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ROCKF`ALL MITIGATION DESIGN SPECIFICATION3
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THE VALLEY, PHASE VI, YAIL� COLORADO
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Prepared For ( I
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Mr. Ed ��neimer
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Prepar�d By �
• - Arthur I. Mears, P.E. , �nc. �
' Gunn�son, Colorado ;
September, �990 ' r
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1 OBJECTIVEB AND LIMITATI�NS
This analysis of xockfal� mitigation techniques and perfarmance
specificatians af mitigation design was requested by Mr, Ed �
Zneimer and has the following objectives:
a. CalcuZation of design rockfa2l bounce heights, �
veloci�ies, and momentum at three locatians requiring
rockfall mitigation design; i
b. Calculatzons of the �ailure probabilities ot rock- �
fall defenses at various locations, and
c. Specificatian of the locations, sizes, and possible
forms of rockfall protection.
The conclusions and recammendations of this repqrt ar� site
specific, the�efvre they may not apply at other sites. Further-
mare, any substantial changes to b�ilding positions from those '
shown on Figure 5 of this report may invalidate the recommenda-
tions af this study.
Some of the conclusi�ns of this study and the application of the �
Colorado Rockfall Simulation Program (CRSP) depend on observa- '�
tions and field measurements made on June 21, 1990. These
observations were reported in the '�Rockfall Hazard Analysis, The
Va11ey, Phase V2, Vail, Golorado, " submitted to Mr. Ed Zneimer on
June 25, �990.
2 APPLICATION OF "CRSP" AND ROCKFALL DESIGN PARAMETERS '
i
Rockfall mitigation design requires information about rock size E
and mass, rockfall velocity, and rockfall bounce heights at the �
position of the mitigation device. These design parameters are �
determined by field observations and through application of the �
CRSP computer mod�l, a stocastic model that outputs a statistical
distribu�ion of rock behavior at positions alang the rockfall
path. D�sagn rock size, rockfa�.�. source l�cation, slope incZina-
t�.on, and ground hardness and roughness must be measured and
�stimated in order to apply. CRSP. Design rock size was deter- ,
mined to be a 2-foot diameter rack during the field inspect3on oF
June 21. During the field wark the rockfall source locations
were locatedr the slopes of the most likely rockfal.3, paths were �;
surveyed, and the ground roughness and hardness were estimated.
Historic (and pre-historic) rock�all runout distances were also
� mapped and used to ca].ibrate roughness and hardness. . ;
i
Three analysis pos�tions were considered for mitigation and used '
a.n the CRSP application: (A} a possible catching-fence locatian
60 feet above builda.ng envelopes 3 and 4, (B) the uphill walls of
buildings 3 and 4, and (C) a possible� ber�n location near the '
bottom of the valley. These positions are shown in Figure 5.
Detinitions of rockfa�l bounce heights and velocities are def ined �
di.agrammatically in Figure 1. �he statistical distribution of �
rockfall bounce heights and velocities at each of the three !
mitigation pvsa�tions are summarized on Figures 2 , 3 , and 4. The �
��
. � • ' . . • �
bounce height and v�loci�y� distributions are given as "excee-
dence" probabiZitzes, therefore the probabzlity that a randflm
roekfall event will exceed a given value has been shown on these
graphs. For example, the bounce-height graph on Figure 2 ind�-
cates a 25a probability that bounce he�gh�t will exceed 4 feet at
the fence ].ocation but only a 10°s probability that it will exceed
6. 5 feet. Velocity probabilities have been similarly determine�l
and grap�ed. The distributions shown res�alt from 1�0 simulated
2-foot dzameter rockfall events along the pa�hs determined in the
field survey. The computer prin�outs for the 100 rockfall
simulations at each analysis pasition are given in appendices A -
C, at the end of this repart. It must be emphasized that the
probabilities gi.v�n in Figures 2 , 3, and 4 apply to the 2-foot
d�.ameter "design" rock only. Smaller rocks will a�so roll down
the slope, but will attain lesser velocities and bounce heights;
many small rockfalls wi�.l stop on the upper slopes and not reach
the proposed building locations.
Rockfall design speGifications are based on the lflo exceedance
probabilities shown in Figures 2 , 3 , and 4 . This means that
there exis�.s orae chance in ten �hat the d�sign rockfall event
will exceed mitigation desic�n capaci�y. This is a reasonably
conservative approach because the design rockfall event is
exp�cted only once in several decades and when this rare event
does occur ther� is a 90o chance the mitigation will work. OF
course, an even grea�er level of protection can be achieved by
insisting on a smaZler exceedence pzobability.
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3 RQCRFALL MITI�ATION FOR BUII�DINGS 3 AND 4 ` J
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Two forms of external rockfall defenses have been cons�dered �or ' �
protection of these two building sites: (A) a "flexible-post" �''��� �`'Zt
fence, and (8) rockfall-wall barriers. -'
A FLEXIBLE-POST FENCE �,_L P�
The flexible-post fence coul�i be located as shoran by line A - A
on Figure 5, approximate�y 60 feet north ot the buildings. Fence
design parameters, based on the 10% exceedance probability are: �
bounce height - 6.5 feet; velacity = 37 ft/sec. Fence height,
therefore, must be 7 . 5 feet (measured verticaZly) , and is equal
to the bounce height (6. 5 feet) plus the rock radius (1. 0 feet) .
