HomeMy WebLinkAboutReview of Potential Rockfall Hazard 07.20.20K+rf åï'r1l.*trf;ißffniliii.*"5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 94s-8454
email: kaglenwood@kumarusa.oom
An Employeo Ownsd Compcny wwwkumarusa.com
Ofüce Locations: Denvø (tIQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, ând Sünmit County, Colorado
Jttúy 20,2020
Cindy and Royal Stillman
580 JB Court
Glenwood Springs, Colorado 81601
clstilly@smail.com
Project No. 19-7-121.01
Subject: Review of Potential Rockfall Hazard,Proposed Auxiliary Dwelling Unit, 580 JB
Court, Garfield County, Colorado
Dear Mrs. and Mr. Stillman:
As requested Kumar & Associates performed a review of the potential rockfall hazaú, at the
subject site. Our comments and recommendations are presented in this report. The review is
supplemental services to our agreement for geotechnical engineering services to you dated
January 25,2019. We previously conducted a subsoil study for foundation design of the
proposed auxiliary dwelling unit (ADU) at the site and presented our findings in a report dated
May 8, 2019, Project No. 19-7-121.
Proposed Construction: The ADU \Mill be a single story wood frame structure looated on the
site roughly in the area shown on Figure l. Grading for the building pad will be relatively minor
with cut depths expected to range between about 3 to 5 feet. Foundation loadings for this type of
construction are assumed to be relatively light and typical of the proposed type ıf construction.
When building location and grading information have been deveþed, we júould be notiflred to
review the recommendations presented in this report.
Site Conditions: The natural terrain at the site slopes down to the west at about 20yo grade
below steep hillside further to the east. Canyon Creek is to the west. The building site is vacant
and has been graded with cuts and fïlls to create the relatively flat building pad, sJe nigure Z.
Maroon Formation bedrock spines and fins outcrop on the steep hillside to ihe east and northeast
of the site. The building area has been cleared of vegetation.
There is a near-vertical cut up to about I feet high on the uphill, eastern side ofthe graded
building pad. Several large irregular shaped angular boulders, up to about 4-feet in size, were
observed on the natural, moderately steep slope above the cut faıe beginning about 4 feet from
the top of the cut.
A fïn of the Maroon Formation bedrock is located about 150-feet upslope of the northern portion
of graded building pad. The bedrock in this area appeaxs to be moderately weathered wittl three
plus random joint sets of 2 to 4 foot spacing with no infilling.
Vegetation on the moderately steep slope between the bedrock outcrop and the proposed
building area consists of low native grass and weeds, with scrub oak ánd scauered iinyon pine
trees.
Potential Rockfall H¡zard Assessment: Based on or¡r field observations and experience in the
a{ea, the risk of potential rockfall impact to the proposed ADU residence appears relatively low.
The risk is higher in the northern portion of the building pad and lower in tlhã southern portion.
-2-
The boulders located near the top of the building pad cut slope are partially buried on the uphill
side and have mature lichen growth (about 3-4 inches in diameter). It does not appear lhat a
recent rockfall has occurred at ttre site, however, the building pad grading may have removed
any signs of recent rockfall impacts. Although the probability that a rockfall will impact the
residence site is likely low, a rockfall has the potential to cause damage to the building with
possible risk of harm to the occupants. Moving the building site to the south (right side of top
picture on Figure 2), further away from the bedrock fin outuop on the slope and below the
thicker vegetation above the graded building pad, would help to reduce the risk of potential
rockfall impact to the proposed residence.
Keeping the uphill wall of ths residence at least 10 feet west of the bottom of the building pad
cut will also fr¡rther reduce the risk of potential rockfall impaot to the proposed residence by
creating a small 'catchment area' fbr potential rockfall. If practical, extending concrete
foundation walls approximately 5 feet above grourrd surface and rninimizingthe number of
windows and their size on the east side of the building will provide some mitigation if the
structure were to be impacted by a small boulder. If the residence is placed against the uphill
building pad cut, mitigation to reduce the rockfall risk such as constructing a berm across the
slope uphill of the building should be evaluated.
Limitations: This review was conducted according to generally accepted geotechnical
engineering principles and practices in this area at this time. We make no warranty either
express or implied. The conclusions and recommendations submitted in this report are based
upon our field observations and c¡ur experience in the area. We are not responsible for technical
interpretation of our findings by others. 'We should review the selected building location ancl
grading plans for additional recommendations as needed.
If you have any questions or if we rnay be of further assistante: please let us know.
Respectfully Submitted,
X(t*l*wnt & hs*o*ist*s, Ttzt,
W 6t\
Robert L. Duran, P. E.
Reviewed by:
David A. Young,
RLDlkac
attachments
Figure and Boulders
Cc: Kurtz Engineering - Brian Kurtz - (hyf-1X;3lg*ççt:!i*Xf:Z]:çl:-,1:nf:)
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T-1"-t^Lts
Kum¡¡ & Aseociatûs, lnc. ir Project No. 19"7 -121.41
580 JB Courl
APPROXIMATE SCALE_FEET
l9-7 -121.0 Kumar & Associates SITE PLAN Fig. 1
9-7-121.0 Kumar & Associates VIEW OF SLOPE AND BOULDERS Fig. 2