HomeMy WebLinkAboutDRB17-0171_17-7-193 (02-23-17) Geo Hazards signed_1497301200.pdf FI -P--WUMAR 5020 County Road 154
Glenwood Springs,CO 81601
Geotechnical Engineering I Engineering Geology Phone:(970)945-7988
Materials Testing I Environmental Fax:(970)945-8454
Email: hpkglenwood@kumarusa.com
Office Locations: Parker,Glenwood Springs,and Summit County,Colorado
GEOLOGIC HAZARDS ASSESSMENT
LOT 30,VAIL POTATO PATCH SUBDIVISION
807 POTATO PATCH DRIVE
VAIL, COLORADO
PROJECT NO. 17-7-193
FEBRUARY 23,2017
PREPARED FOR:
SCOTT RYAN
2398 SW 76T11 LANE
OCALA, FLORIDA 34476
srvannazefon.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - 1 -
PROPOSED DEVELOPMENT .- 1 -
SITE CONDITIONS - 1 -
PROJECT AREA GEOLOGY - 2 -
REGIONAL ROCKFALL INFORMATION - 2 -
GEOLOGIC HAZARDS ASSESSMENT - 3 -
EARTHQUAKE CONSIDERATIONS - 3 -
LIMITATIONS -4 -
REFERENCES - 5 -
FIGURE 1 - PROJECT SITE LOCATION
FIGURE 2 - SITE PLAN
FIGURE 3 - PROJECT GEOLOGY
H-PiKUMAR
Project No. 17-7-193
PURPOSE AND SCOPE OF STUDY
This report presents the findings of a geologic hazards assessment of Lot 30, Vail Potato Patch
Subdivision, 807 Potato Patch Drive, Vail, Colorado. The purpose of our study was to evaluate
the geologic hazards at the project site and assess their potential impact on the proposed
development.
A field reconnaissance of the project site was made on February 22", 2017 to observe the
geologic conditions and collect information on the potential geologic hazards present at the
project site. In addition, we have reviewed relevant published geologic information, looked at
aerial photographs of the project area, and reviewed previous studies in the area. This report
summarizes the information developed by this study, describes our evaluations, and presents our
findings.
PROPOSED DEVELOPMENT
The proposed development is in the preliminary phase. Our understanding is that a single-family
residence will be located approximately in the upper to middle part of the lot. When building
location, type, and grading conditions have been developed, we should be notified so that we
may reevaluate the information submitted in this report and perform additional analysis as
needed. Due to very steep slopes and the guard rail along Potato Patch Drive, access to the lot is
not currently feasible with drilling or excavation equipment without creating an access trail into
the building area or grading drill pads off of the road right-of-way.
SITE CONDITIONS
The 0.4-acre parcel is located about 0.35 miles to the north of Interstate 70 on the south side of
Potato Patch Drive, as shown on Figure 1. The site was visited on February 22", 2017. At the
time of the visit, the site was mostly covered with around 6 inches of snow. The site is located
on a very steep, south-facing hillside to the east of Red Sandstone Creek. The site lies at an
elevation of around 8,590 feet. The existing topography is shown by the contour lines on Figure
H-P--KUMAR
Project No. 17-7-193
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2. The slope across the proposed development area is about 50 percent. The hillside transitions
to a moderately sloping bench with an average slope of about 20 percent to the north of the site.
Vegetation on the proposed development area is grass and scattered scrub oak with a few aspens
near Potato Patch Drive.
PROJECT AREA GEOLOGY
The main geologic features in the project area are shown on Figure 3. This map is based on
regional mapping by Scott and Others (2002).
The project site is underlain by interbedded arkosic sandstone, conglomerate, and shale in the
middle Pennsylvanian-age, Minturn Formation. The formation rock bedding in the area dips to
the west to northwest at angles between around 15 to 40 degrees. The Red Sandstone syncline is
to the north and east of the project site. At the project site the formation rock is mostly covered
with pre-Bull Lake-age glacial till, but scattered outcrops of the sandstone may be present. The
onsite rock blocks in the northern part of the site are associated with the till.
