that have developed in historic times. Because of the complex nature of the evaporite-related sinkholes, it will not be possible to avoid all sinkhole risk at the garage site. You should be aware of the sinkhole potential, since early detection of building distress and timely remedial actions are important in reducing the cost of building repair should a subsurface void start to develop into a sinkhole after construction. Earthquake Considerations: The largest historic earthquake in the project region occurred in 1882. It was located in the northern Front Range about 113 miles to the northeast of the project site and had an estimated magnitude of about M 6.2 and a maximum intensity of VII, see Figure 2. Historic ground shaking at the project site associated with the 1882 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 garage , 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 garage should be designed to withstand moderately strong ground shaking with little or no damage and not to collapse under stronger ground shaking. For firm rock sites with shear wave velocities of 2,500 fps in the upper 100 feet the U. S. Geological Survey 2002 National Seismic Hazard Maps indicates that a peak ground acceleration of 0.06g has a 10% exceedence probability for a 50 year exposure time and a peak ground acceleration of 0.23g has a 2% exceedence probability for a 50 year exposure time at the project site (Frankel and Others, 2002). This corresponds to a statistical recurrence time of about 500 years and 2,500 years, respectively. The sail profiles at the garage site should be Job No 107 0861 Ggcgtech Scott Steele March 31, 2008 Page 5 considered as a Class D, stiff -soil sites as described in the 2006 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 and our experience in the area. The information presented in this report has been prepared exclusively for our client and is an evaluation of potential risk to the proposed garage associated with the geology. We are not responsible for technical interpretations by others ofour information. If you have questions, or if we may be of further assistance, please let us know. Respectfully Submitted, HEPWORTH - PAWLAK GEOTECHNICAL, INC. Ralph G. Mock Engineering Geologist Reviewed by: Daniel E. Hardin, P.E. RGM/vad Enclosures: Figure 1 Figure 2 Figure 3 04,74. ,4011111114111" 4mfitsiookt EAG\ Geologically Young Faults and Larger Historic Earthquakes Western Colorado Evaporite Region Landslides in Vicinity of Lot 12 Garage Site Job No 107 0861 Scott Steele March 31, 2008 Page 6 REFERENCES Frankel, A. D. and Others, 2002, Documentation for the 2002 Update of the National Seismic Hazard Maps: U. S. Geological Survey Open File Report 02-420. Hepworth-Pawlak Geotechnical, 2008, Observations of Excavation, Proposed Garage, 4985 County Road 113, Cottonwood Hollow, Garfield County, Colorado: Prepared for Scott Steele, Carbondale, Colorado (Job No. 107 0861, January 29, 2008). Kirkham, R. M. and Rogers, W. P., 1985, Colorado Earthquake Data and Interpretations 1867 to 1985: Colorado Geological Survey Bulletin 46. Kirkham, R. M. and Widmann, 13. L., 1997, Geology Map of the Carbondale Quadrangle, Garfield County, Colorado: Colorado Geological Survey Open File 97-3. Kirkham, R. M. and Scott, R. B., 2002, Introduction to Late Cenozoic Evaporite tectonism and Volcanism in West -Central, Colorado, in Kirkham R. M., Scott, R. B. and Judkins, T. W. eds., Late Cenozoic Evaporite Tectonisim and Volcanism in West -Central Colorado: Geological Society of America Special Paper 366, Boulder, Colorado. Kirkham R. M. and Others, 2002, Evaporite Tectonisim in the Lower Roaring Fork River Valley, West -Central Colorado, in Kirkham R. M., Scott, R. B. and Judkins, T. W. eds., Late Cenozoic Evaporite Tectonisim and Volcanism in West -Central Colorado: Geological Society of America Special Paper 366, Boulder, Colorado. Tweto, 0. and Others, 1978, Geology Map of the Leadville 1 EX 2E Quadrangle, Northwestern Colorado: U.S. Geological Survey 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. Job No 107 0361 G�'�cPitech j .,--: . ", t I« Missouri ' { It yei9hts :ltiol,l2—��. Cottonwood • `H0Elow Missouri • • 1 . !P Explanation: 011 Younger Landslides: EOlder Landslides: Contact: Approximate boundary of map units. Modified from Kldcham and Widmann (1997) 0 2000 ft. 1 i 1 Scale: 1 In. = 2000 IL Contour Interval: 40 It. March 2008 107 0861 HEP10111H-PAWLAK GeOlECHNICAL Cottonwood Hollow Subdivision Landslides in Vicinity of Lot 12 Proposed Garage Site Figure 1 115.1 Intermountain Seismic Bell Lafarge Min. 1064 M 5.5 VI 4 1 Walden d Fort oc- H. Fraa . i 5.2 �M N aa 195T - NI NIoMI xf«nn1na 7 M3.211014 5.3 Meeker Project Silo 0 Eagle Dent Manual . x1 `C) o ' .Z_ \056a crafran Ridge Gamboa 1960 0 IA Lake — 913 1955 n vt 4,1 O Carta CO. Pe9or+ *MP a - o DvMrga odes 1955 k45.1 Explanation: Post•Glaclai Faults: ▪ Fault younger than about 15,000 years. Larger Historic Earthquakes: Earthquakes with maximum intensity greater than Vi or magnkude greater Bran M 5.0 from 1807 to present. Nuclear Explosion: Large underground nuclear explosion for natural gee reservoir enhancement I� Historic Seismic Zones: 1.. Areas Win hhtorlcaly high seismic activity. M Local, surface wave or body wave magnitude VI Modified Mercaili Intensity References: Widmann and Others (1998) U. S. Geological Survey Earthquake Catalogs 0 50 m1. t Scale: 1 In. = 50 mi. 107 0861 G Gtech HEANaa1H—pANWc 050I eMCAL Cottonwood Hollow Subdivision Lot 12 Proposed Garage Site Geologically Young Faults and Larger Historic Earthquakes Figure 2 4 Sawatch Range Anticline 1 c a. to a! ' • —Cco 0 coS TA 0 LW I z 107 0861 Ptech NEPNORlHE-PARIAK CE01E0NIC/VL Cottonwood Hollow Subdivision Lot 12 Proposed Garage Site Western Colorado Evaporite Region Figure 3