HomeMy WebLinkAboutPEC17-0041_Geo Hazard Study, East Vail Parcel 06.19.17-signed_1503941965.pdf
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ROCKFALL HAZARD STUDY
East Vail Parcel
Vail, Colorado
Report Prepared for:
Mr. Kevin Hopkins
Vail Resorts Development Company
PO Box 959
Avon, CO 81620
Project No. 17.5029
June 19, 2017
17.5029 Rockfall Hazard Study, East Vail Parcel 06.19.17 i
ROCKFALL HAZARD STUDY
East Vail Parcel
Vail, Colorado
Report Prepared for:
Mr. Kevin Hopkins
Vail Resorts Development Company
PO Box 959
Avon, CO 81620
Project No. 17.5029
June 19, 2017
Report Prepared by:
Julia M. Frazier, P.G.
Senior Geologist
CESARE, INC.
17.5029 Rockfall Hazard Study, East Vail Parcel 06.19.17 1
TABLE OF CONTENTS
1. INTRODUCTION ......................................................................................................................... 3
2. SCOPE OF WORK ........................................................................................................................ 3
3. SITE CONDITIONS ..................................................................................................................... 3
4. GEOLOGIC SETTING ................................................................................................................ 11
4.1 REGIONAL GEOLOGY ............................................................................................................... 11
4.2 SITE GEOLOGY ........................................................................................................................ 12
4.2.1 ARTIFICIAL FILL (AF) ....................................................................................................... 12
4.2.2 COLLUVIUM (QC) ............................................................................................................. 12
4.2.3 LANDSLIDE DEPOSITS (QLS) ............................................................................................. 12
4.2.4 PINEDALE TILL (QTP) ....................................................................................................... 12
Robinson Limestone Member (Pmr) ....................................................................................... 13
Lower Member (Pml) ............................................................................................................ 13
5. GEOLOGIC HAZARDS ............................................................................................................... 14
5.1 ROCKFALL ............................................................................................................................... 16
5.2 LANDSLIDE ............................................................................................................................. 16
6. ROCKFALL ANALYSIS ............................................................................................................... 18
6.1 ROCKFALL STUDY SECTION ...................................................................................................... 18
6.2 ROCKFALL MODELING - CRSP ANALYSIS .................................................................................... 24
6.3 ROCKFALL ANALYSIS RESULTS ................................................................................................. 26
6.4 DISCUSSION OF ROCKFALL ANALYSIS RESULTS ......................................................................... 26
7. LANDSLIDE HAZARD MAPPING ............................................................................................... 27
8. CONCLUSIONS AND RECOMMENDATIONS .............................................................................. 28
8.1 ROCKFALL CONSIDERATIONS ................................................................................................... 28
8.1.1 PLACEMENT OF THE ROCKFALL CATCHMENT STRUCTURE ................................................... 28
8.2 LANDSLIDE CONSIDERATIONS ................................................................................................. 29
8.3 DEBRIS FLOW CONSIDERATIONS.............................................................................................. 30
9. LIMITATIONS .......................................................................................................................... 30
TABLES AND DIAGRAMS
DIAGRAM 1. Cross Section D-D’ .................................................................................................. 14
TABLE 1. CRSP Simulation Parameters ....................................................................................... 25
TABLE 2. Slope Profile Parameters .............................................................................................. 25
TABLE 3. Summary of Rockfall Analysis Results ......................................................................... 26
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17.5029 Rockfall Hazard Study, East Vail Parcel 06.19.17 2
FIGURES
SITE LOCATION MAP ........................................................................................................ FIGURE 1
TOPOGRAPHIC MAP .......................................................................................................... FIGURE 2
OFFICIAL ROCKFALL HAZARD MAP, TOWN OF VAIL, COLORADO .................................... FIGURE 3
OFFICIAL DEBRIS FLOW HAZARD MAP, TOWN OF VAIL, COLORADO .............................. FIGURE 4
GEOLOGIC MAP ................................................................................................................. FIGURE 5
LEGEND FOR FIGURE 5 GEOLOGIC MAP ........................................................................... FIGURE 6
LANDSLIDE EXTENTS MAP ................................................................................................ FIGURE 7
STUDY SECTIONS MAP ..................................................................................................... FIGURE 8
ROCKFALL STUDY SECTION .............................................................................................. FIGURE 9
LANDSLIDE STUDY SECTION .......................................................................................... FIGURE 10
SLOPE MAP ..................................................................................................................... FIGURE 11
APPENDIX
REFERENCES ................................................................................................................. APPENDIX A
ROCKFALL HAZARD ASSESSMENT AT BOOTH FALLS CONDOMINIUMS AND PROPOSED
MITIGATION (COLORADO GEOLOGICAL SURVEY) ....................................................... APPENDIX B
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1. INTRODUCTION
This report presents the results of a rockfall hazard study for an undeveloped lot located on the
east side of Vail, Colorado and owned by the Vail Resorts Development Company (Vail Resorts). It
is Cesare, Inc.’s (Cesare’s) understanding that a preliminary rockfall hazard analysis is desired prior
to potential development of the western portion of this site, along with other geologic hazards
which may have a significant impact on the proposed development. The site is located directly
north of the I-70 East Vail interchange. Geologic hazards, such as rockfall, debris flow, and
avalanche are recognized by the Town of Vail and delineated in the project area. The rockfall
hazard has been identified and addressed on the neighboring development to the west (Booth
Falls Mountain Homes), with multiple existing catchment structures.
2. SCOPE OF WORK
The scope of services for this rockfall hazard study generally included:
1. Review of available information, including published geologic maps, aerial photography,
and readily available studies performed on nearby sites.
2. Site reconnaissance to verify geologic and geologic hazard conditions on and upslope
from the subject site, with a focus on rockfall. This involved mapping the geology and
geologic hazards by traversing the site on foot, and through photography and video of
the site using an unmanned aircraft system (drone).
3. Modeling of the rockfall hazard potential using a critical cross section through the
project site and input into the Colorado Rockfall Simulation Program (CRSP).
4. Preparation of this report presenting our findings and preliminary recommendations
relative to the rockfall hazards potentially impacting the site, including conceptual
techniques that might be used to remediate and reduce the rockfall hazard. Also
included in this report are applicable figures, tables, and cross sections.
3. SITE CONDITIONS
The project site is located directly north of the I-70 East Vail interchange on the north side of Fall
Line Drive (Figure 1). Pitkin Creek Townhomes (formerly named Falls at Vail) is located
immediately adjacent to the site in the southeast corner, and Booth Falls Mountain Homes (Booth
Falls) and Vail Mountain School are located on a neighboring property to the west-northwest. The
site is rectangular in shape and is located in the southeast 1/4 of Section 2, Township 4 South,
Range 80 West of the 6th Principal Meridian in Eagle County, Colorado. The approximate center of
the property is situated at latitude 39° 38’ 46” N and longitude -106° 18’ 25” W.
Cesare performed site reconnaissance to characterize and map the geologic and geologic hazard
conditions during May 2017. The site is currently undeveloped with a variably sloping ground
surface ranging from about 7 to over 45 degrees (Figure 2). The elevation ranges from about 8375
feet in the west side of the site to about 8940 feet in the northeast corner, an elevation change of
about 565 feet across the site. The site is bound by undeveloped National Forest Service land to
the north, northwest, and east. Fall Line Drive and the I-70 Frontage Road bound the site along
the southern edge. Pitkin Creek forms a deeply incised drainage immediately to the east of the
eastern site boundary. Booth Creek, also deeply incised, is located about 3,200 feet to the
northwest of the site. Gore Creek is located on the opposite side of I-70, about 580 feet to the
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south at closest approach. A retaining wall borders the site along Fall Line Drive near the East Vail
I-70 off ramp in the area of the shuttle stop. Design or construction details for this retaining wall
were not available at the time of this study. Based on site observations, this retaining wall is
constructed of wood cribbage, with gravel placed directly behind the wood facing. The wall
appears to generally be in good condition, with one exception near the east end where the wall
has bulged out. An unpaved, single track road traverses the site along the edge that borders Fall
Line Drive and is barely visible in some historic aerial photographs. Multiple utility service
manholes were observed along this single track road and the manhole covers are labeled with
“electric utility”.
Vegetative cover at the site includes grasses, shrubs, and aspen trees. The western part of the site
and the area upslope of the western part of the site are incised with a network of drainages which
contained flowing water at the time of our site visits. This western area is generally more densely
vegetated with low shrubs and aspen trees than other parts of the site and upslope areas. Refer to
Photographs 1 through 8 for views of these onsite features.
Photograph 1. View of the project site. Photograph taken from the eastbound lane of I-70 looking east
across the site. The photograph shows the relatively steep slope of the site and the rock outcrops present
upslope from the site.
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Photograph 2. View of
retaining wall located along
edge of site that borders Fall
Line Drive. Town of Vail shuttle
stop is visible in the left side of
the photograph.
Photograph 3. View of distressed part of the retaining wall along the edge of the site that borders Fall Line
Drive. The slope rises steeply upward to the north at the top of the wall. This photograph was taken near
the east end of the wall.
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Photograph 4. Aerial view of the west side of the site. The single track road that traverses the site is visible, along with one of the drainages onsite (with flowing water). The white Cesare truck is parked at the beginning of the access road for the rockfall berm, constructed on the neighboring property to the west (Booth Creek). Large, gray limestone boulders which have come to rest on the lower slope are visible in the photograph.
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Photograph 5. View of limestone boulders which have come to rest near the base of the slope in the
western part of the site. Boulders are about 3 to 4 feet in longest dimension, embedded in the soil,
surrounded by mature vegetation, and show lichen on the surface.
Photograph 6. View of large sized limestone boulder located in the southern area of the site. Boulder
measures about 21 feet long by 16 feet wide by 6 feet high. A survey marker has been placed on this
boulder (Eagle County Survey Control, 1998).
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Photograph 7. View of the western part of the site. Note the dense vegetative cover, flowing water, and
exposed bedrock outcrops near the top of the slope.
Photograph 8. View of flowing
water in the western part of the
site.
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Rock outcrops are present upslope from the site and are rockfall source zones which have the
potential to impact the site and future planned development. Rockfall is a recognized hazard in the
site area, as depicted on the “Official Rockfall Hazard Map” for the Town of Vail (Figure 3). A
significantly sized rockfall catchment berm and basin, located about 1,300 feet to the northwest at
closest approach, has been constructed to reduce the rockfall hazard above the Booth Falls
development. It is Cesare’s understanding that this consists of an earthen berm ranging in height
from about 10 to 15 feet, and an upslope catchment area spanning about 20 feet where the
natural slope has been laid back. An access road leading up to the catchment area begins at Fall
Line Drive near the western point of the project site. Additional rockfall remediation structures are
located upslope from Booth Falls Court and are visible in the aerial imagery. These rockfall
remediation features are shown in Photographs 9 through 11.
Debris flows are also a recognized geologic hazard for the area, as shown on the “Official Debris
Flow Hazard Map” for the Town of Vail (Figure 4). As shown on Figure 4, the site is not within a
debris flow hazard zone, although moderate and high hazard areas are delineated along Pitkin
Creek to the east-southeast of the site.
Photograph 9. Google Earth image of Booth Falls Mountain Homes to the west of the project site.
Examples of existing rockfall remediation structures are labeled.
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Photograph 10. View of rockfall catchment berm and basin, upslope from Booth Falls Mountain Homes.
View looking west toward Booth Creek. The berm is between 10 and 15 feet high, and the ditch is about 20
feet from crest of berm to backslope.
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Photograph 11. View of rockfall catchment berm and basin upslope from Booth Falls Subdivision. View
looking east toward the project site.
4. GEOLOGIC SETTING
4.1 REGIONAL GEOLOGY
The site is included in the Southern Rocky Mountain physiographic province in an alpine setting
with elevations ranging from 8000 to 9000 feet. The site is located along the western flank of the
Gore Range, a northwest-southeast trending mountain range situated in north-central Colorado.
The Gore Range is separated from the Front Range Mountains to the east by the Blue River Valley
and Williams Range thrust zone. The core of the Gore Range is comprised of crystalline basement
rock uplifted during the Laramide mountain building event (orogeny) about 70 to 50 million years
ago (Ma). The Laramide orogeny also uplifted thick sequences of sedimentary units deposited
during the occupation of an inland sea in parts of Colorado. The sedimentary units are comprised
of shale, claystone, siltstone, sandstone, conglomerate, and limestone.
The Gore fault is located about 500 feet northeast of the site at closest approach and is not
considered active (Figures 5 and 6). The Gore fault is characterized as a zone of high angle
reverse faults. These faults have had at least five episodes of movement that span from
Precambrian (older than 540 Ma) to late Oligocene and younger (about 28 Ma), although most of
the displacement likely took place during the Laramide orogeny (Kellogg and others, 2011). A
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gentle regional tilt of 5 to 15 degrees down to the south-southwest, characterizing the
sedimentary bedrock in the site vicinity, is interrupted adjacent to the Gore fault. Beds of the
Minturn Formation are steeply dipping and overturned where located close to the Gore fault, as is
the case upslope and to the northeast of the site.