REGIONAL ROCKFALL INFORMATION
The Town of Vail (2000) regional rockfall map shows much of the Vail Potato Patch
Subdivision in a high or medium severity rockfall area. Lot 30 is shown in a medium severity
rockfall area, but based on our site-specific review, in our opinion, Lot 30 is not in a rockfall
hazard area. The primary rockfall hazard is associated with the prominent sandstone outcrop
located to the north of the subdivision. Lot 30 is not located in this potential hazard area. A
secondary rockfall hazard is associated with the local sandstone outcrops and till related rock
blocks on the steep hillside in the vicinity of Lot 30. This secondary hazard does not present a
potential threat to development of Lot 30 because the proposed construction will be in the
potential rockfall source area. Construction should reduce the potential risk to down slope
building if precautions are taken during construction to prevent dislodgement of the rock blocks
and unstable outcrops on the hillside near construction sites.
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Project No. 17-7-193
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GEOLOGIC HAZARDS ASSESSMENT
Potential geologic hazards which could impact the project site consist of instability of the steep
slopes and rockfall. The site has had a relatively stable recent geologic past and the steep slope
should remain relatively stable provided site development grading is properly designed and
constructed. There will be a risk of construction induced instability. Surface grading and
drainage normal for building on steep slopes should be used to protect the residence from surface
runoff. Although Lot 30 is in medium severity rockfall area according to Town of Vail (2000),
based on our site-specific review, in our opinion, Lot 30 is not in a rockfall hazard area. There is
a potential for local instability of the scattered boulders when excavating for the project.
Temporary rockfall protection should be placed downslope of the proposed construction in order
to protect downslope infrastructure and buildings from potential dislodged rocks. Geologic
conditions in the project area should not present an unusual risk to the proposed development.
EARTHQUAKE CONSIDERATIONS
Historic earthquakes within 150 miles of the project site have typically been moderately strong
with magnitudes of M5.5 and less and maximum Modified Mercalli Intensities of VI and less.
The largest historic earthquake in the project region occurred in 1882 (Kirkham and Rogers,
1985). It was apparently located in the northern Front Range and had an estimated magnitude of
M6.2± 0.3, and a maximum Modified Mercalli Intensity of VII. Historic ground shaking at the
project site associated with the 1882 earthquake and the other large historic earthquakes in the
region does not appear the have exceeded Modified Mercalli Intensity VI (Kirkham and Rogers,
1985). Modified Mercalli 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 project facilities should be designed to withstand moderately strong ground shaking with
little or no damage and to not 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 2014
National Seismic Hazard Maps indicates that a peak ground acceleration of 0.06g has a 10%
exceedance probability for a 50 year exposure time and a peak ground acceleration of 0.22g 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,
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Project No. 17-7-193
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respectively. The soil profile at the project site appears to be Class C, as described in the 2015
International Building Code and should be further evaluated as part of the site specific
geotechnical study.
LIMITATIONS
This study was conducted according to generally accepted geotechnical and 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
currently 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 hazards and their
potential influence on the proposed development. We are not responsible for technical
interpretations by others of our information.
H-P KUMAR
Robert L. Duran, E.I.
Reviewed by:
P
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Steven L. Pawlak, P.E .46 z/z 9 ,r
RLD/kac
cc: Pierce Architects—Bill Pierce (bill vaiiarchitects.com)
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Project No. 17-7-193
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REFERENCES
Kirkham, R. M. and Rogers, W. P., 1985, Colorado Earthquake Data and Interpretations 1867
to 1985: Colorado Geological Survey Bulletin 46.
Scott, R. B., Lidke, D., J., and Grunwald, D. J., 2002, Geologic Map of the Vail West
Quadrangle, Eagle County, Colorado: U. S. Geological Survey, Miscellaneous Field
Studies Map MF-2369, Version 1.0.
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.
Town of Vail, 2000, Official Rockfall Hazard Map, Town of Vail: Prepared by the Town of Vail,
Vail, Colorado (Adopted by the Town Council on October 17, 2000).
H-P;KUMAR
Project No. 17-7-193
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17-7-193 H-P` KU MAR Lot 30, Vail Potato Patch Subdivision - Project Geolozy Figure 3