The fence will stap 2-foot diameter rocks or w.ill reduce their
speed so they will not cause damage.
The fence posts (at approximate�y 30-faot intervals) consist of a
laundle of cab3.es (similar to 3/8" ar 1/2" guy wires) encased in '
4" diame�er galvanized steel tubing (Figur� 6) . A section of the
cable bundle approximately 12" to 18" long is open and allowed to
flex upon impact. The rock momentum, therefare, is dissipated �
gradually as the fence bends downslope at impact, instantaneous
impact forces are reduced, and massive, expensive structures are
avoided.
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Although these fences are not in widespread use, pratotypes have
been tested extensively by the Colorado Highway Department and
have been very successful in stopping rockfall. Estimated cos�s
for a 7 . 5-high fence is $60 - $1(}0 per foot of fence length. A
330-foot long fence as shown in Figure 5 would cost approximately
$20, 000 to $33 , 000. The fences are not mass produced at the
present time. Rarrr materzals would need to be purchased and
ass�mbled at the site.
B ROCKI'ALL WALL �ARRIERS
Rockfall.-wall barriers would �liminate structural damage from the
design (10%-probabi�.ity} rockfall event if built on the uphill
sides ot buildings 3 and 4 (Figures 5 and 7} , These barriers
must be 6 . 5 fe�t high (5. 5-foot bounce height plus 1. 0-foot rock
radius) . They should con��_st af �teel �rames covered with coarse
wire mesh and filled with unconsolidated gravel ar�c� small rocks.
The structures should be approximately 2-te�t thick. Pre-fabri-
cated "Gabion" baskets, in commnn use along highways, could be
substituted. Similar to the flexible-post fences discussed
above, the rock momentum will be dissipated by the barrier and
damage to the wall eliminated.
Cons�ruction and ins�.allation of �he rockfall-wall barriers could
be done locally. Protection of buildings 3 and 4 would requi�e
an estimated 70 feet of length and a volume of 45 yd3 per build-
ing. Casts would depend upon local construction casts and the
cost and transportation of the unconsolidated grav�l fill materi-
a�.. As n�tec3 above, Gabian rock-filled baskets could be substi-
tuted. Loca1 cost for these baskets, which should maintain the
he�ght requirements, have not �een determined.
BUILDING-WALL REINFORCEMENT
A third altez-native form of protection for buildings 3 and 4 is
ta allow rockfal�. impact with the uphill building walls and
design internal wall structural members tv resist rock impact.
As in the above sys'tems "A" and '�B, '� the major construction
elements in the building (beams, cross-bracing, etc) , must absorb
rackfall momentum. Fo� flexible constxuctian, �tructural d�f�ec-
tion must be considered in computing th� impact �orce, P.
Equating the baulder kinetic energy with work expended in bending
deflec�ion yields the relationship
F = (M VZ K}: �5� (1) '
where M is boulder mass (691/32 .2 = 21. 5 lbs-ft/secZ} , V is
ve�.ocity (36 ft/sec at �.Oo probabzlity level [Figure 3] ) , and K
i.s a stifFness factars. For a simple beam,
K = 48 EI/L�, �2� �
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where EI is beam stiffness and L is beam length. The relation-
ships expressed in (Z) and (2) inc�icate that flexiblr� structural
members are more effiCient in resisting impact than stiff ones.
The actual expression for stiffness, K, wi11 probably be differ-
�nt than (2) , d�pending on structural-engineering de�ails. In
general, rockfall protection at buildings 3 and 4 m�ast
a. avaid windows wi�.hin the lawer 5.5 feet;
b. assure structur�l e�Ements can resist P; and
c, assure racks wiZl not penetrate walls betwe�n the beams.
The additional cast for reinforcing the building walls wi1.�
d�pend on s�ructural-engineering and archi�ectural details which
will b�come known in final design.
Of the 3 structural-mitigation options d�scussed in this section,
only the flexible-post fence wiZZ prevent rocks from reaching the
buildings. The ather two optians (rockfa�i-wall barriers and
building-waZl reinforcement) obviously allow rockfall impact with
the building and would also allow rocks to ro�l between the
builclings and acxoss Lions Ridge Loop. The resa.dual risk to
persons in the ar�a, th�e�efore, is larger with the latter two
mitigation opt�ons. Even this residual risk i� small, how�ver,
because �najor rockfalls are expected to occuz only once every few
years or �ess, thus the probability of encounter with a p�rson
who may be outside and exposed just when the rockfall vccurs is
very smal�.