4.2 SITE GEOLOGY
The site is underlain by surficial units comprised of artificial fill, colluvium, landslide deposits, and
till of the Pinedale glaciation (Figure 5 Geologic Map). The bedrock underlying the site is mapped
as Minturn Formation (Kellogg and others, 2003; Kellogg and others 2011). Artificial fill is
associated with the construction of Fall Line Road along the southern border of the site and likely
with the unpaved, single track road (with buried utilities) in the southwest part of the site. A
wedge of colluvium is mapped mid-slope in the western half of the site, however, the colluvium
was actually observed to completely cover the site and largely obscure bedrock outcrops. The
eastern half of the site is predominantly landslide deposit and Pinedale Till underlies the
southeastern corner of the site. Bedrock of the Minturn Formation underlies the surficial deposits
at the site. Descriptions of these units are described below, from youngest to oldest. Refer to
Diagram 1 for a geologic cross section near the site.
4.2.1 Artificial Fill (af)
Artificial fill is associated with the ground modifications that have occurred within and adjacent to
the site boundaries. Based on site observations, artificial fill is likely associated with the single
track utility road in the southwestern part of the site, construction of Fall Line Drive, and
construction of the shuttle stop and retaining wall in the southeast part of the site.
4.2.2 Colluvium (Qc)
Colluvial deposits (Holocene and upper Pleistocene; 126,000 years ago to present) cover most of
the slope in the site area based on site observations. Colluvium is characterized as unconsolidated,
generally non-stratified deposits mantling slopes less than 50 degrees. Colluvial deposits are
comprised of pebble, cobble, and boulder sized rock and fine grained material mixed together by
downslope movement. Colluvium is typically less than about 30 to 45 feet thick.
4.2.3 Landslide Deposits (Qls)
Landslide deposits (Holocene and upper Pleistocene; 126,000 years ago to present) underlie most
of the eastern half of the site. Kellogg and others (2003) characterize these mapped deposits as a
range of chaotically arranged debris to intact slump blocks of bedrock. The middle member of the
Minturn formation (Pmm) is notably susceptible to landsliding, although slope failures can occur in
most sedimentary units where over steepening of the ground surface has destabilized slopes.
Largescale landslide deposits may be up to about 120 feet thick.
4.2.4 Pinedale Till (Qtp)
Glacial till of Pinedale age (upper Pleistocene; 126,000 to 11,000 years ago) underlies the
southeast corner of the site and also a majority of the slopes to the east-southeast, and the area
upslope to the north of the site (in part). Pinedale Till is characterized as unsorted, unstratified,
and boulder. It tends to form hummocky topography with common depressions and small ponds.
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Till deposits were observed upslope from the site and were bouldery (sedimentary and igneous
composition) and poorly sorted. This unit has been mapped as high as 900 feet above the present
elevation of Gore Creek, with thickness up to about 90 feet.
4.2.5 Minturn Formation
The Minturn Formation (middle Pennsylvanian; 315 to 307 Ma) underlies the entire site and
general vicinity. This unit is generally comprised of conglomerate, sandstone, siltstone, claystone,
shale, and stratigraphically distinct layers of limestone and dolomite. The Minturn Formation is
divided into multiple units, two of which directly underlie the site:
Robinson Limestone Member (Pmr)
Marine limestone and dolomitic limestone, gray to yellow gray, fine to medium grained,
and locally contains fossils. Comprised of four separate sequences (each about 60 feet
thick) of limestone interbedded with pinkish tan, light tan, cross bedded, mica rich
sandstone and grayish pink sandy siltstone and shale. The sandstone, siltstone, and shale
layers weather in rounded forms, and the limestone and dolomite beds weather in
relatively angular forms. Outcrops of the Robinson Limestone member are visible in the
steep cliffs northwest and are also exposed directly upslope from the site. One large
boulder dislocated from upslope and came to rest near the base of the slope along Fall Line
Drive is sandstone containing purple gray coral, possibly representative of a reef facies
within the Robinson Limestone member. The Robinson Limestone member is about 360
feet thick north of Gore Creek.
Lower Member (Pml)
Conglomerate, sandstone, siltstone, and shale, pinkish gray, gray brown, gray green,
mottled maroon, and gray green. The Lower member may contain clasts of Proterozoic age
granite (2,500 to 541 Ma). This unit is generally obscured by vegetation onsite and
outcrops were not identified during our site visits. The Lower member of the Minturn
Formation can be up to about 1,200 feet.
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DIAGRAM 1. Cross Section D-D’
5. GEOLOGIC HAZARDS
The current study focused on the geologic hazard related specifically to slope stability, including
rockfall and landslides in particular. Rockfall was analyzed using the Colorado Rockfall Simulation
Program (CRSP) for one study section located on the west side of the site where development is
most likely (per client communication). The landslide hazard was characterized primarily through
review of published maps and site reconnaissance to verify the nature, extents and evidence of
recent movement. Debris flows are a significant potential hazard in the site vicinity, although
debris flow susceptibility has not been determined for Vail or Summit County to date. The site is
not included in the Official Debris Flow Hazard Map for the Town of Vail, although Pitkin Creek
located near the southeast corner of the site is considered to have moderate to high hazard
potential. One debris flow located on the east-facing slope of Booth Creek (about 3,700 feet from
the western site boundary) and visible from the site is shown in Photograph 12.
Cross section D-D’ excerpted from the Geologic
Map of the Vail East Quadrangle (Kellogg and
others, 2003). This cross section is located
immediately east of the project site and
schematically depicts the surface and
subsurface geologic conditions in the site area.
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Photograph 12. View looking west toward Booth Creek. The project site is located beyond the trees in the
right side of the photograph. Features are labeled.
Debris flows and rockfalls have damaged buildings in the Gore Creek area since development
increased in the 1960’s. Debris flows can be triggered by intense summer rainstorms or rapid
melting of deep snowpack. Debris flows generally form on fan deposits, such as those composed
of glacial till. Freeze-thaw cycles in the spring tend to pry rocks loose, resulting in rockfalls of
varying magnitude and runout distance. The rockfall hazard is also related to a combination of
weak shale beds between harder sandstone and limestone beds, joints, and a regional bedrock dip
toward the valley. Large boulders from cliffs comprised of the Robinson Limestone member of the
Minturn Formation fell and damaged several residences in the Booth Falls subdivision in the
1980’s. As a result, the homeowners and Town of Vail created a Geologic Hazards Abatement
District (GHAD) which aided in construction of a rockfall catchment ditch and berm that has
generally proven to be an effective protection measure (shown in Photographs 9 through 12).
The exception would include the event in 1997 when a large scale rockfall skirted around the
western end of the catchment structure, rolling downslope, and damaging structures below. This
event resulted in the construction of mechanically stabilized earth (MSE) walls to add protection
for the downslope condominiums (some of which were not included in the original GHAD). A
report issued by the Colorado Geological Survey (CGS; undated) summarizes the event:
“At 11:20 p.m., a ledge of Minturn Formation limestone at the highest exposed
outcrop of the upper cliff, just below the exposure of glacial till, failed similarly to
that shown in Figure 3 of Appendix A. The ledge dimensions that detached and
toppled is roughly 20’ x 8’ x 8’. As it fell, it impacted and broke additional rock
blocks from outcrops below. The rock mass broke apart as it tumbled down the cliff.
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As it fell down the slope, the rock fragments randomly fanned out such that the
path of the rockfall formed a swath more than 500 feet across where they came to
rest. […]
Approximately one third of the swath of rolling rocks were retained by the ditch and
berm. […] The remaining two-thirds of the event came to rest, scattered around
the condominiums.”
5.1 ROCKFALL
Rockfall is a potential hazard for the site and poses a risk to the property. Rockfall is the fastest
category of slope movement and is common in mountainous terrain near cliffs of broken, jointed,
or faulted rock, on steep slopes comprised of rocky material, or where cliff ledges are undercut by
erosion or human activity. Stability of a rock mass is generally influenced by the underlying
support provided to that rock mass and the structural nature of the rock, including the orientation
and spacing of discontinuities. After a rock dislocates from a rock mass, the controlling factors for
how far that rock will travel downslope include characteristics of the falling rock (composition, size,
and shape), characteristics of the slope (form, length, and angle), the presence or absence of
obstructions on the slope, and the height of the initial fall. The rocks exposed upslope from the
project site are comprised of the Robinson Limestone member of the Minturn Formation. The rock
exposures contain fractures and thin layers of siltstone and shale. As time passes, cracks can be
enlarged by weathering of the rock, accumulation of soil or vegetation growth, and the forces
associated with freezing-thawing of moisture within the cracks.
5.2 LANDSLIDE
Landslide deposits in the area occur on unstable slopes typically underlain by Minturn Formation
shale, siltstone, claystone, or glacial till, and are largely considered inactive. The extents of a large
landslide onsite were mapped during field visits, and the published boundaries were verified and
refined using available light detection and ranging data (LiDAR). Refer to Figure 7 for the
approximate landslide extents mapped for this study. Geomorphic features across the landslide
have been masked by heavy vegetative cover, and obscured and smoothed by natural processes.
The block sliding mechanism responsible for parts of the landslide mass enable large, relatively
intact bedrock masses to slide downslope. These masses may appear to be in-place, when in fact
they have moved downslope from their original position. Based on the high level of detail offered
by the LiDAR view, Cesare has confidence in the mapped extents of the landslide as depicted in
Figure 7.
The toe of the mapped landslide deposit is abruptly cut off by Fall Line Drive. The downslope
extents and western flank of the landslide are steep and form a recognizable break in slope shown
on the topographic map (Figure 2) and on the LiDAR (Figure 7). Photograph 13 is a view of the
landslide toe and western flank, looking eastward. The retaining wall built near the Town of Vail
shuttle stop is about 10 feet high and the slope above the top of wall is relatively steep (30
degrees or greater). According to Kellogg and others (2011), a large landslide was activated on
the north side of I-70 due to undercutting from highway construction. The landslide is located
about 1.5 miles west of the project site on I-70, involves the Minturn Formation (same unit that
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underlies the subject site), and is failing by combination of shallow earth sliding and deep
rotational movement.
Photograph 13. View looking eastward from the western flank of the landslide toe. The ground surface is
relatively steep along the toe and flanks of the slide mass, visible in the photograph.
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6. ROCKFALL ANALYSIS
6.1 ROCKFALL STUDY SECTION
Cesare analyzed one rockfall study section through the west part of the site (Figure 8). The
location of this rockfall study section is representative of the slope on the west side and passes
through the area of the project site most likely to be developed in the future. The rockfall study
section is considered a reasonable representation of the slope in the western part of the site. The
section profile was derived from topographic maps available through the USGS, the Town of Vail,
and a topographic map for a portion of the western part of the site provided by the client. The
rockfall study section is depicted on Figure 9 and shown in Photographs 14 and 15.
Photograph 14. View looking upslope along the rockfall study section. Notable features include the
limestone bedrock exposures visible at the top of the slope and the dense vegetation on the slope. The
limestone bedrock forming the cliffs at the top of the slope are considered the primary rockfall source zone.
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Photograph 15. View looking downslope along the rockfall study section. Notable features include the rock
exposures visible at the top of the slope, the steepness of the slope, and the density of the vegetation. Fall
Line Drive, I-70, and East Vail are visible in the background.
The rockfall study section begins upslope above the primary rockfall source area exposed in the
cliff comprised of Robinson Limestone and extends southward to Fall Line Drive, with a total
elevation change of about 760 feet over a profile length of 1,530 feet. The analysis for the rockfall
study section assumes the rockfall source zone is located in the exposed cliff face upslope from
the site at an elevation of about 9040 to 9080 feet. Photographs 16 through 18 show the
limestone bedrock exposed in the cliff face upslope from the site. Bedrock exposures (potential
rockfall source zones) were not observed further upslope from this area, although the glacial till
deposits above the primary rockfall source zone may be eroding and contributing to the rockfall
hazard. The slope above the western part of the project site is incised with active drainages and
covered in aspen trees, tall shrubs, and scattered boulders and outcrops.
Rocks deposited along the rockfall study section slope are primarily blocky to slab shaped, and
comprised of gray limestone interbedded with thin layers of sandstone, siltstone, and shale.
Boulders comprised of sandstone were also observed. The rockfall study section appears to be an
area of more recent rockfall events, compared to other areas of the site. A number of rocks in the
rockfall study section area display a comparatively “fresh” appearance, relative lack of lichen or
vegetative overgrowth, and some with minimal soil embedment. For other parts of the slope, a
majority of the boulders are more deeply embedded in the soil and overgrown with lichen and
vegetation (indicating much older rockfall events). Refer to Photographs 19 through 23 for
examples of boulders observed on the ground surface in the area of the rockfall study section.
