HomeMy WebLinkAboutDRB120116 SUPPORTING DOCS.pdf Arthur I. Mears, P.E., Inc.
Natural Hazards Consultants
555 County Road 16
Gunnison, CO 81230
Tel/Fax: (970)641-3236
November 15, 2007
Mr. Brennen Scott Fitzgerald, AIA
Turnipseed Architects DRAFT
P.O. Box 3388
Eagle, CO 81631
RE: Site visit, The Valley Phase IV" rockfall mitigation, Lots 1 — 3.
Dear Brennen:
This letter, as requested, evaluates the rockfall hazard potential at building sites
1, 2 and 3, recommends mitigation, and discusses the feasibility of mitigation.
My conclusions are based on a site visit last Friday, evaluation of detailed
topographic maps and my experience with rockfall hazard evaluation and
mitigation at various Colorado locations.
1. Rockfall Hazard
All of Lots 1, 2, and 3 appear to be
included in a "high severity" rockfall
area on Town of Vail maps produced
- s - - in 1984. My site inspection last
t a week and experience with a rockfall-
hazard evaluation completed by me
in 1990' suggests that rockfall
hazard is not "severe" by Colorado
or general mountain standards; it
r~ should be classified instead as
"moderate."
Rockfall hazard originates in sandstone outcroppings of the Minturn formation.
This formation crops out roughly 150-300 feet vertically above the building sites
staked on the ground. The outcrops are small in aerial extent and have produced
some rockfall in the past. Figure 1 shows a typical example roughly 2 feet in
length found on the slope. Judging from the intersection of joints and bedding
plains in the rock and isolated rocks that have stopped on the slope, this is
'A.1. Mears, P.E., Inc., "Rockfall Hazard Analysis, The Valley, Phase IV, Vail, Colorado", prepared
for Ed Zneimer, June 25, 1990.
typical of the size of rock to be considered in mitigation design. According to the
A.I. Mears (1990) report, rockfall has occurred in the past several decades (prior
to 1990) and has reached the valley bottom. Because of the limited rockfall
source area, rockfall events will probably be relatively infrequent (one event per
decade or less) and will probably consist of only one or two boulders. Most will
stop on the slope above the building sites.
Because moderate rockfall hazard does exist, structural mitigation for rockfall is
recommended, as discussed below.
2. Rockfall Mitigation
Two mitigation techniques are feasible at these sites: (a) construction of barriers
at or directly above the proposed buildings, and (b) construction of small rockfall
energy-absorbing fences on private property below the source areas. Building
sites on lots 1, 2 and 3 should all be protected because rockfall paths may be
somewhat unpredictable.
Rockfall barriers at the buildings, while feasible from a technical perspective,
are probably not desirable. Building walls lined on the uphill sides with
deformable gabions or planter boxes could be made to absorb rockfall energy
and distribute impact forces over a large area, thus avoiding point loads from
rock impact and structural damage. The heights of the required structures at the
building have not been determined but should be based on the newly-available
Town of Vail GIS topography and updated rockfall modeling. Previous work
suggests the wall heights might be up to 6 feet at the proposed houses. While
this technique could be effective it may not protect the entire house because of
the locations of doors, and it may have an undesirable appearance.
The second feasible mitigation technique would be construction of small rockfall
energy-absorbing fences on the slope, a short distance below the source
areas. The design heights and strengths of the fences would be modest
because energy and bounce heights will also be modest. They would probably
be fairly short at each of the two rock outcroppings2.
As discussed at the start of this report, one objective has been to discuss the
feasibility of rockfall mitigation. Rockfall mitigation does appear to be feasible at
these sites, and should, in my opinion be assumed as such during the permitting
process.
Sincerely,
Arthur I. Mears, P.E.
2 As a rough estimate (not based on detailed analysis)fences may need to be no more than 50
feet in length at possibly two locations between the trail switchbacks but on private property.