4 ROCRFALL MITIGATIpN FOR BUILDZNGS 9 - 13
Buildings 9 - 13 are loca�ed across the flat valley �loor, near
the southern limit of rockfall po�.ential (�igure 5) . No rocks
whieh cauld be c�early identified with the source outcrop on the
hill could be found at the proposed building locations, and the
CRSP simula��.on indicated that onl�r 350 �f the rocks rolled as
far as the buildings. The simulated rocks that did reach the
buildings were rolling (not bounczng above the ground) , with
typzcal rolling velocities of approximately 20 ft/sec �� less
(Appendix C) . Roekfall events of this character could dent the
lower part of building siding, but would not �ndanger the struc-
ture or its occupants. Furthermore, suc� major events are
exgected on�.y once every decade or �onger.
�
Because roc}cfall �nergies will be small at �he mid--valley loca-
ti.an, there�ore the fill bank on the nc�rth side of �he proposed
subdiv�ision road, located directl.y north of Lots 9 - 13 , will
stop a�most all rocks. In my opinion, this raad .is sufficient
miti.gation.
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Additional str�ctural mi�igation for these rare, relatively low-
energy rockfall events is not recommended unless occupants of
these buildings demand camplete protec�ion from rockfall. If
such complete protectian is desired, it can be attained by
building a 3-foot high rackfall barrier near the center of the
vaZley, as located on Figure 5 and diagrammed on Figure 8 . The
rockfall baxrier shown on Figure 8 consists of a retaining wall
wi�h a vertical face toward the rockfall direction consisting of
railraad ties or simi�ar weather-treated timber of �arge cross
section. These ties should be braced on the uphill side w�th
steel fence posts driven into the ground at approximately 4-foot
intervals, and should be filled on the downh�ll side. This will
stop the 10%-probability rockfall event (Figure �) , which has a
bounce height of �. 2 �eet and a �elocity of 26 ft/sec.
5 SCALING: REMOVAL OF ROCKFALL SOURCE
This type of rockfall mitigation is sometimes used where obvious,
active rockfall source areas can be identified and removal of
rocks can c�early reduce the hazard. Removal of all the loose
rocks could b� completed abov� the prapased subdivisian, probably
within 10 - 15 man-days of work. Field observations of the
bedrock outcroppings and lower slopes indicate that most of the
loose rack couZd probably be remaved by blasting and/or prying
rocks loose and forcing them to roll down the slope. Traffic
eontrol on �he L�ons Ridge Loo� road would obviously be required
during scaZing.
Although scaling would red�ce rock�all risk to an acceptable
level immediately after the work is completed, i� is not a
permanent so�ution at this location. In �ime, the �ormal weath-
ering process will produce additianal saurc� material and the
rockfall hazard will gradually in�reas� with time. With the.
houses in place, additional sca�ing to reduce th� rockfall hazaxd
could nQt be completed.
Report submitt�d by,
�-��� ��. 4�'�� .
Arthur 2. Mears, P.E.
Rock t�ajectory \ -
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FIGURE l. Definitions of velocity and bounce height
at the analysis point. The statist�.cal distribution
of these values are comp�-�ed by the CASP rockfall model
and are shown for 100 s�mulated rockfalls of 2-foot
diameter rocks at 3 different slope positions in Figures
- 2� 3� and 4.
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FIGCIRE 2. Exceedance probabil.it�.es for bounce heights and velocities
of 2-foot diameter rock at possible fence location, approxima,tely 60
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FIG[h?E�. Exceedance probabilities for bounce heights and velocit�es
of 2-foo� diameter rock at possible honse locations at Lots 3 and 4�.
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FIGURE �. Exceed.a.z�ce probabilities for bounce heigY�t� and. velocities
at b�rm location in val�ey bv�tom �o protect Lots 9 - 13.
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/ �� ,- FIG�. R��o�ended locat�ons of flexib�e--po�t �
. fence ���, up�1�l rackfall-barrier wall (B) � North `
, and
railroad�ear�hen barrier C
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are from Sept,ember 18, 199� site plan,
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_• FIGUR� 6. Flexa.ble-post rockfi'all �ence located 60 feet uphill of buiZdings �- --
3 and 4. Exposed cable neax base of� fence pasts enable fence to flex upon �
rock impac� and reduce forces on structure membexs anc� foundations. Design
fence heigh� �7.5 f�} is based on 10f exceedence probability at location
plus design rock radius (i.d it) .
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FIGURE 7. Rockfall proteetion at uphill walls of houses on Lots 3 and ►�.
Rockfall protection barrier shoula consi.st of unconsolidated, coaxse- `
grained, xell-drained g�cavel and sma1.1 rocks tha.t wi1� absoxb �ock �
momentum, D�sign height (6.5 ft� is based on lOJ exceedence probabi.lity
at loeation plus desi�n rock radius.
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--er . Rockfall Direction � �_s�:: .
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FIGURE 8. Op�ional rackfall barrier to protec� units 9 - 13. Vertical
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upha.11 face is made o�' railroad tiss or similar ma.terial and is braced
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• � . • , Appendix A. C1� output fvr des�gn of flexibl�s� rock�all. fence
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