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Photograph 16. View of limestone bedrock exposure at the primary rockfall source zone. Note the eroding
shale partings and vertical fractures (spaced about 10 to 15 feet apart).
Photograph 17.
Close-up view of
primary rockfall
source zone
bedrock. Gray, hard
limestone
interbedded with
thin, weak shale
layers.
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Photograph 18. Aerial view of the rockfall source zone. This photograph shows the steep cliff forming exposures of Robinson Limestone member of the Minturn Formation, dense vegetation in the form of trees and large shrubs, and flowing water in one of the drainages on the west side of the site. The bedrock exposures are fractured, blocky, and ledge-forming.
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Photograph 19. View of limestone
boulder, embedded. Blocky, angular,
and about 3 feet in diameter. Boulders
like this one are common on the
property and are either embedded in
the soil (older, ancient rockfall events)
or are sitting on top of the soil with
minimal soil embedment or vegetation
overgrowth.
Photograph 20. Limestone boulder,
embedded, lichen growth. Blocky,
angular, and about 4 foot by 3 foot by
2 foot.
Photograph 21. Limestone boulder,
minimal soil embedment. Blocky,
angular, and about 3 feet in diameter.
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17.5029 Rockfall Hazard Study, East Vail Parcel 06.19.17 23
Photograph 22. View of large, angular, slab shaped boulders near the base of the slope within the area
most likely to be developed in the future. Boulder sizes were observed to be at least (1) 12 foot by 8 foot by
5 foot, (2) 7 foot by 7 foot by 3 foot, and (3) 21 foot by 12 foot by 9 foot. These boulders are embedded in
the soil and have been resting here for some time.
Photograph 23. Aerial view of lower slope in western part of the site. North is toward the top of the
photograph. Notice scattered boulders as large as about 7 to 8 feet in longest dimension and slab shaped.
Most boulders are 3 feet or less in dimension and are embedded in the soil, representing older, ancient
rockfall events.
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17.5029 Rockfall Hazard Study, East Vail Parcel 06.19.17 24
6.2 ROCKFALL MODELING - CRSP ANALYSIS
Factors which influence the runout distance, mode of travel, speed, and energy of a rock traveling
downslope include:
Type, size, and shape of the rock.
Type, length, height, and angle(s) of the slope.
Potential launch points along the slope.
Presence of obstructions on the slope (including trees, shrubs, and existing boulders).
Height of the initial fall.
Based on site observations, the types of rocks traveling down the slope are comprised primarily of
blocky to slab like limestone. Rocks are also comprised of sandstone to pebble conglomerate and a
minor percentage of small, granite boulders (derived from the glacial till capping the slopes above
the cliff-face rockfall source zone). Sizes generally range from about 2 to 6 feet in diameter, but
can be as large as 20 to 30 feet in longest dimension. The larger dimension rocks are slab shaped,
irregular, with angular corners. The falling mechanism for the slab shaped rocks would be
primarily sliding after detachment from the source rock, although these rocks may roll downslope
end-over-end along the shorter dimension. Based on our experience with similar conditions, site
observations, and on opinions presented by the CGS for the rockfall hazard at Booth Falls to the
west of the project site, the limestone rocks falling from the cliff source zone tend to break apart
during their descent downslope. Cesare opines that some of the larger blocks on the scale of 20 to
30 feet in diameter may have been entrained in block slide movement of the landslide complex
onsite.
CRSP requires that the section analyzed be divided into regions (cells) based on areas with
uniform slope and characteristics. Cell boundaries are determined based on characteristics, such as
slope angle, material comprising the slope, and the presence of obstructions. Surface roughness
was estimated with consideration for the size of the rock and the irregularity of the slope surface.
The surface roughness (S) is defined as the perpendicular variation of the slope within a slope
distance equal to the radius of the rock. This value varied based on rock size analyzed. Based on
site observations and available topographic maps, there are no significant launch points below the
rockfall source zone along the section.
The tangential coefficient of frictional resistance (Rt) for the rock is the component of velocity
parallel to the slope, which is slowed during impact. The tangential coefficient was chosen with
consideration for the material which comprised the slope, as well as the amount of vegetation
characteristic in each cell. Vegetation would tend to increase the frictional resistance in the
direction parallel to the slope, thus decreasing the tangential coefficient. The normal coefficient of
restitution (Rn) considers the change in velocity of the falling rock normal to the slope after impact,
compared to the normal velocity before impact. For both the Rt and Rn coefficients for each cell,
Cesare referred to the CRSP manual which provides ranges of suggested values based on different
material types.
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17.5029 Rockfall Hazard Study, East Vail Parcel 06.19.17 25
Cesare calibrated the model using the current conditions of the slope (no rockfall barrier, native
condition) and using rock sizes and shapes based on site observations. Simulation and slope
profile parameters are listed in Tables 1 and 2, respectively.
TABLE 1. CRSP Simulation Parameters
Parameter Study
Section A
Length of section analyzed (ft) 1,530
Elevation difference across section (ft) 760
Total number of cells 6
Analysis Point 1 (x-coordinate) 1,000
Analysis Point 2 (x-coordinate) 1,200
Top starting zone (y-coordinate) 9,080
Base starting Zone (y-coordinate) 9,040
Number of rocks simulated 500
Starting velocity (x) 1 ft/sec
Starting velocity (y) -1 ft/sec
Material density of modeled rock 160 lb/ft3
Rock shape Spherical
Rock dimension (diameter) 10
Starting cell number 2
Ending cell number 6
TABLE 2. Slope Profile Parameters
Cell Begin
(x,y) Rt Rn
Approx
Slope
Angle
(°)
Description of Slope Geologic Unit
1 0,9140 0.65 0.15 35 Vegetated slope above rockfall
source zone. Glacial till (Pinedale).
2 100,9080 0.85 0.20 Near
vertical
Cliff face, rockfall source zone,
approximately 30 to 40 feet high.
Robinson Limestone member
of the Minturn Fm.
3 110,9040 0.70 0.15 30
Vegetated slope below rockfall
source zone, runout accumulation
zone.
Colluvium overlying
Robinson Limestone/Lower
members of the Minturn Fm.
4 930,8540 0.60 0.15 20 Vegetated slope, accumulation
zone.
Colluvium overlying Lower
member of Minturn Fm.
5 1180,8438 0.60 0.15 8 to 16 Vegetated slope, accumulation
zone.
Colluvium overlying Lower
member of Minturn Fm.
6 1411,8382 0.90 0.60
Paved
roadway
(flat)
Fall Line Drive, asphalt paved
roadway. Not applicable.
Rt: Tangential coefficient
Rn: Normal coefficient
Surface roughness varied based on rock size analyzed.
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17.5029 Rockfall Hazard Study, East Vail Parcel 06.19.17 26
6.3 ROCKFALL ANALYSIS RESULTS
The results of the analysis using the current condition of the slope are summarized in Table 3.
Reported are results for common rock sizes observed at the site (3 feet diameter) and an
estimated maximum case (10 feet diameter). Although boulders as long as 30 feet in longest
dimension were observed embedded near the base area of the slope, these are considered more
likely to have been placed during block sliding of the landslide mass.
The rocks were modeled as spherical in order to represent the worst case scenario. Rocks which
are spherical will tend to have longer runout distances and higher velocities and kinetic energies
associated with them. Elongate, angular rocks will tend to lose momentum sooner than a rounded
rock as they travel downslope. Analysis Point 1 was placed about 200 feet upslope from the
property boundary and Analysis Point 2 was placed right at the upslope property boundary. Based
on observed runout and accumulation zones and calibration analysis results, it is Cesare’s opinion
that the input values listed in Tables 1 and 2 adequately model the slope in question. Rockfall
analysis results are listed in Table 3.
TABLE 3. Summary of Rockfall Analysis Results
Number
of Rocks
Passing
AP
Velocity
(ft/sec)
Bounce
Height (ft)
Kinetic Energy
(ft-lb)
Kinetic Energy
(kilojoules)
Max Avg Max Avg Max Avg Max Avg
Rock Shape = spherical; Rock Size = 3 ft (2,262 pounds),
AP1 492 37.6 19.2 4.3 0.7 65,545 18,906 90 26
AP2 21 16.9 8.0 0.3 0.1 13,957 3,649 19 5
Rock Shape = spherical; Rock Size = 10 ft (86,394 pounds)
AP1 499 52.9 35.7 3.9 1.1 4,570,623 2,240,805 6,197 3,038
AP2 497 33.2 20.8 2.7 0.7 1,846,786 800,467 2,504 1,085
Rock Shape = discoidal; Rock Size = 12 ft diameter by 5 ft thick (90,478 pounds)
AP1 499 46.7 37.6 3.4 1.0 4,112,846 2,861,685 5,588 3,880
AP2 499 33.8 24.7 2.6 0.8 2,243,475 1,270,950 3,042 1,723
AP = analysis point
ft/sec = feet per second
ft-lb = foot-pounds
6.4 DISCUSSION OF ROCKFALL ANALYSIS RESULTS
The CRSP analysis results show that a 10 foot diameter, spherical limestone boulder rolling
downslope along the rockfall study section from a source zone between 9040 and 9080 feet
elevation will have an estimated maximum kinetic energy of 1,846,786 foot-pounds (ft-lb), an
equivalent of about 2,500 kilojoules, at the upslope property boundary. The slope gradually
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17.5029 Rockfall Hazard Study, East Vail Parcel 06.19.17 27
decreases between Analysis Point 1 and 2, resulting in a decrease in kinetic energy of a rolling
rock between these points. The area of Cell Number 4 along the profile is a zonal transition from
rockfall runout in Cell 3 to rockfall accumulation in Cell 5.
For comparison, the worst case scenario considered in the CRSP analysis performed by the CGS for
Booth Falls was a spherical boulder 7 feet in diameter with an impact force of 5,000,000 ft-lb
(about 6,800 kilojoules). This estimated energy is extreme when considering rockfall fences
(flexible mesh barriers) currently on the market are rated for impacts up to a maximum of 8,000
kilojoules. The ground surface in the area of the slope analyzed at Booth Falls is generally steeper
and vegetatively bare compared to the section analyzed for this study. CGS recommended the
design height for the proposed rockfall mitigation structure be at least 12 feet, if placed at the
analysis point located 30 feet upslope from the existing condominiums. An added option to
mitigate for smaller rock fragments which tend to break from larger rockfalls, included adding a
fence to the top of the berm or wall to be constructed. Cesare understands that for Booth Falls, a
pair of soil walls reinforced with geotextiles and sized 8 feet high by 10 feet thick and 12 feet high
and 12 feet thick were constructed after the 1997 rockfall event.
The nature of the ground surface at the project site acts to dissipate rockfall energies compared to
the slope above Booth Falls. The ground surface on the west side of the site is comparatively less
steep, heavily vegetated with aspen trees and large shrubs, dotted with scattered, embedded
boulders, with incised drainages that act to channel and slow rockfalls. Vegetation, incised
drainages, and embedded boulders act to increase surface roughness of the slope, creating
obstacles which decrease rockfall energies. Comparison of the ground surface characteristics and
the CRSP results for both the project site and the neighboring Booth Falls indicates the rockfall
hazard is higher for the Booth Falls area than for the project site.
7. LANDSLIDE HAZARD MAPPING
The extents of a large landslide complex were mapped on the east side of the site (Figure 7). A
landslide study section passes through the middle of the landslide, location shown on Figure 8 and
profile shown on Figure 10. The landslide study section begins upslope above an exposed outcrop
comprised of Robinson Limestone at about 8900 to 8920 feet elevation and extends southward to
Fall Line Drive, with a total elevation change of about 588 over a profile length of 1,220 feet. The
elevation of the Robinson Limestone bedrock exposure can be correlated to the rock exposures to
the west which are the primary rockfall source zone for the Booth Falls subdivision, although the
outcrop on the subject site is not as pronounced or as exposed as areas to the west. Based on the
landslide morphology visible in the LiDAR image, this bedrock exposure at about elevation 8900
likely slid down from a higher elevation upslope.
The LiDAR bare earth surface and the landslide study section both display a benched and
hummocky pattern characteristic of landslide terrain. The flatter parts of the benched areas range
from about 15 to 20 degrees, while the toe areas of the benches range from about 30 to 40
degrees. A slope map is shown on Figure 11 and depicts the range of slope angles across the site
and surrounding area.
CESARE, INC.
17.5029 Rockfall Hazard Study, East Vail Parcel 06.19.17 28
Cesare understands that the Pitkin Creek townhome development located southeast of the site and
also at the toe of the mapped landslide extents has not reinforced the slope above the residences.
