HomeMy WebLinkAboutEngineer's Letter - Review of Rockfall Condition 9.15.2017H-PKUMAR
Geotechnical Engineering 1 Engineering Geology
Materials Testing 1 Environmental
5020 County Road 154
Glenwood Springs, CO 81601
Phone: (970) 945-7988
Fax: (970) 945-8454
Email: hpkglenwood@kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, Summit County, Colorado
September 15, 2017
Joe Koronkiewicz
P.O. Box 45
Silt, Colorado 81647
lamelody 03 vyahou.corn
Subject:
Dear Joe:
Project No. 17-7-233
Review of Rockfall Condition, Lot 80, Spring Ridge Reserve PUD, Phase 4,
Garfield County, Colorado
As requested by Leslie Koronkiewicz, the undersigned representative of H -P Kumar observed
the hillside above the proposed residence at the subject site on September 12, 2017, for potential
rockfall impacts to the proposed construction. We previously conducted a subsoil study for
design of foundations at the site and presented our findings in a report dated March 24, 2017,
Project No. 17-7-233. The current review was performed as additional services to our
Agreement for Services with you dated March 14, 2017.
Proposed Construction: We understand the proposed residence will be a one or two story,
wood frame structure, possibly with a walk -out basement level. The building will be located in
the southwestern, front portion of the building envelope, generally between the boring locations
as shown on Figure 1 of the referenced subsoil study report. Foundation excavations on the up-
slope side of the lot of up to about 10 to 12 feet may be necessary to accommodate a basement
level.
Project Site: The subject lot slopes up to the east-northeast at about 20 to 25 percent across the
building pad portion of the lot. The building pad area is bordered to the east by an abandoned,
east to northwest trending irrigation ditch with a small berm bordering the ditch on the uphill
side. Above the ditch, the terrain generally slopes up at about 20 percent with localized slopes of
up to about 40 percent near the crest of the slope. The slope is about 350 feet in length from the
abandoned irrigation ditch to the crest. The uphill berm above the ditch is generally about 2 to 4
feet in height. Surficial soils on the hillside above the proposed residence consist of relatively
shallow colluvium, comprised of silty, clayey sand with gravel, cobbles and small boulders,
overlying sandstone bedrock of the Maroon Formation. Scattered, mostly flat (tabular) shaped
rock fragments, typically up to about 1 foot in size, are exposed on the hillside above the
residence. An area of cobbles, larger boulders and the remnants of a bedrock outcrop occurs
near the crest of the ridge, with boulders ranging from 1 foot to several feet in size. The boulders
Joe Koronkiewicz
September 15, 2017
Page 2
are generally tabular in shape, and typically partially embedded in the overburden soil. Drainage
on the slope above the proposed residence appears to be by sheet flow.
Conclusions and Recommendations: Based on our observations and professional experience,
we conclude that the rockfall hazard to the proposed construction is low and rockfall mitigation
measures are not warranted.
Although low in probability, down-slope rock movement, if it occurs, will likely originate from
the steeper, rocky zone located near the crest of the slope. Cobbles and boulders in the rocky
zone are typically tabular in shape and not prone to down-slope movement. Scattered cobbles
and boulders with sub -angular to sub -rounded shapes were also observed, and will be more
prone to movement down-slope. Storm and seasonal events, resulting in erosion of surficial
soils, may cause instability of rock fragments on the hillside. The berm on the uphill side of the
irrigation ditch, as well as the ditch, should provide some measure of mitigation for smaller rocks
moving downslope, but the level of mitigation is indeterminate based on visual observations
alone. If the owner is concerned with the potential for down-slope rock movement, we
recommend periodic observation of the slope by the owner to identify rocks undermined by
erosion and prone to potential movement, and we should be contacted if further investigation is
desired at that time.
If alterations to the slope above the residence are planned in the future, we should be consulted to
review the planned alterations for potential rockfall hazard.
If you have any questions or need further assistance, please call our office.
Sincerely,
Ps KU MAIC
mes A. Parker, P.E., P.G.
Reviewed by:
Steven L. Pawlak,
7)1S)/
JAP/kac t.,
-Th....,; y,.'
AP
cc: Garfield County - 'man r- P velopment — Dave Argo (dargo@garfield-county.com)
H-P%KUMAR
Prosect No. 17-7-233