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Home My WebLink AboutRockfall Hazard Analysis - June 25 1990 (2) � : �- � � � . . ROCKFALL HAZARD ANALYSIS THE DALLEY, PHASE VI, VAIL, COLORADO Prepared For Mr. Ed Zneimer Prepared By Azthur I. Mears, P.E. , �nc. Gunnison, Col.orado June 25, 1990 . �� • • � �� . 1 08JECTIVEB AND LIMITATIONS This report on potenti.al rockfall hazard at The Val3.ey, Phase VI, was requested by Mr. Ed Zneimer and has the following objectives: a. Identification nf rockfall source areas and roll- out distances in the area of potentia3 deve�opment; b. Applicat�.on of the CaZorado Rockfall Simulation Program (CRSP) to predict rockfall roll-out distances, velocities, and bounce heights at various positions; c. Discussion of the rockfall risk; and d. Discussion o� general rockfall mitiqation methods that could be applied to protect certain building sites. The analysis and recammendations are site-specific. Therefore the recQmmendations may not apply tv other locations. Further- more, any substantia� - changes to building pasitions from those shown on Figure 1 may invalidate the recommendations of this study. 2 ROCRFALL HAZARD AND $ZTE DEVELOPMENT The rockfall-hazard areas and the approximate positions of �2 buildings are shown on Figure �.. The building positions were scaled �rom the Peter Jamar Associates, Inc. conceptual land-use plan as madi�ied by Mr. Ed Zneimer on June 20. The rockfaZl saurce areas, where rocks become detached and begin the downslope mdtion, are located approximately 500 feet north af the Lions Ridge Loop Road at 8, 450-to-8, 55Q feet elevation within sandstone eutcroppa.ngs of the Minturn Fnrmatian. Although rockfall is not a frequent problem, the source areas are active and many loose raoks up to approximately 2 feet in diame- ter are perched in metastable equilibrium within and below the source-area outcroppings. Major rockfall events certain�y do not occur annually, but substantial rock�alls that cross the Loop Road can be expected roughly once per decad�, approximately. During the past decades roekfall has occasionally oecurred and rolled beyond the center of the field at the bottom of the valley. Some events have reached approximately to the farest on the southern edge of the property where some buildings wil.l be lacated. Inspection of the source area and the rocks in the field suggest a rock diameter of 2 feet should be used in rock- fa�.l analysis and mitigation. Therefore, 2-foot radius, spheri- ca], rocks were used in the CRSP rockfall simulation discussed in Section 3 of this report. Larger rocks have been excavated during construction of the Loop Road, but these large bou�.ders do not appear to be of rockfall origin. � . �� • � � x' 3 RESIILTB OF ROCRFALL SIMULATION (APPLICATION OF CRSP) The Colorado Rockfall Simulation Program (CRSP) is a research computer program recently developed by the Colorado Highway Department to predict rockfall velocities, bounce heights, and roll-out distances. This was applied to The Valley project to provide an estimate of rockfall velvcities and bounce heights at the proposecl building locations. The computer program was applied as follows: a. First the rockfall ro1.1-out distance in the field was observed and recorded to pravide an end point for a real rockfall event. b. Second the slope was subdivided into segments of constant slope, roughness, and hardness by field mea- surements; and c. The CRSP model was app�.ied on 2 profiles on the slope and forced to pravide a "mean" roll-out distance corresponding with the field measurements. The slope profiles are shown on �he topographic map (Figure 1) . The following conclusions result from application of CRSP and analysis of the field observations: a. Rockfall can reach buildings 8, 9, 10, 11, and 12, but 2-foot diameter rocks will be rolling (not bounc-� ing) at a veloaity of ZO t�et/sec or less at the build- ings. These rocks cauld b� stopped by building a berm or vertical barrier 3 feet high near the center of the field, possibly on the north side of the propased access road. However, rockfall protection is not a high priority here because events are infrequent (pos- sibiy once per decade) , velocities at� the building sites wi21 be moderate, and substantial. damage to buildings will not occur. b. Rockfall wi11 also reach buildings 3, 4, and 5, which are located at the 8�300-foot level on the hills- lope. Mean rockfall velocity h�re is 25 feet/sec with bounce heights ranging from 1-to-6 feet. These 3 buiZdings should be pratectad because the desiqn rock- fall event can produce kinetic energy of 21, 000 ft-lbs and may produce substantial damage to the uphill walls. Protection can be achieved by installing a rockfall fence above the building or by specially reinforcing the lower 6 feet of the back building walls. Mitiga- tion desiqn details are beyond the scope of this analy- sis, but are feasible at this Iocation. c. Buildings 1, 2, 6, and 7 are not affected by rock- fall. I _ � � 4 INCREASE IN ROCKFALL HAZARD AT A RESULT OF DEVELOpMENT Because the proposed development wi�l bring an increased number of people into this rockfall area, the overall risk will increase in proportion to the larger number of persons exgosed. Further- more, there exists some small probabi�ity that the persons one wishes to protect may not be in a sat'e location when a rockfall occurs. An additional and related hazard increase will occur on the Lions Ridge Loop Road because the probability of a car hitting a fallen rock (or a rock hitting a moving car) wi11 also increase as a result of larger traffic volumes. Although the potential hazard increase should be understaad and accepted by area residents, all rvckfall risks are small when compared ta rockfall danger we normally accept without question at other locations, such as in Glenwood Canyon, for example. Area residents can reduce the small risk even further through education about the rockfall process. Such education is recom- mended. Report submitted by, ��^�- ��,Qa'/� Arthur I. Mears, P.E. � / . � / � � � _ � .� PyqS� � � 2�.�S . ' � � � Q�� l . . � i 'S' � �. � h' � " � � . 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