It was beyond the scope of this study to research potential landslide movement causing distress to
the Pitkin Creek development townhomes, and at this time Cesare is not aware of landslide
movement or related structural distress in the southeast area of the site. Chen and Associates,
Inc. (Chen) issued a soil and foundation investigation report for the proposed Pitkin Creek
Townhomes (dated September 20, 1978) which included subsurface exploration using test pits to
a maximum depth of 10 feet. The soils encountered were described as 1 to 3 feet of topsoil over
dense, sandy gravel, with cobbles and boulders to the maximum depth explored. Groundwater
was not encountered in the test pits. The Chen report mentions how the slope of the site rises
steeply to the north and that several large boulders were observed on the ground surface, but
does not discuss landslide or rockfall hazard or potential.
8. CONCLUSIONS AND RECOMMENDATIONS
This report presents findings of a geologic hazard study specifically focused on rockfall. During the
course of the study, a significant landslide hazard was identified and is discussed in this report.
8.1 ROCKFALL CONSIDERATIONS
Based on the CRSP analysis results and existing rockfall mitigation structures on the neighboring
site to the west, a rockfall barrier or wall at least 12 feet in height is recommended. Based on site
conditions, including such aspects as slope angle and property boundaries, a rigid wall would be
more ideal than a flexible fence or berm/basin. The flexible fence system would require a
downslope buffer zone for flexure during rockfall events. A berm and basin system would require a
significantly sized footprint on the slope, something this project site does not necessarily have
flexibility towards. Cesare’s CRSP model represents an estimate of rockfall energies at the analysis
point placed at the upslope property boundary along the section line and is not representative of
other locations on the slope. Changing the placement of the rockfall barrier will require changing
the location of the analysis point. Rockfall energies were modeled to be significantly higher at
Analysis Point 1 located 200 feet upslope from the property.
A catchment zone large enough for accumulation of boulders and for equipment to access the area
behind the barrier will be necessary, a width of at least 10 or more feet. It is the responsibility of
the wall designer to provide criteria for a wall that will withstand impacts with the sizes and
energies predicted by the CRSP analysis, and one which will allow for successful implementation of
recommended maintenance requirements. For rigid rockfall walls similar to those constructed at
the Booth Falls site, the height to width ratio is typically a 1:1 relationship. The rockfall catchment
will be reducing the rockfall hazard for a potential residential development and should be designed
with consideration for the nature of the structures (full-time occupancy).
8.1.1 Placement of the Rockfall Catchment Structure
Factors which influence the placement of the catchment structure include the rockfall energies,
sizes, shapes, and bounce heights estimated in the CRSP model for that analysis point on the
slope. Other considerations include site topography, site boundaries, and the spatial footprint of
the proposed rockfall catchment structure. The mitigation structure must provide an adequately
CESARE, INC.
17.5029 Rockfall Hazard Study, East Vail Parcel 06.19.17 29
sized catchment zone behind the wall and a buffer zone in front of the wall. The catchment zone
behind the wall must be sized to allow for accumulation of large boulders on the scale of 10 feet in
diameter, as well as access for equipment to remove accumulated debris from behind the wall.
Design considerations should include access for excavation equipment and adequate surface
drainage. Based on topography, the west side of the property provides adequate access for a track
mounted vehicle from Fall Line Drive and possibly a rubber tire vehicle (although access depends
on actual site development/grading plans, not available at the time of this study).
An adequately sized buffer zone in front of the wall is necessary in order to allow for a certain
amount of potential outward deflection in the event of an impact. The amount of deflection
depends on the type of wall to be constructed. The downslope buffer zone must be designed and
maintained as an open, empty space. The type of catchment structure has not been decided, and
may vary from a flexible barrier to a more rigid design, so it is important that this buffer zone is a
consideration during design stages. A flexible catchment fence will require more consideration of
outward deformation than a rigid wall, and will require a conservatively sized buffer zone. The
intent of flexible barriers is to slow the velocity and decrease the energy of the falling rock, not
necessarily to stop it completely. Rigid barriers have the limitation of being prone to damage
during high energy events, but this is generally the case with most constructed rockfall barriers.
The barrier should be designed to withstand the types of energies predicted by CRSP analysis
results described in this report. The catchment structure will require periodic and routine cleaning
of the accumulation areas to remove debris.
The rockfall remediation should be designed, constructed, and maintained to ensure hazards
impacting adjacent or downslope properties are not aggravated. In its current condition, the
western half of the site is impacted by rockfall consisting of boulders the size of 10 feet or more.
These boulders have historically rolled and slid down the slope from the steep cliff faces exposed
upslope from the site. The vegetative cover on the slope above the project site acts to slow
rockfall events in its current condition. If this vegetative cover were to be removed for some
reason (e.g. clear cutting, wildfire), these obstacles would be removed and the rockfall hazard
would increase.
8.2 LANDSLIDE CONSIDERATIONS
Cesare did not observe evidence of recent landslide movement at the project site. The retaining
wall for the Town of Vail shuttle stop which is located at the toe of the landslide, appears to be
performing adequately. The landslide area displays benched and hummocky topography with over-
steepened toe and flank areas, however, fresh landslide features, such as tension cracks, scarps,
slumps, and other features, were not observed. Figure 7 shows the bare earth land surface and
provides a convincing depiction of the landslide extents. Cesare is not aware of landslide
movement causing distress to the townhomes in the Pitkin Creek subdivision notched into the toe
near the southeast corner of the site.
Based on the lack of evidence of recent landslide movement as observed onsite and through aerial
photographs and LiDAR imagery, Cesare does not recommend monitoring of the landslide at this
time. Slope stability should be a primary consideration if ground modifications and development
CESARE, INC.
17.5029 Rockfall Hazard Study, East Vail Parcel 06.19.17 30
are planned in or near the landslide mass. The landslide has the potential to destabilize if the
ground is disturbed or modified in adverse ways. Slope stability of the over-steepened toe and
flank areas, as well as large-scale global stability should be considered. In addition, the bedrock is
dipping gently out-of-slope, exacerbating the slope instability issue.
8.3 DEBRIS FLOW CONSIDERATIONS
Although the site is not within the limits of the Town of Vail Debris Flow Hazard zone, there exists
the potential for debris flows at the site. Material and debris which could mobilized in a debris flow
event cover the slopes at and above the site, including glacial till capping the ridge above, and
rock talus and colluvium on the slope above the site. Incised drainages actively flowing with water
are present on the west side of the site, and ground surface patterns visible in the LiDAR imagery
suggest erosive processes are underway in this area. A significant precipitation event has the
potential to trigger or increase the probability of a debris flow event, additionally, ground
modifications may alter or increase this debris flow hazard in some areas. Cesare recommends the
debris flow hazard potential be considered in future development stages.
9. LIMITATIONS
This report has been prepared for the exclusive use of our client for specific application to the
project discussed and has been prepared in accordance with generally accepted geologic and
geotechnical engineering practices. No warranties, either expressed or implied, are intended or
made. In the event that changes in the nature, design, or location of the project as outlined in this
report are planned, the conclusions and recommendations contained in this report shall not be
considered valid unless Cesare reviews the changes and either verifies or modifies the conclusions
of this report in writing.
FIGURE 1
Site Location Map
PROJECT NO:
PROJECT NAME:
DRAWN BY:CHECKED BY:
17.5029
RAB JMF
Rockfall Hazard Study, East Vail Parcel
DWG DATE:06.16.17 REV. DATE:--
W:\2017\Summit\17.5000\17.5029.A Vail Valley Rockfall Analysis\ACAD\Figure 1 Site Location Map.dwg 6/15/2017 4:48 PMBACKGROUND IMAGE FROM GOOGLE EARTH02000'4000'1" = 2000'LEGEND:SITE BOUNDARY
0 250 500 750 1000 ft
Site Boundary
Topography
10' Contour
2' Contour
Roads
Fall Line Drive
Road
LEGEND 2JMF17.5029Rockfall Hazard Study, East Vail ParcelLAG5/22/17Topographic Map
0 0.25 0.5 0.75 1 mi
Site Boundary
Town Boundary
Parcels
Rock Fall
Approved Mitigation
High Severity Rockfall
Medium Severity Rockfall
LEGEND 3JMF17.5029Rockfall Hazard Study, East Vail ParcelLAG5/22/17Official Rockfall Hazard Map Town of Vail, ColoradoGoogle Earth Imagery
0 0.25 0.5 0.75 1 mi
Site Boundary
Town Boundary
Parcels
Vail Debris Flow Hazard
High Hazard Debris Avalanche
High Hazard Debris Flow
Moderate Hazard Debris Flow
LEGEND 4JMF17.5029Rockfall Hazard Study, East Vail ParcelLAG6/2/17Official Debris Flow Hazard MapTown of Vail, ColoradoGoogle Earth Imagery
FIGURE 5
Geologic Map
PROJECT NO:
PROJECT NAME:
DRAWN BY:CHECKED BY:
17.5029
RAB JMF
Rockfall Hazard Study, East Vail Parcel
DWG DATE:06.16.17 REV. DATE:--
W:\2017\Summit\17.5000\17.5029.A Vail Valley Rockfall Analysis\ACAD\Figure 4 Geologic Map.dwg 6/15/2017 4:53 PMMap Source: USGS Kellogg, Bryant and Redsteer0600'1200'1" = 600'LEGEND:SITE BOUNDARY
FIGURE 6
Legend for Geologic Map Figure 5
PROJECT NO:
PROJECT NAME:
DRAWN BY:CHECKED BY:
17.5029
RAB JMF
Rockfall Hazard Study, East Vail Parcel
DWG DATE:06.16.17 REV. DATE:--
W:\2017\Summit\17.5000\17.5029.A Vail Valley Rockfall Analysis\ACAD\Figure 5 Legend for Figure 4 Geologic Map.dwg 6/15/2017 4:57 PMMap Source: USGS Kellogg, Bryant and Redsteer
0 250 500 750 1000 ftSite Boundary
Landslide Boundary
LEGEND 7JMF17.5029Rockfall Hazard Study, East Vail ParcelLAG6/2/17Landslide Extents Map
0 250 500 750 1000 ft
Site Boundary
Town of Vail Topography
10' Contour
USGS 40' Contours
Study Section Lines
LEGEND 8JMF17.5029Rockfall Hazard Study, East Vail ParcelLAGRockfall Study Section
5/22/17Landslide Study Section
Study Sections Map
830084008500860087008800890090009100920093000 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500Elevation (ft)Length of Profile Line (ft)Rockfall Study SectionRock OutcropFall Line Dr, Study Area BoundaryStudy Area BoundaryCell 6: Frontage Road Cell 5: Accumulation ZoneCell 4: Runout/Accumulation ZoneCell 3: Runout Zone Cell 2: Source Zone Cell 1: Upper Slope917.5029Rockfall Hazard Study, East Vail ParcelLAG JMF6/1/17Rockfall Study Sectionφ=36°φ=75°φ=33°φ=28°φ=20°φ=16°φ=8°φ=1°
830084008500860087008800890090009100920093000 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300Elevation (ft)Length of Profile Line (ft)Landslide Study SectionRock OutcropStudy Area BoundaryFall Line Dr, Study Area BoundaryToeBenchBench1017.5029Rockfall Hazard Study, East Vail ParcelLAGJMF6/1/17Landslide Study Sectionφ=29°φ=38°φ=37°φ=17°φ=36°φ=20°φ=28°φ=27°
0 250 500 750 1000 ft
Site Boundary
LEGEND 11JMF17.5029Rockfall Hazard Study, East Vail ParcelLAG6/2/17Slope MapSlope Angle
APPENDIX A
Documents and Drawings Reviewed
References
CESARE, INC.
17.5029 Rockfall Hazard Study, East Vail Parcel Documents and Drawings Reviewed, References, Appendix A 1
DOCUMENTS REVIEWED
DOC1. Chen and Associates, Inc., Soil and Foundation Investigation for Proposed Pitkin Creek
Townhouses Near Interstate Highway 70, East Vail, Eagle County, Colorado, Project No.
17,046, dated September 20, 1978.
DOC2. Chen and Associates, Inc., Geologic Hazards Reconnaissance, Lot 11, Block 1, Vail Village
12th Filing, Vail, Colorado, Project No. 25,474, dated January 26, 1983.
DOC3. Colorado Geological Survey, Rockfall Hazard Assessment at Booth Falls Condominiums,
and Proposed Mitigation, prepared for the Town of Vail, Colorado, undated.
DOC4. Nicolas Lampiris, letter re: Unit #13, Pitkin Creek Townhomes, prepared for Nedbo
Construction Company, dated September 12, 1987.
DRAWINGS REVIEWED
DWG1. Topographic Map of a Portion of the South 1/2 of the Southeast 1/4 of Section 2,
Township 5 South, Range 80 West, Town of Vail, Eagle County, Colorado, prepared by
Peak Land Consultants, Inc., dated January 10, 2017.
REFERENCES
REF1. Kellogg, K.S., Bryant, B., Redsteer, M.H., 2003, Geologic Map of the Vail East Quadrangle,
Eagle County, Colorado: U.S. Geological Survey Miscellaneous Field Studies Map MF-2375,
Version 1.1.
