HomeMy WebLinkAboutRockfall Hazard Analysis 07.02.2026Ir CTL THOMP ON
July 2, 2026
IRMW, LLC
P.O. Box 1037
Carbondale, CO 81623
Attention: Don Vandevander
Subject: Rockfall Hazard Analysis
Parcel 1, Lot 1A, Building 1
1058 County Road 100
Garfield County, Colorado
Project No. GS07130.000-130-L1
CTLIThompson, Inc. (CTLIT) has completed a rockfall hazard analysis for Parcel 1, Lot
1A, Building 1, located at 1058 County Road 100 in Garfield County, Colorado. A commercial
storage building is proposed for the site. The project site is adjacent to a near -vertical,
excavation face at the base of a steep slope. Rockfall from bedrock outcrops higher on the
slope is a potential hazard for the property. Results from our rockfall hazard analysis include
estimates of rock velocity, bounce height, and kinetic energy. The scope of our rockfall hazard
analysis was set forth in our Proposal No. GS-26-0104.
Site Reconnaissance
James Parker, P.E., P.G., of CTLIT performed a site reconnaissance on June 12, 2026.
The reconnaissance included walking the slope above the proposed building site to observe the
slope configuration, surface conditions and locations of rock outcrops on the slope. Parcel 1, Lot
1A, Building 1, is located south of County Road 100, approximately 1 mile east of Carbondale in
Garfield County, Colorado. A vicinity map with the location of the site is attached as Figure 1.
The proposed building site is in the north -central portion of a 57.8-acre parcel. The
location is on a cut -fill bench at the base of the north -facing slope at the south side of the
proposed building location. The near -vertical excavation face is approximately 25-foot high. An
unpaved access road is north of the building location. Photographs of the site are included in
the attached Appendix A.
Slope Configuration and Rockfall Sources
The natural slope above the near -vertical, cut -slope is relatively steep. The overall slope
grade above the site is approximately 60 percent (31 degrees) in the upper two-thirds of the
steep slope, and about 40 percent (22 degrees) in the lower one-third of the slope. The overall
slope topography is incised with a broad drainage that funnels down toward the building site.
Vegetation is predominantly sage and grasses with pinyon and juniper trees, and thickets of oak
brush.
Based on our recommendations, the client will excavate the near -vertical excavation
face to an approximately 1 horizontal to 1 vertical (1 H to 1 V) slope prior to construction of the
building. This proposed geometry was used for our rockfall analysis model.
CTLIThompson, Inc.
Denver, Fort Collins, Colorado Springs, Glenwood Springs, Montrose, Pueblo, Summit Count; — Colorado
Chevenne, Wyoming and Bozeman, Montana
Geologic mapping by the U.S. Geological Survey (USGS) titled, Geologic Map of the
Carbondale Quadrangle, Garfield County, Colorado, by R.M. Kirkham and B.L. Widman (dated
2008) indicate the slope above (south) the subject site is predominantly composed of Eagle
Valley Evaporite in the upper portion of the slope and quaternary colluvium in the lower portion.
A younger quaternary debris fan is mapped at the mouth of the drainage above the site,
overlying the proposed building footprint. The relatively gentle slope above the steeper slope is
mapped as miocene sedimentary deposits, composed of fluvial sand, gravel, and cobbles.
Numerous cobble -size rocks originating from the fluvial deposits were observed on the slope
above the building site. The Eagle Valley Evaporite is exposed as outcrops in the central,
eastern and western portion of the slope. These outcrops are the source of potential rockfall
boulders that could affect the subject site.
Tvaical Rockfall Boulders
We noted several boulders on the lower slope resulting from previous rockfall. The
boulders were generally about 2 to 4 feet in diameter and sub -angular to sub -rounded in shape.
The boulders were partially buried and did not appear to have been due to recent rockfall
events. Large boulders derived from rockfall were not noted in the excavation cut -face.
Photographs of rockfall boulders encountered at the site are shown in the attached Photo Log.
Rockfall Analysis Approach
CTLIT evaluated the potential rockfall hazard for Parcel 1, Lot 1A, Building 1, using the
Colorado Rockfall Simulation Program (CRSP) developed by the Colorado School of Mines
(2004). We analyzed three slope sections (A -A' through C-C') shown on the attached Figure 3,
to represent the three primary rock outcrops observed during our site visit. The program
requires input regarding slope geometry and slope material properties, as well as coefficients to
determine rock energy loss upon impact. It is also important to estimate the size, shape, and
weight of rocks that would comprise a potential rockfall event.
Our site observations indicate the rockfall boulders are generally sub -rounded to sub -
angular in shape. Due to the constraints of CRSP, we analyzed the rocks as spherical boulders.
This is conservative because a spherical shape represents the "worst -case" because a sphere
comprises the most mass for a given radius. Our analysis assumed a rock unit weight of density
of 165 Ib/ft3, which is a conservative estimate for the evaporite bedrock. We evaluated rocks
with diameters of 4 ft, 6 ft, and 8 ft. The diameters of the rocks used for analysis were
determined by our site observations and our experience with similar rock types and slope
configurations.
