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Home My WebLink AboutGSA Map Change Request - Buffehr Creek Lot 1 - ROCKFALL MITIGATION REPORT - stamped 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 � 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 feasibitity 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 r ,-�a , � : a ,u� F / "'�: � s . �� } Y� ;�a .�t �" �1 ..0 +� ,�_4s.y' �.,'.;, �`ti � �x}` f i�+ � lj. �� r�f��` ��:�s ; ` y`� � � � �� All of Lots 1, 2, and 3 appear to be ` � �r ��K- .�i. ,s.�� -} �.�. .r^, �-. ;��A �-� °��- .� �� . �� Tf' � included in a "hi h severit " rockfalt x ,.��.. x:��...� .�.,,;_�!. 9 Y �µ`�# `�� `' �� ` area on Town of Vail ma s roduced , �.� ��,.-' �ry:: � ��.==� -' '}" � p p �� y.:��,�'�,� .'`' �� t�, ' in 1984. My site inspection last b r�s.%��-�. �.k� ,;�� ' y,., 4'' ;s�Y �r�,,, ,�� ,� �� week and experience with a rockfall- �y�.�.� � , o . c�-..F�„ �-.°+ ��, �� .-' �'�=�� s,. z: ��.,�� hazard evaluation completed by me r .� g � � ;� �?'� r;�� '��'� 1.`�, __��s ;�� in 1990� suggests that rockfall � i��='�'-�4�'� �'��`f�. - �``� � s��`�Y h a z a r d i s n o t "s e v e r e" b y C o l o r a d o � t a��'.� .�- r� �`�� ".� `� �`��>� or general mountain standards� it 3�; �'►`,�,�, k . �'�,� �t,, , r :� e;�b 4w . � `. r`k`.. . ". - � �� '_� �'t= . r . , .` �� a-.� � � �, should be classified instead as ��, = �; � ag t�r,, � � '�'� '�` « „ M�� w,h��.� � g� ��-��,.R�� , ..�� moderate. .a r� � ._ ._. . � - ._ •Y y.v>`�".w. _. -� 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.I. Mears, P.E., Inc., "Rpckfall Hazard Analysis, The Valley, Phase IV, Vail, Colorado", prepared for Ed Zneimer, June 25, 1990. typi�ai of fhe size of rock fo be considered in mitigation de�ign . According to the A . I . Mears ( 1990) repor�, rockfall has occurred in the pa�t �everal decade� (prior - to 1990} � nd has re� ched the valley b�ftom . �ecause of the limi�ed rockfall sour�e area , rockfall evenfs will probably be rel�tively infrequent (one event per decade or les�) � nd will probably c� nsist of only one or ivvo bot� lders . fU� ost will stop on the slope above the building � ites. Because rr� oderate rockf'all hazard doe � exisf, s�ructural mi�igation for rockfall is recommended , as discussed below . .�� a< � ;�$ 2 . Rockfall Mitigati,'on Two mitigation technique� are feasible at fhese sites : (a) construction of barriers at or directly above the proposed buildings , and (b) construcfion of small rockfall energy-absorbing fence� on private property below the source areas . Building sites on lots 1 , 2 and 3 should all be protected because rockfall paths may b � somewhat unpredictable . Rockfall barrier� a� the builti � ngs , while feasible from a technical perspective , are probably not desirable . Building walls lirted on �he uphill � ides 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 impac� and s�ructural damage ., The heigh�s of the required �truc�ures at the building have not be� n determined but shauid be based on the newly-available Town of Vail GIS topography and updated rockfall modeling . Previous work suggests the wall heights rnight be up to 6 feet at the proposed house� . While this technique could be effective it may not pro�ecf the entire house b�cause of the locations of doors , and it may hav� an undesirable appearance . T'he second feasible rr� itigafion technique would be construction of srnall rockfali energy -absorbing �ences on the slope , a shor� distance below the �ource areas . The desi� n heights and strengths of the fence� would be mod ��t bec� use energy and bounce heights will � Iso be modest. T'hey woutd probably be fairly short at each of th � �viro rock outcroppings2 . as di �cussed at th � starfi of this r�port , one obj �ctive has been to discuss the feasibility of rockfall mitigation . Rockfail mitigation does appea�to be feasible at these site� , and should , in my opinion be assumed as su rr�� __ _ ermitting process . �;> � $�:� � � �� ,�, Q��' � � �c�► �°�� . ,�, ;�-, � � � � � � � � ��_ Sincereiy , ,� �,� : :,�° � �� - , � �� '�' '�, b �;� �"�°. � �4�Gu� � 1�2� � � ; . �{ ;.; ,�.� :, �, : �1R �. :r+ .. . t� � _; �� Arthur I . M �ars , P . E . � � � A �� ' � , , ��.� � : , ,�: �`��y > � ��v � �. �� ,� � 2 �� , , As a rough e�timate (not based on detailed anaiysis) fences may ne e. �han 50 feet in length at possibly two locafions between the trail switchbacks but on private property .