REF2. Kellogg, K.S., Shroba, R.R., Premo, W.R., Bryant, B., 2011, Geologic Map of the Eastern
Half of Vail 30’ x 60’ Quadrangle, Eagle, Summit, and Grand Counties, Colorado: U.S.
Geological Survey Scientific Investigations Map 3170.
APPENDIX B
Rockfall Hazard Assessment at Booth Falls Condominiums and
Proposed Mitigation
(Colorado Geological Survey)
ROCKFALL IIAZARD ASSESSMENT AT BOOTH FALLS
CONDOMINIUMS
AI\D PROPOSED IIdITIGATION
prepared for
The Town of Vail, Colorado
by
Jonathan L. White
Colorado Geological Suwey
l3l3 Sherman Street" Room 715
Denver, CO 80203
ph. (303) 894-2167
fax (303) 894-2174I
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CONTENTS
Introduction
March 26,1997 Rockfall Event
Hazard Assessment
Rockfall Mitigation Options
Rockfall Analysis and Design Criteria
Recommendations
Current and Future Actions
Appendix A. Booth Creek Rockfall Hazard Arca
by Bruce K. Stover
Appendix B. Rockfall Mitigation
by Jonathan L. White
List of Figures and Photos:
Booth Crcck Rocldall RepoG Pagc I
Page
2
2
Figure #1
Figure #2
Photo #1
Photo #2
Photo #3
Photo t#4
Photo li5
Site map and location of March 26,1997 rockfall.
Screen dump of CRSP slope profile
Booth Creek rockfall source arrea
Top Cliffrockfall source area
Close-up of top cliffsource area
Location of pioposed mitigation at Condos
Lower cliffabove district to be monitored
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"** ,"* *kfau Rcpo4 Pase 2
INTRODUCTION
The Colorado Geological Suwey has assisted the Town of Vail in assessment of the rockfall
hazard at Booth Creek since May 1983, when a severe rockfall event occurred there. Since then the
town and property owners in Vail Village Filing l2 formed 4 Geologic Hazard Abatement District
(GIIAD). The Disfiict has mitigated much of the hazard by the constnrction of a ditch and berm on
the slope above the residential area As far as the Survey lnows, the ditch and berm configuration
has been 100% effective forrocks that continually fall from the cliffs of the Mintum Formation. On
March 26, 1997, another very serious, potentially lethal, rockfall occurred that incuned substantial
damage to the Booth Falls Condominiums that exists to tle west of the GHAD and outside the
protection envelope provided by the ditch and berm. Under the auspices of the Critical Geologic
Hazards Response Program and our concerns expressed in earlier involvement, the CGS can assist
the Town of Vail in assessment of the hazard that the condominiums bear, options for mitigation for
that portion of slope west of the ditch and berm terminus, and design criteria for said mitigation
systems. Included in this report are two appendices. Appendix A, Booth Creek Rocldall Hazard
Area by Bruce Stover, is a report on the general geology, geomorphology, and the mechanism of
rockfall for the Booth Creek site. Appendix B, Rocldall Mitigation, is a short paper on types of
rockfall mitigation systems that are available.
THE MARCH26,I997 ROCIGALL EVENT
At I l:20 p.m., a ledge of Mintnm Forrnation limestone atthe highest exposed outcrop of the
upper cliff, just below the exposure of glacial till, failed similarly to that shown in Figure 3 of
Appendix A. The ledge dimensions that detached and toppled is roughly 20'x 8'x 8'. As it fell, it
impacted and broke additional rock blocks from outcrops below. The rock mass broke apart as it
tumbled down the cliff. As it fell down the slope, the rock fragments randornly fanned out such that
the path of the rockfall formed a swath more than 500 feet across where they came to rest. See
Figure #l of this report. The location of the rockfall source is shown by arrow in Photo # 1 and#Z
and the scar easily seen in Photo #3.
Approximately one third of the swath of rolling rocks were retained by the ditch and berm.
See Figure #1. The remaining two-thirds of the event came to rest scattered around the
condominiums, The condo stucfures received three rock impacts and several near misses- Rock
sizes ranged from 2 to 5t feet in average diarneter. Surrounding the condos several items were also
damaged or destroyed, (i.e., small haul trailer, trampoline frame, small wooden deck and chafus,
wood walkway). Of the tbree impacts, one was minor and the other two major. The minor impact
was from a -3 foot diameter rock that obviously had slowed almost to a stop upon impacting ttre
westemmost condo structure. The rock came to rest, ominously so, next to a large boulder from an
eaflier rockfall. A major impact, also about 34 feet in diameter at high velocity, had jus missed the
ditch and berm catchment. The rock impacted and smashed the comer of the eastemmost condo,
snapped offthe side balcony support, and destoyed atrampoline frame along its path before coming
to rest in the subdivision below. The third and worst impact was a 5* foot block that broadsided the
easternmost condo. Sufficient rock velocity enabled the boulder to smash through the outside wall,
interior walls, and the floor, finally being caught in the crawlspace below. Luckily the resident,
whose bedroom this rock smashed throug[ was not home at the time of the rockfall.
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Bootll Creek Rockt'all Report, Pagr
I
Booth Creek Rockfall Hazard Area
Vail, Colorado
Areal extent of rockfall impacts from
1 l:20 pm, 3126197 event.
Rockfall Source: Limestone bed at highest
point of upper cliff. See companion photos
in report. Location not shown on town GIS
map.
i one inch = 200 feet
.r The berm was 100% effective for that
I portion of the 3126197 event that fell into ic
15t1.2
6f.o.6 |Xl
Jisure
#1.
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Booth Falls Rockihll Report, page 4
The CGS made an initial inspection of the site Thursday, March27,l997. Our pretiminary
assessment was that it appeared that the ledge broke away relatively clean and the hazard risk in no
greater or less than the day before the rockfall; which is to say that rockfall can occur from this
source area anytime. It was on our preliminary inspection of the ditch and berm where we
discovered that an earlier rockfall event occurred, either earlier this year or sometime after the town
last cleaned the ditch out. Several rocks (<4 foot diameter) had fallen and, by lithology, could be
differentiated from the March 26 event (sandstone vs. limestone). This rocKall occurred without
anyone's knowledge because the entire event was contained within the ditch and berm. Friday,
March 28, 1997 anaeial recormaissance was conducted of the source area and while the preliminary
assessment has not changed, we reiterate that rockfall of similar magnifude will continue at this
site. During this inspection we did see several loose rocks on the slopes and rock features with
questionable long-term stability.
HAZARD ASSESSMENT
In a ranking ofa rockfall hazard the parzrmeters zue source area, a steep acceleration zone,
proximity of structures to both, and history of rockfall impacts. In two aspects the condominium
location is worse than most of the special district to the east because the upper cliff is more fully
exposed at this location (it is mostly soil covered to the east) and the slope between and below the
cliffs steepen where the slope curves around into Booth Creek Valley. See Photo #l and Figure #1
map in Appendix A. !n.rrF. . .
The main source area
for Booth Falls
Condominiums is the upper
cliff. The exposed, lower
cliff of sandstone reduces in
height as it trends to the
northwest. Photo #l and a
close-up photo #2 show the
extent of the upper cliff
where it is not soil covered.
They reveal a benchy cliffo
beds of limestone, thin shales,
and minor sandstone. It is the
dense, hard, gray limestone
that creates the largest
rockfall boulders in the Booth
Creek area. The report by B.
Stover in Appendix A
provides fuither in-depth
discussion on the source areas. Photos #l and#2 also show the exposed shale slope, between the
cliffs, steepening to the left. The general lack of soil and vegetation suggests that this slope is harder
and smoother, compared with the right. A further close-up, Photo #3, reveals limestone blocks,
pedestals, and ledges, defined by the crisscrossingjoint pattern, being undermined by the quicker-
Photo #1. Booth Creek rockfall source area. Note enlargement of upper cliff
exposure and conesponding rockfall source area, northwest ofthe ditch and
berm terminus.
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Boorh Falls Rockfall Report. page 5
eroding interbedded shale partings. Also in Photo #3 are several slumped and isolated limestone
blocks on the rock slope that have not yet falien. The history of reported rockfall events at Booth
Creek and the physical nature of the slope merits our assessment that, Booth Falls Condominiums
is in a severe rockfall hazardous area,
Photo #2- Top cliff rockfall source area. white anow marks location of March 26, lg9'. rockfall.
$:".
Photo #3. close-up aerial view of source area. Note ledgy appearance with joint defined blocks
undennined by eroding shale panings. White anow A marks scar liom March 26. 1997 rockfall. White
arrow B marks rock pedcstal that rvas hit by rockfall and may be destablized. Note loose blocks, rnarked
by black anows.
Rock
Weieht
Rock
Size
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Booth Fa.lls Rocldall R€poft, Page 6
ROCKFALL MITIGATION OPTIONS
Appendix B contains most of the recognized forms of rockfall mitigation and protection
devices commonly used. Rockfall mitigation is divided into two types: stabilization ofthe rock mass
at the source area to prevent rocks from falling; and rockfall protection systems that acknowledge
that rocks will fall but structures or public areas are protected from the impacts. At the Booth Creek
site stabilization ofthe rock mass at the source area is not being contemplated for several reasons.
They include:
l. The soruce area is in the USFS Eagles Nest Wildemess Area;
2. Source area stabilization at this site would need to cover a large area, be labor intensive,
require technical rock climbing skills, and helicopters for mobilization that would make the
project cost prohibitively high;
3. Source area stabilization consbuction activity would present unacceptable risks that rock
could be inadvertently knocked down, by workers or equipment, onto the residential areas.
Rockfall protection systems that will be considered at this site are ditch and berm
configurations and impact barrier wall systems. Fences will not be considered because they can have
high maintenance cost and generally cannot withstand high impact forces without being destoyed.
ROCKFALL ANALYSIS And DESIGN CRITERIA
Proper analysis of the hazard for design purposes requires accurate slope geometry and a
determination of appropriate rockfall sizes. Forthe slope geomety we used information gained from
our earlier investigation for the special distict mitigation, the Town of Vail GIS 1:2400 scale maps,
photos, and the USGS l:24,000 scale map. For the rockfall size using the maximum size boulder
that is found on site would be prudent. We used the Colorado Rockfall Simulation Progam (CRSP)
ver. 3.0a for our analysis. Four to seven foot diameter boulders were modeled, and weight was
calculated using the unit weight of limestone. The analysis seemed to bear out observable results
of rockfall in the area. Bounce heights were highest on the cliffs and at the transition to the lower,
softer slopes the rocks begin just to roll. The critical design factor is the high impact energies
developed by these larger rocks. A screen dump is shown on Figtre#2 of the CRSP program slope
profile. An analysis point was chosen 30 feet upslope from the condominiums where the slope
breaks to a grade of 40o/o to 50%o. In modeling rockfall with CRSP we arrived at the following
bounce heights, impact kinetic energies (K.E), and velocities at this analysis point.
Bounce K.E.(max.) K.E.(avg.)Vel.(max.) Vel-(avg.)
ft. ftlbs . ft.lbs ff:/sec ft/sec
4' sphere
5' sphere
6' sphere
7' sphere
4'x7' cyl.
5'x6' cyl.
6'x6' cyl.
6'x7' cyl.
5058
9878
17069
27106
13272
11775
25600
30000
3.0 1,000,0002.r r,900,0002.0 3,000,000r.7 4,600,000|.7 2,500,0001.9 3,600,0001.9 4,900,000
t.8 5,700,000
800,000 98 83
1,400,000 95 8t
2,300,000 96 78
3,300,000 89 74
1,700,000 93 74
2,400,000 94 76
3,500,000 89 '74
3,700,000 90 72
Booth Falls Rockfall Repon, Page 7
Figure 2. Screen dump of CRSP program of Booth Creek-west side. Analysis point arrow is 30 feet above
condominiums. Horizontal and vertical are not at the same scale.
RECOMMENDATIONS
The following recommendations and design criteria are based on modeled rolling rocks
analyzed at 30 feet upslope from the condominiums, so are only valid at that point on the slope.
Mitigation design should not only insure that rockfall is contained but also the impact structure
remains sound and does not require costly reconstruction afterwards. The CGS recommends that
design criteria for mitigation at the condominiums should be capable to withstand and retain a worst
case scenario, which is believed to be a boulder in the 6 to 7 foot diameter range. An examination
ofthe source area, the most recent rockfall, and earlier research done by Stover and Cannon for work
the CGS did in 1988 seems to confirm this scenario. That translates to a rolling rock with an impact
force of 5,000,000 ft-lbs at the analysis point. Besides withstanding the impact force the mitigation
system would need to prevent any rock that encounters it from climbing and overtopping, or
bouncing over. The impact face should be vertical and have an effective height that prevents
overtopping. Design height will be specific to siting of the structr.ue. At the analysis point it should
be no less than 12. These design parameters do not take into account smaller rock fragments that
separate from larger boulders. During inspection of the site following the March 26, 7997 event
there was evidence of smaller rocks snapping off the tops of Aspen trees, 25 feet high, near the
condos. These rock fragments do not reflect actual bounce heights but display the high rotational
velocity of the rock and the centrifugal force acting on fragments as they detach. Options to mitigate
these highly random rock fragments are limited to moving the protection system farther up the slope
(which will change design criteria) or constructing a low capacity rockfall fence at the top of the
berm or wall.