The CRSP program simulates "rolling" of rocks of the specified size and shape down the
modeled slope geometries at each section. Parameters that must be entered for the slope
sections include estimates of surface roughness, as well as tangential and normal coefficients,
which allow estimates of rock trajectory and energy loss upon impacts as the rocks bounce
while rolling down the slopes. The analysis ultimately predicts the number of rocks that would
pass designated analysis points. It also provides estimates of rock velocities, bounce heights,
and kinetic energy of the rocks at these points.
CTLIT set two analysis points for each of our three slope sections. We modeled a
potential rockfall mitigation structure consisting of a 6-foot high earthen berm at the south
(uphill) terminus of the planned 1 horizontal to 1 vertical (1 H to 1 V) layback of the toe of the
IRMW, LLC Page 2 of 4
PARCEL 1, LOT 1A, BUILDING 1
CTLIT PROJECT NO. GS07130.000-130-L1
slope. We modeled the berm with a base width of 15 feet such that the upslope and downslope
faces of the berm would have slopes of 1 H to 1 V with a 3-foot wide bench on the top of the
berm. Analysis Point 1 for each section was set at the base of the upslope (south) face of the
berm. Analysis Point 2 was set below the base of the downslope face of the berm, between the
berm and the proposed building location.
Rockfall Analysis Results
CTLIT utilized the CRSP program to simulate rolling of 100 rocks down each of the three
modeled slope sections. Our analyses assumed rocks starting at a velocity of 1 ft/sec upon
separation from the source rock at the outcrops. Results of our analyses are summarized on the
attached Table 1
The first data set for each slope section indicates the rock velocities, bounce heights,
and kinetic energy at Analysis Point 1. The second data set is for Analysis Point 2. As indicated
on the Table, a significant number of rocks were predicted to reach the base of the rockfall berm
below slope sections A -A', B-B', and C-C'. The analysis showed none of these rocks passing
the berm and reaching Analysis Point 2 for any modeled rock diameter.
Opinions and Recommendations
Based on the analysis results, the highest rockfall risk to the proposed building location
on Parcel 1, Lot 1A, Building 1 is within the central and east areas. The greatest rock velocities,
bounce heights, and kinetic energy would be below slope section A -A' in the western portion of
the slope. Mitigation of rockfall hazard is required for safe development of the subject parcel.
Our analysis indicates that a 64eet high earthen berm, constructed at the crest of the
laid-back excavation face. The upslope and downslope faces of the berm at 1 H to 1 V, would
prevent rocks from traveling past the berm and into the building envelope. Other mitigation
systems can be considered, provided they are capable of absorbing the calculated maximum
kinetic energies summarized on Table 1. We can provide additional assistance to the design -
build contractor, if requested.
Limitations
The analysis methods, opinions, and recommendations in this letter are based upon
consideration of many factors including, but not limited to, site topography, geologic conditions,
and the proposed site development. We believe our analysis was performed in a manner
consistent with that level of care and skill ordinarily used by geotechnical engineers practicing in
this area at this time. No warranty, express or implied, is made.
CTLIT is available to discuss the contents of this letter. Please contact us if you have
questions or need additional information.
IRMW, LLC Page 3 of 4
PARCEL 1, LOT 1A, BUILDING 1
CTLIT PROJECT NO. GS07130.000-130-L1
CTLITHOMPSON, VIC.
7-j02A6
James A. Parker, E., P.G.
Principal Engineer
iaparker@cifthompson.com
Attachments: Figure 1 —Vicinity Map
Figure 2 —Aerial Photograph
Figure 3 — Slope -Cross Sections
Table 1 — Results of Rockfall Analysis
Appendix A — Reconnaissance Photographs
Via email: don.vandevanderLamail.com
IRMW, LLC
PARCEL 1, LOT 1A, BUILDING 1
CTLIT PROJECT NO. GS07130.000-130-L1
Reviewed by:
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ames D. Kellogg
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Principal Ei
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APPENDIX A
RECONNAISSANCE PHOTOGRAPHS
IRMW, LLC
PARCEL 1, LOT 1A, BUILDING 1
CTLIT PROJECT NO. GS07130.000-130-L1
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Looking east across natural slope
Slope above the proposed building site
IRMW, LLC A-2
PARCEL 1, LOT 1A, BUILDING I
CTLIT PROJECT NO. GS07130.000-130-LI
r7, ff M
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Looking east across central part of slope above proposed building area
Rock outcrop on west flank of broad drainage
IRMW, LLC A-3
PARCEL 1, LOT 1A, BUILDING I
CTLIT PROJECT NO. GS07130.000-130-LI
Closer view of rock outcrop on west side of drainage
Looking cross -slope at rock outcrop on east flank of drainage
IRMW, LLC A-4
PARCEL 1, LOT 1A, BUILDING 1
CTLIT PROJECT NO. GS07130.000-130-L1
J
Closer view of rock outcrop on eastern flank of drainage
Boulders approximately 150-feet up -slope from proposed building location
IRMW, LLC A-5
PARCEL 1, LOT 1A, BUILDING 1
CTLIT PROJECT NO. GS07130.000-130-L1
Approximate 4-foot diameter boulder about 40 feet from cut -slope
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Fractured and weathered boulder above east portion of proposed building location
IRMW, LLC A-6
PARCEL 1, LOT 1A, BUILDING 1
CTLIT PROJECT NO. GS07130.000-130-L1
General view of site and slope
IRMW, LLC A-7
PARCEL 1, LOT 1A, BUILDING 1
CTLIT PROJECT NO. GS07130.000430-L1