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Booth Falls Rockfall ReDort. pase 8
Only a stout protection
system can be designed at the
criteria stated above. Both
ditch and berm systems and
inertial impact barriers, or a
combination of both, can be
dcsigned for the site and be cost
effective. No rockfall fence on
the market can probably
withstand the impact forces that
are being contemplated. The
rockfall protection must be
designed to begin at the road
and extend to the southeast to a
point where sufficient overlap
exists with the existing berm
above, a length no less than 350
feet. Rocks that skirt the edge
ofthe top berm must be caught
by the lower. See Photo #4. At
the high impact velocities and
conesponding impact forces both ditch and berm and reinforced impact walls will need to be
carefully designed. In a ditch and berm option a careful look will be needed to determine whether
the berm of only compacted soil will have the strength to withstand these forces. The earthen berm
may need to be reinforced with geotextiles. A rockfall impact barrier or earth wall will need to be
reinforced with geotextiles in lifts of 8-12 inches and have a width no less than 10 feet. We
recommend that the Town of Vail retain the CGS for review of the mitigation design and our
approval be a condition for design acceptance by the town.
CIIRRENT AND FUTURE ACTIONS
Adverse or highly variable weather prevented the CGS from doing a site inspection of the
source area immediately after the March 26 event. Later this spring we plan to conduct this site
inspection where the failure occurred and examine those impacted rock features below that may be
of questionable stability. During ow aerial inspection we also found a rock feature above the special
district ditch and berm that may require long term monitoring. See Photo #5. While we believe this
feature will not be a threat for many years it bears watching because of its size. If this feature were
to fail the vohrme of the fall would quickly overwhelm the capacity of the ditch and overtop it. We
will provide the Town of Vail a supplemental report based on our f,reld studies later this summer.
For the interim. residents of Booth Falls Condominiums who are concerned about their safety
can take precautions to lesscn their exposure to rockfall hazards. As stated the larger rocks are
basicallv rolling when they reach the condos. The safest area in these condos presently is the top
floor on the side facing downhill. The worst case rockfall impact can put a big hole through a
Photo #4. Location of proposed impact barrier or berm site. Note
accumulation ofrocks in existing ditch. The largest are 5 feet in diameter.
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Booth Falls Rockfall Report, Page 9
Photo #5. Lower sandstone cliff above district ditch and berm. The CGS will visit this
feature this spring and install movement gauges for future monitoring.
structure and possibly condemn it, but probably will not tear it down. Our advice to residents is that
they not establish living areas where they spend the bulk of their time, such as bedrooms and the
sitting areas of living rooms, against the exterior wall that faces upslope. Bedrooms shouid be
moved upstairs and/or beds placed against the wall facing dorvnhill. Do not place beds directly in
front of, or below, windows that face uphill. The Home Owners Association and Town of Vail
should act quickly so that these structures are protected from the next rockfali of similar masnitude.
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APPENDIX A
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Boors Cnnpr Rocrmr,r, Ilaann AREA
Broce K. Stover
Colorado Geological Survey, 1313 Sherman Street, Room715, Dem'er, C0 80203
Rcsidcoccs sinratcd u thc basc ofthc ralleywall at thc nouth
of Booth Gc€t in Vail Vallcy arc crpccd to varriry dcgrccs of
roctdall hazard (Frgurc 1). Ile hazard ranges fron hm to
Dodcf,ate for structurcs ncar thc limits of the runout zonc o thc
hazard sas thus aot idcntifcd priu to dcvelopocot
In tho years since tbc odginal bazard ilvesdgadon sas coo-
ductd sercral nce signiEcant roddall events harp occured;
boulden havc dcstrolrcd tinber patic and logr€tahhgwals'
Tho tocm of Vail ald aficctcd propcrty wacf,s arc crrrcrt'
$pursuing a mcans aad framcryork for adninisteringdccrge ald
constnrction of protectivc rocKall structues atrd batders in a!
attcmpt to safeguard thc residentid area
Geologr of RoeJdrll Sourtc Artas
sandstone beds about 12 m thick rcsting on a weah fssilg rapid-
ly eroding black to
promincnt joint sets
combine to scparate
visible from thc valley beloyr. Above the saudstoni is a soft, fri-
able coarse sandy congtomeratic bed I m thick which weathers
to a smooth rounded ledge and continually undercuts a 0.6 to 1
ra thi* dcnsc, bud grry limcstone Eit rcsling abotE iL The
limcstocbjointcd sothatsrbangular blocb (5r.6x1 n) coo-
tinuously dctach frm tts bcd atd fal oE the slorping difi cdgc.
lacsc lincstmc ffi ac cmnolyimrohrcd in the roqc ftc-
qucutty reorring cmnts tbat can oftcn causc dapage to struc-
turcs in thc ruout zoa
A thic& sbab udt bctstco thc uppcr aad lowcr dift has
srcarhcrcdbac& to a68 pcrccntdopc.Ttc sbale is soft' dayey'
aadsho*scvirieaceoeiocatizcdstippageadsoallslopefailures
or arc rcstirg rGar pofots of initial biha
Above this soft aodiDg shalc is a thictcr cJifi-forming unit of
thc Robinson Lincstona This bcd of deose, harrd' gray linc-
stone nrics ftom 15 o 10 n tliclr h thc study arca ald is thc
sourcc for thc largcst rocldall bouldcrs cncountercd in thc
nuout zora Thc lincstoc bouldcn'that detael froa the clifi
arc quitc resistant and tcnd not to breat up or shancr on their
*ay dornslopc, Thc targest boutdcrs fourd in thc runout zonc
appcar to bc dcrivcd fron this uppcr diE'fmning limcstone.
acating pedcstal-lilc blods cfrich ctteltraly topplc off their
pcrctras.lte linestooc isjointed such that blods approrimate-
ly3 nxl2mxl2 m arc scparatcd fron the clifi and tilt out-
qard tonnrd the difi cdge. Thiuer beds withia the lirncstonc
cliff producc oore stabby bloc.ks that, if rct tiirncd onto thcir
cdges by chancc during thc initial fa[ renain flat-sidc do*l on
tbe ste,cp slopcs-
An eroding slope io glaci"l till rasts directly above thc cliff-
forming upper limc,*one in the nortbcrn Part of the study area.
Tbc croding slopc periodically shcds smoot\ rounded granitic
boulders which tumble down the cliff into the runout zone.
Other areas oftbis till farther east along the diff appear rclativc-
ly stablg and are not actively shedding largc rods to the slopes
bclow.
proxinately9,450 ft-
cD-
n)
ontothe
zone pcriodically detac"h from the difi and free fall
aad bound domslopc and ofr the lowcr cliff. Most
rocks do not shatter, but renain as htact aP
proxinately 8 by 5 ft (25 by 15 m) limestone
boulden which arc capable of reaching the fartbest
limits of the runout zone. (Figure 4)
Eroding uppcr till slope - Glacial till resting on top
of the uppcr cliff shcds routrdcd granitic boulden
D)
E)
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D
I-rgpr€ 1. Lo€don map of soily arta, scalg 124'0fl)
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--/aesoEfiTtALsr"ucTrnEs
8OT.|LDERS N
Ritrqrr zollE
GLTdAT TILL
{i ?;:
I,P?ER
r_FsroraE ct.FF
?;':E
*gglTALL SOT'RCE AREA
T.EDGE
.:.-.-tj
lf!ft.-\ ^e,ffi].{.ffiS IfDSTO1{G \ m )f6t4)4:.-+ ++r'.t =;-,;j-rt---
CN.IFFS
uxvEn '/rwE t E
sHAr..E
@LU fltH ot{
ACCELEnAmfl - -&=--; -- --:--::t-a-;;.a--:::--;-- -i--,i--; J--;=
RUNOUT ZotIE
ngo"r 2. Geologk rliagrrm of conpoond roclr&ll slopes h shdy ena. Dram to scab wi& no rcrdcal cmftuadoo' No0c dip of
sEata tonrd rzlleY.
=j-,:ajj5,=5_:=---'F;F;j:=:---
domslope urtich roll and hll ofithc cliF<. This dl
slopc is considercd to bc a part of thc uPper source
arca-
Rocldall Mcclanisns
of Gorc Creek in the study area- Thesc faaors iadudc ioint pat-
terns, difiercntial weathering ofvariors rock typcs, dip of srat4
ald thc slopc of cliffs and acccleration zoncs-
Joindng and Difiercnttrrl Weatherlng of OilI Facrs
Oncc a slab has dctachcd from the scdimentary bed' it bcgi6 to
creep outwards owitrg to gravity and frost wedging in the joins.
The joins widen with time, and are often wedged farther aPail
by trcc roots, atrd smaller rock that fall isto the crack formed
bythcjoiun (Figure3)
ger adjaccnt u$table parts of thc diff to fall as *cll
Dip ofSbata anrl ToPograPhY
Ttc dip of the roct lcdges makiag up the sourcc arca also
contributes to rockfdl dong diffs ia the study area- The strata
in thc two cliffs dip approrimately 15 dcgrees into tbe tzllcy'
r bouldcrs on the ledgcs to
of thc 16 m vertical clifi.
their bcds byjoiating and
weatbcring creep down toward tbe nalley along thasc dippmg
be&ock surfaces (Figurc 5)- Rounded glacial cobbles and gravel
t:ii':
ItsrrG3.To'fDlhg$ebftlbtScgoca1hl6"1g6co-tnn6or.a lXftrafralrtathcrlryo($ftsDab bcgluto !ld.r.ot
nrsslrc Of faoq d.DqI. Jobfr oDrn ril! rftLD {bc to dopc ctccp ud foct Gddr& SDrhgF lssoc hon contact bcocath dIE
3.Unrfumtdrycodlp3l.Jobtrildclal{arcrc&pr!o'ncltyroelhrrodrs,causlryshbtotlltoutrn'd,s.4. Slsbfalsfron clitr
facc 66to *ceqrOo dopc* hfgbg rton ortdyllg to.tt 5. SLb toppla enil shatfcr:' shortrlng taDout toDG belol with
bouldcq ald cqlclry xrdltrhce to codoo.
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Ilgure 4. Limcstone stabs rtsdng oD ftaL shalc ptdcstalst
uppcr clilf soucc ana.
Ffuurc 5. Slopc rrtep causlng llnestore blocls to nove dorn
bcddbg plarcs aurl ofr lortr dlff edga Blocls are genentty 2
fr x 3 ft- Thts ncchanlsn ls responsible for frequent rocl( fals
h thc sbrdy arta.
?::
':---, =--?,;:==
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ROCKFALL
ao|ruvtr
rffir
ttgure6. Physlcal diEeracos bctrccurocldalaldglada[tdcpcltcdDoldcrrh moutzooaRoddallbosftlcrsartdlll@c
or sandstone' lhilc gladal bouldcrs are mostf nurdcd gr.lllc or Drfrno4hlc [thologb. No0c that soll dsts b.tov m&ll
bold.rrr trtlh lt b rDccot b.rcdh drdd boldcrc.
DEPRESSIO}I
tN sotL
GRANITIC BOULDERS PITIED
WEATIfRED
SURFACE
It't TILL
SOIL PFOFILE
slougb down dolg the dip slopcs ad oreotually fall into opcn
crac&s forned Sioints, wdgiry dab6 fartb€r eart
Thc glaciatedvalg6of GorcandBooth Cree&sbothpcscss
rclatively f,at bouos and stccp rcadywrtical sidcs. Tbc slopcs
are so stcep tlat once a bouldcr or slab topplcs fron thc aiff\
it usuatty cauot cmc to rcst Etil it rcactcs tbc lo*rr fooalopcs
of thc rralley *zll Aa cxanioation of tbc rurout zooc sho*s that
large bould€rs ard slabs havc tradlcd mto ald acrs pdts of
the vzlley floor duc to the treoendous moncntun tbcy aoquirc
in the aelcratim zma
Factors TiigFrirg Rotralls
Moct of thc roctdalls rcportcd in rhi( arca aPpeat' to bc re-
lated to altcrnating frcczc-thaw coaditiols. Brcnts harc oc-
currcd at n'ght in winrcr, sp'ring ald hq aftcr warm days of
melting harre introduccd runofi into johts aad fracturcs Upon
freeziog the icc e.pads in thc cra& suffcicntly to topple aa
urstable blocls Somc cvents barc also occtrrcd on the othcr side
of the cyclg as sunshinc tha*s the 6oeca cliffs, rclcasing a
precariously perched bloct or boulder.
Hazard Classilicadou ald Zonatlon
The rockfall hazard associated with gcologic and
topographic conditions and the proximity of duelling as
dcssibed aborc is considercd to be severe. Tle majority of large
boulders found anong structurcs h thc runout zooc havc hllcn
from thc clilfs. Ficld study indicates tbat the qucstion is not,
"Will sigEificant roctfall occur?", but rathcr, 'What is thc rcsr-
rcuce interyal bctween signfficant rockfall events?",
Acceleration slopes are so ste€p aud smooth that rock
trawrsing them arc frce to deflcct aad skittcr latcrally ia any
OLDER ROCKFALL BOULDER FRESH
NO DEPRESSIOII
BOULDER
HCOT(i| TEXT
DtscoLotATpIS
EDGES EX?OsCD
dirccfio radiatiog fro6 hc point ofinitial fall Thc pattcm c
traicctcy a girco bouldcr c@ld follow is so unpcdicdlc tbat
It b impraaical to dclircatc individual bazard aocs bccd o
tDc fiyfcal oditions of rarbus scgncotr of tic diEhccs. Ir
thc prcscot situation, hazard zoocs arc ncc practically relatcd
to haizolat.dista$cfiomthcsourccarcas, mcsbrthEarry
cqcricnchg a snalcr probability of bcfug cocompasscd by a
givcacrcotTtis ap'Fo€ctyicldsanappruinatclyradflalsaics
of zocs radiadry out from thc sourcc arca; thc mor! seeGrc
ba&sc dftuslydccst totnc dift Itsbould bcpohtcd
oEt, hoxrcrrr, tbat any uca within thc cxcot of thc ronc ac
is dicd o smo dcgpc of rocldal haard.
BarardZmc Ddlncadoo
Varyi"g degrcas of roctdall hazard scncrity caa bc ap
prorinated by cxaniration of thc aaturc and pcitios of
borldcn and slabe in tlc nrnout zona Eacl largE bon|rlcc was
exanincd to dcterninc scveral factors vfiic.h wrc uscd to ag
pruinatc thc crlcat of tbc ruout zonc, 6ad esrinrerc 1f,6 rinc
spans sbcc cach roc.Ifall boulder cane to rest Thcsc hctqs
arq,
1) Whaher or aot a boulder was of rocldall cfin or
dadillydcpodtcd
Wbethcr or not a roclfall bouldcr *zs rcsting udis-
turbcd in its orighal pcitioa or had bcco Eortd by
hunal activities.
llephpical naturc of rudisturbcd rocldallboldccs
with respect to basal contacg (rcsting on srfacc, cm-
bcddc{ partiatly corrcrd ac) and lichc4 mcq
and wcathering patterns on eJeoscd surfaccs
The comparative sizc distributioos of boldcrs
sithin 1[s run691 z6ag.
Rockfall Versus Glacial Origin of
In order to determinc the €tccnt of the rockfdl runout zone,
it is nccessary to determine Yfiether boulden en@untered
belowthe cliffs in Vail Village have fallcn from one of the source
areas and come to rcst on tbe surfacg or if tbeywere Eansported
in occnc glacia-
tio thc ctraracter
of bouHers fouad cnbcdded in rmdisturbcd glacid dcposits
with the limestone and sandstone bouldcrs derived fron the
cliffs (Frgurc 6). Glacially dcpositcd boulders arc mostly
roundcd to subrounded snooth granite or EetaEorphic rocks
whic.b are inb€dd€d in the surrormdinggfacial deposits Thc ex-
posed mrfaces of thcsc boulders are alnost totally covered with
iichens and -ots. ite hear' lichen corrcr and other well
developed sruface rocl weathcring featurcs sucb as pis-and
etched relief of individual nincral grains' nrggost that thcse
bouldcrs haw bccn in placc for Z) to'10 thousand years Tbe gla-
duc to thc fact that the ody sourcc uea stcrc valcy glaciers
of largc bouldcrs of rocffall origir and detcrminc thc ap
prorinate limits of the runout zona
DisturtertYer$s Unilisturtcd Roddrlt Boollcrs
reliabla
go
aaa,iooay th"Qs and ticben growth p"n"'og if -y, -" io' I
consistent with the prescot orientations of the boulders, indicat-
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bccnpushed s ofteu leave trails.or a
marks whcre the ground creatilg I
a small bcrm of their basal edges' r
UDdistu$ed roddall boulders do not sbow fresh gouges or
scrapes, hawconsistentlichen and mossgrowthpanerng do uot I
shoursoil discolorationsontheirsides or tops, aad are often sur- l
rounded by young bushes, aspcn trces, or natural vcgetatioq
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torest in their clitrs
Factons Used ,"tt-"' I
of Major RocJdrlt E\rcDts
Certain characteristics cfribited by uadisturbea
'ocfAaU I
bouldcrs and slabs iu tbe nmout zone, srggest approxinate or
rclative time spals sincc theY
a roug! csimate of thc
failure cvpnts. The contad madc
suggests how loug the roc& has
tioa- As tbe length of timc
into thc ground, ald sloPe vasb'
will aa t6 fll h arormd the base of the roc'k with soil materials'
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dircdty a*.
trecs th .*t tu*l
bencarh thc cdgcs of such a roc.k
Older rocks also harc more consistent lichen grovnh pancns
tbal receotly noncd rocts rvhich bave dctachcd from the-"lttrl
Recentlynond rocls maypossess difrerentially weatlered sur- |
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dismloration and crcatc a aew uaiform srrfai:e color on theroch l
Distribution of Rocldall Evcnts
F-xaminatiot of the sourc€ area and rurout zorre rcwtlt th"l
two basic typcs of rocldall cncns tate place in thc study area'r
Thc 6rst atrd Eost conmoq involvcs soaller iadividual boulders
lcoerAty in the (05 x 1 n) size range yhi$-detach fr-o{
i"an""t"tyU"a" aad eventu lv fall from the clilfs These falfl
commonlyinrchrc seraral boulders, many of which are set ia ne
tion afteibcing struct by thc initial falling rock * O*
|
o
Boulders
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rninor rockfall is common, and bascd ou crqunination of thc
runout zoqe and cliffs aborrc, can be cxp€ded to occur eecry otre
to thee years. This is the tlpe of rockfall which occurred in the
reported cvents ofMay 1983, January 1986, ard September 198i/,
damaging several structures. Many rockfall eyents go uD-
reported unless eignifiqat d^megc to structures occrrrs.
The second type of rocldall is much less frequent, bu of far
greater danger and destructive potcatial It involraes massivc slab
failures of the clifi faccs, alongjoints u,hich libcrate largc (a5 x
6 n) slabs aad (L5 x 15 m) limcstoae boulders, shocrerhg then
onto the acoeleration slopes bclw. Tbe noc roclfall of this nag-
uitude will drnost certainly result in extensive dam.ge ot
d*truction to structure in thc nrnout zonc bclos.
An imprecise prclininary qstimatc of rccorrencc intcruls for
thcse large slabfailurc cvcnts, bascd on cxamination of the
source arca and undisturbcd roddall boulders in thc nnout
zone, is ou thc order of ,10 to m
'ears.
Iarge boulders sct in
motion during thesc cnents can travcl tbrougb tbc ruaout zoe
as far as the naximum probable linit" An cstimatc of the last oc-
omcoce of this tlpe of crrcnt, bascd on the freshcst, undisturbcd
roctfall bouldcr in thc nraout zonq and weathcring pancrls on
15s clifrc, is on the ordcr of40 to 60 years ago.
Potcntial Solutions to Roddall Bazards
the feasibility of protcctiw structures and o,thcr prarcntivc
measurcs wcre evaluatcd during thc study.
Snaller boulders comno,nly falling off thc lwrr difi could
probably be arcstcd by protccliw structurcs built mr the
lowcr acceleration zooc (m propcrty withi! tlc planal srb-
divisioo The stnrcurcs mu* be capable of abaorbing thc cncr-
gics of one tonbouldcrs tmrelilgat 50 nph, andwldprobab-
ly involve energr abcorbing naterials held within timber o roclt
critSing Maintcnarcsof thesructureswouldbcaccessarycach
time a boulder is so'ppcd, since the encrg dissipation will
rlrrnage or deform that part of tbc structure invohtEd. It is
probably not feasible to build an arnoring wall or othcr typc of
sructure which attcnpts to arr€st the boulden throng! dgid
strengt\ due to thc cxtrenely high momentun rocls gain
througb tbe acccleration zone. Tte unpredictablc patbs ald pat-
terns followed by rocts skincring down slope matcs it di$crilt
to determhe the bcst places to sitc tle protective structures
One approach would be to coDstruct individual protective struc-
tures for """5 6uilding within the nrnout zone. Alternatively, a
<ingle large structure above thc subdivision might provide as
much protection and create lcss overall disturbance to the area.
The structure would have to bc carefully desigped ard con-
stnrcted to be fr"6 iftrining aad to prcrreEt adrrcrse snow or ice
accuhulations from forming above the protcaive barrier. Siting
a community type protective structurc appears to bc feasiblc if
bascd on tbe detailed siting studies which would bc lccessary to
detcrmine thc most suitable location In cither casc, csts for
thesc structurcs are estimated to bc on the order of 0.75 to one
milion do[ars, and could bc higber. Unfortruately' thcsc struc'
turcs would do little to prevcnt largcr bouldcn or slabs dcrirrcd
throWh toppliag failures from dcstroying structurcs in the
rurout zonc. Tbc cneryics posscsscd by such slaba or boulders
arc simply too great to contain within the rcstricted spacc avzil'
able betwccn thc source arcas and cxising residenccs
Rnrnnnncns
Mcarg A.L, 19?9, Colorado snos-aralarche area studics and
guidclines for analanchc-hazard plaaniag: Colorado
Gcological SurvEySpccial Publicatiou 7, f,14 p.
Robinson, CS, aad Associatc'sr Gcological CoDsultants' 1915,
Geologic hazard maps for cwironmcotal and land'usc plaa-
ning Eagle Couaty, Colorado.
Rogcrg W.P, et al- ilta, Guidclincs and criteria for i&utfi-
cation and laad-use conrols of gcologic hazard and mincral
resourcc arcas Colorado Geological Suwey Special
Publicatiou6, 146p.
Shelto'n,D.C, 194, Rocldalt rariables which determire the
hazard Unpublished reporg Oolorado Gcotogical Sur€'y
Geologic hazard filc'g De,nver, C.olorado.
Tvetq Ogden, ard Invcring TS, lfi, Geologtof the Min-
turn l5minute Quadranglc, Eaglc and Summit Couaties,
C-olorado: US. Gcologicat Sunrty Professional Papcr 956,
96 p.
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APPENDIX B
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O Ro.KFALL l\trrrcATrt
Jonathan L. White
Colorado Geologicd Survey
INTRODUCIION
Rockfall is a geologic hazard that is catastrophic
iD nature. For the most part it is viewed as a nui-
sance by highway maintenance personnel wbo
are rcquired to clean the debris off the roadway
and periodically clean out the fallen rocks with-
in the roadside ditches. Wben rockf,all occurs in
populated areas or areas frequented by people,
lethal accidents can occur.
In general, roclfall occurs where there is.
source of rock and a slope. Within the rock
mass, discontinuities (bedding planes, joints,
fractures, etc.) are locations where rock is prone
to move, and ultimately, fail. Depending on the
spatial orientation of these planes of weakness,
failures occur when the driving forces, those
forces that cause movement, exceed the resisting
forces. The slope must have a gradient steep
enough that rocks, once detached from bedrock,
catr move and accelerate down the slope by slid-
ing, falling, rolling, and/or bouncing. Where the
frequency of natural rockfall events are consid-
ered unacceptable for an area of proposed or
current use, and avoidance is not an option,
there are techniques of mitigation that are avail-
able to either reduce rockfall rates and Prevent
rocks from falling, or to protect strucftres or
areas of use from the threat.
There have been important techaologicd
advancements in rocldall analysis and mitigation
techniques in the last several years. They
include rocldall sirnulation software, rock
mechanics softwarc, aod rcsearch and develop
ment in new, innovative mitigation techniques.
This paper emphasizes mitigation techniques.
. Therc are. many factols that influence a
selection and design of a mitigation system to
reduce or eliminate a rocKall hazard. They
include:
I . The rock source (lithology, strength, struc-
ture, and weatherability) and expected re-
sultant fallen rock geometry (size and shaPe);
2. Stope geometry (topography);
3. Slope material characteristics (slope surface
roughness, softness, whether vegetated or
basen);
4. Proximity of the structure requiring Protec-
tion to source area and rocldall nrn-out zone;
5. Level of required rockfall protection (the
acceptable degree of risk);
6. Cost of the various mitigation options (con-
struction, project management, and design);
7. Constnrctability (mobilization dfficulties,
eEripment access, and other constraints);
8. Future maintenance costs.
For any public or private land use proposal,
in steep sloping areas, the geologic hazard
investigation should initially recognize those
physical factors listed above. If rockfall has
been identified as a hazard then a detailed rock-
fall hazard analysis is warranted. The conclusion
of such analyses, in addition to the determina-
tion of the factors above, must include:
1. An accurate dercrmination of anticipated
risk and frequency of rocldall at the loca-
tion of the proposed land use, and;
2. Site specific calculations of the velocities,
bounding heights, and impact forces for the
range of anticipated rockfall events.
Once all physical characteristics and calcu-
lated falling rock dynamics are determined then
the appropriate engineering and desigl can be
completed for mitigation of the rocKall threat-
ROCKRALL MITIGATION
TECHNIQUES
The available techniques in effective prevention
and mitigation of roclfall, fall into two cate-
gories. One is stabilization of the rock mass at
the source to prevent or rcduce roclfall occur-
rcnces. The other is the acceptrttss that haz-
ardouS rocKall will occur, but wi$ the place-
ment of protective devices to shield structures,
or public areas, from the threat of impact. There
is a third category that, while not a form of miti-
gation, is a method that can diminish the cata-
strophic nature of rockfall. It is rocKall waming
and instrumentation systems. Systems, electrical
and mechanical, that either will indicate that a
rocKall event is imminent, or has just occurred.
Stabilization and Reinforcement
Techniques that require in-situ or surficial treat-
ments of the slope to induce additional snbility
to the exposed rock mass are termed rock and/or
slope stabilization and rcinforcement. Stabiliza-
tion can be accomplished by any combination of
the following: removing unstable rock features,
reducing the driving forces that contribute to
instability and ultirnate failure, and./or incrcasing
the resisting forces (friction or shear strength).
1. ge-ling (hand scaling, mechanical scal-
ing, and fin blasting)- 5saling is the
removal of loose and potentially unstable
rock from a slope. On slopes of poor rock
conditions scaling is generally viewed as a
continual mainteuance procedure because
the loose rock removed exposes the rock
underneath to furtber weathering'
2. Reduce slope grade. Layiag a slope back
can prevent rocks from falling from a
source area
3. Dewater or dnin rock slope to reduce
water pore pressures. The installation of
drainage holes in rock can reduce the pore
pressrue in rock fractures-{ne of the dri-
ving forces mentioned above.
4. Rock dowels. Rock dowels are steel rods
that are grouted in holes drilled in rock'
generally across ajoint or fracture in the
rock of unfavorable orientation- It is a pas-
sive system in which loading or stressing of
the dowel occurs ooly if the rock moves
(slides) along thejoint plane. (See Figure
r.)
5. Roc.kbolts Rockbolts are installed much
like dowels but are usually loaded or
stressed, which imparts a compressive force
on the rock. The loading of the steel rod
during the installation increases the shear
strength of the joint or fracturc and pre-
vents movement, reinforcing the exposed
rock mass. There are wide varieties of rock-
bolts, including mechanical, groute4 and
binary epoxy resin systems.
6. St€el strapping. Steel smPPing, also called
mine strapping, is a strip of steel that
bridges between offset rockbolts or dowels
to supPort the rock mass between them.
7. Anchored wire mesh or cable nets. Fence
wire or, depending on loading criteria,
cable nets are draped on a rock slope and
anchored to the rock mass by the bearing
plates ofrock dowels or rock bolts. The
anchor pattem is set so that the wire mesh
or cable nets are.in continuous contact with
the rock face so that there is complete con-
finement of the loose rock material. (See
Figure 2.)
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Fipre 2. Anchored mesh or nets.
Figure 1. Rockbbtb ad dowels"
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8. Shotcret€. Shotcrete is the sp'rayed applica-
tion by compressed air of concrete on rock
or rocky soil slopes forreinforcement and
coDrrinrnent. Shotcrcre applications can be
stnengthened by the addition of nylon or
steel fibers to the coDcrete mixture, or the
placement of a wire grid on the rock slope
pnor to application. Weep holes are usually
drilled into the shotcrete to ensure that the
contained material is ftee draining. (See
Figure 3.)
Figure 3. Shotcretc.
9. Buttresses. Butte.sses are used wherc over-
hanging or undermined rock features
become potentially unstable and re4uire
passive restainL Brttesses can be con-
stnrcted from many qaes of marcrial- For
coDcrete buttresses, rock dowels are gener-
ally installed into surrounding comPetent
rock to anchor the buttess in place. (See
Figure 4.)
l0.Cable lashings. Cable lashilg is the wrap-
ping ofhigh capacity cables around a
potentially unstable rock feature. The
cables are then attached to anchors (rock
dowels) installed in adjacent competent
rock. (See Figure 5.)
ll.Ground Anctrors. Ground anchors are
generally used to prevent large, potential
landslide-type failures in heavily weathered,
fractured rock and rocky soils. Their
installation requires ftg ddlling of deep
holes and the gouting of thick bundles of
high-strength wire stan4 which are attached
to large load-bearing panels and then shessed
(pulled) to a desired tc,nsional load and
locked off.
Figure 5. Cable lashing.
Rocldall hotection Devices
When stabilization of rock slopes is not practical
aod suffrcient room exists, protective devices or
struchrres can be constnrcted to shield areas from
roclf,all impact
1. Fences. RocKall fences come in a variety of
sryles and capacities. They tend to become
less effective and are damaged if not
destoyed by larger rocKall events. (See
Figure 6.)
Figure 4. Anc.horcd concrete buttresg
Eartben berms. Berms are elongarcd
mormds of fill, commonly used in associa-
tion with dirches to increase the effective
height and catchment of the protection
device. (See Figurc 7.)
Hangingfences, nets, and other attenua-
tion devices. h welldefined rocldall chutes
in steeper rock slope areas it is possible to
anchor cables to span the chute and hang
fence mesh, cable nening, or rock asenua-
tion elements. Rocks that roll and bouncc
down the chutc impact these devices, which
attenuates (reduces) the rock velocity. (See
Figure 9.)
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Ftgure 6. RocKall fcnce.
2. Ditctres. Dirches excavated into slopes can
provide excellent rocKall protection. Care is
needed in analysis and desiga to insurc that
bounding rocks cannot span the ditch width.
(See Figure 7.)
3. lmpaci barriers and walls. Impact barrier
and walls can be made from many types of
materid, from fill mechanically stabilized by
geotextiles, rock gabion baskets, timber,
steel, concrete, or even haybales. Higbway
departnenB comnonly use Jersey barriers
on roadsides to cotrtain smaller falling rock
in the ditch. The inertial systems, able to
absorb the forces of momenntm of the mov-
ing rock, have higher capacities, without
costly impact darnage, compared to more
rigid systems. (See Figure 8-)
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Figure 8. Mechanically stabilized bacHll barrier.
re@\
Figure 7. Rocldal ditch anil bero.
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Figure 9. Tire impact attenuator.
6. Draped mesh or netting. Draped mesh is
similar to the stabilization technique
androred mesh but is only attached to the
rock slope at the top. Rocks from the slope
are still able to fail but the mesh drape keeps
the rock fragment next to the slope where
they safety "dribble" out below to a catch-
rJnt ditch or accumulate as small denital
fans. (See Figure 10.)
Figurc 10. Draped mesh.
7. Rocksbeds and tunnels. Rock sheds and
tunnels are mentioned here only because
they are used mostly for transPortation corri-
dors. They have little or no application in
most types of land use.
AVOIDANCE-
THE lOO PERCENT SOLUTION
Therc is one more mitigation method that is nei-
ther a stabilization/reinforcement system nor pro-
tection system. It is strongly recornrnended at
locations where rocKall hazards ,re very severe,
and/or risks very high. Mitigation designs pre
posed in zuch areas may not afford the necessary
level of protection. Bear in mind that no rockfall
mitigation is 10O percent guaranteed, even in
mild rockfall hazard zones. Avoidance is excel-
lent mitigation and must be considercd where cir-
cu.mstances warrant. Any professional in rocldall
analysis and mitigation (as with any geologic
hazard) must, at times, inform developers, plan-
ners, and the public that a proposed land use is
incompatible with the site conditions.
SUGGESTED READING
Federal Highway Administration, 1989, Rock
slopes: design, excavation" and stabilization:
Rrblication FIIWA-TS-89-045, pepared by
Golder and Associates, Seaule, Washington,
funded by the Federal Highway Adminis-
uation, U.S. Departrnent of Transportation:
Mclran, Virginia Research, Development,
and Technology, Turner-Fairbank Highway
Research Centea [373] P.
Federal Highway Administration, 1 994, Rockfall
hazard mitigation methods, particiPant work-
book hrblication FTIWA-SA-93-085, pre-
pared for the Federal Highivay Administra-
tion, U.S. Deparunent of Transportation
Publication by SM International Resources,
Inc.: Washington, D.C., National Highway
Institute ffiI Course 13219), [357] p.
Hambley, D.F., ed., 1991, Association of
Engineering Geologists, 34th annual meet-
ing, Chicago, Illinois, Sept. 29-Oct. 4, 1991,
Proceedings, national symposiur4 highway
and railroad sloPe maintenance: Association
of Engineering Geologists, 1 80 p.
Hoek" EveG and Bray, John' 1981, Rock slope
angineerhg, (rev.3rd ed.): London, U.K., The
Institution of Mining and Metallurgy, 358 p.
Pfeiffer, T.J., et al., 1995, Colorado rocKall simu-
lation program, ve-rsion 3.0a: Colorado
Depafirnent of Transportation hrblication
CDOT:DTD-ED3-CSM-89-28. Available
from: Colorado Geological Survey Miscell-
aneous Inforsration Series 39, diskette, 60 p.
tOF DOSTAfE
COLORADO CEOLOGICAT SURVEY
Division of Minerals and Ceology
DeDartment of Natural Resources
1313 Sherman Street, Room 715
Denver, Colorado 80203
Phone: (303) 866-2611
FAX: (303) 866-246l
March12,2002
Mr. Russell Forrest
Senior Enviroffnental Planner
Town of Vail
75 South Frontage Road
Vail. CO 81657
-stJ-98-O004
-i t'.. 5 ^ 7' ,.',ir1
DEPARTMENTOFNAIURAI
RESOTIRCES
Bill Owens
Governor
Greg E. Walcher
Executive Director
Michael B. Long
Division Director
Vicki Cowart
State Ceolotist
and Director
RE: Review of Rockfall Mitigation for Booth Falls Condominiums.
Dear Russ:
The CGS was requested by you to provide some additional comments on the completed
rockfall mitigation at the Booth Creek Condominiums in the Town of Vail. At your earlier
request, I inspected the rockfall mitigation structures on October 22,2001after construction was
completed last fall and sent cornments to you in a letter dated November 9, 2001.
A question arose concerning any potential impacts to adjacent owners from the
construction of the inertial banier walls designed for rockfall impact. During my site inspection
last fall I did not note any way in which these structures would adversely impact adjacent
owners, except for a remote possibility to the access road to the Town water tank. There should
be sufficient room to stockpile the snow against the foot of the westem wall if the water tank
road needs plowing for access during the winter.
Also the issue of maintenance and inspection of the structures was raised. The
mechanically stabilized earth impact walls are basically maintenance-free. One concem I raised
last fall was potential for sloughing or slumping of soil into the catchment zone from the bare cut
slopes. If not cleaned out, the soil accumulation could effectively reduce the wall height. The
cut slopes behind the walls (re-vegetated and stabilized as recommended) should be inspected
every spring or after an unusually heavy precipitation event. The ba:rier walls should also be
inspected after any rockfall impacts. Crushed portions of the wall facing after impact should be
quickly repaired. Yenter Companies can provide guidance on recommended repair techniques
for the wall facing.
The only other type offailure ofthe system that could arise is a bearing failure ofthe
native soils that the impact barrier wall is founded on. If tilting or sagging of portions of the
walls is observed, the homeowner's association should inform Yenter Companies and require
their staff to inspect the structure. Slight undulations along the length of the walls by differential
settlement will not effect the performance of the structures. While an unlikely scenario, adverse
tilting of the structures could be more problematic.
Inspection of the walls and catchment zone behind should be part of a normal
maintenance item of the condominium grounds by the homeowners association. I do not believe
this action needs to be conducted by city staffunless distress ofthe wall parallel to the water tank
access road is observed, which could possibly affect the roadway. Again, I believe it is very
unlikely that this would occur.
Enclosed with this letter is a copy of the original rockfall assessment report the CGS
prepared after the March26,1997 rockfall event. If you have any questions, please contact this
office at (303) 866-3551 or e-mail: ionathary/h{e@state.co.us
Sincerely,
Jonathan L. White
Engineering Geologist