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Home My WebLink AboutB13-0187 Rockfall Report Stamped Department of Community Development 75 South Frontage Road �� j� Vail, CO 81657 Tel: 970.479.2128 www.vailgov.com Development Review Coordinator TRANSMITTAL FORM Use this form when submitting additional information for planning applications or building permits. This form is also used for requesting a revision to building permits. A two hour minimum building review fee of$110 will be charged upon reissuance of the permit. Application/Permit#(s) information applies to: Attention: �Revisions �Response to Correction Letter Building Permit B13-0187 Ford Residence Warren, Martin, JR �attached copy of correction letter �Deferred Submittal �Other Project Street Address: 1079 W Sandstone Drive (Number) (Street) (Suite#) Building/Complex Name: Description of Transmittal/List of Changes, Items Attached: Signed and stamped rockfall report Applicant Information (architect, contractor, owner/owner's rep) Contact Name: �ohn G. Martin Address: PO Box 4701 City Eagle State: CO Zip: 81631 ContaCt Name: (use additional sheet if necessary) Contact Phone: 9�0-328=0592 Building Permits: ohn martinmanle architects.com Revised ADDITIONAL Valuations (Labor&Materials) Contact E-Mail: 1 @ y (DO NOT include original valuation) I hereby acknowledge that I have read this application,filled out Building: $ in full the information required,completed an accurate plot plan, and state that all the information as required is correct. I agree to Plumbing: $ comply with the information and plot plan, to comply with all Town ordinances and state laws, and to build this structure according Electrical: $ to the town's zoning and subdivision codes, design review ap- proved, International Building and Residential Codes and other Mechanical: $ ordinances of the Town applicable thereto. X Total: $� Owner/Owner's Representative Signature(Required) Date Received: For Office Use Only: Fee Paid: Received From: Cash Check# CC: Visa/ MC Last 4 CC# exp. date: Authorization # March 18, 2013 Trent Hubbard, Contractor, on behalf of Nate Ford, Owner 1079 Sandstone Drive Vail CO 81657 Re: Rockfall Potential, Proposed Addition to 1079 Sandstone Drive, Vail, Colorado, Dear Mr Hubbard: At your request, I have examined the area that is a concern for potential rockfall damage and/or injury to your proposed house addition. Following are my results and conclusions. SUMMARY There is a small potential for damaging and/or personally injurious rockfall at the subject site. Although the magnitude of potential damage if a single large rock fell could be large, the sources of potential falling rock are few. Rockfall events are sporadic over time, and it is not possible to predict any times of occurrence or the specific paths of any future rockfall events. If a greater level of assurance is desired, one potential rockfall boulder may be removed, and the uphill portion of the addition may be designed for the most likely potential rockfall loadings. SOURCES OF INFORMATION The site was investigated using USGS maps, other published literature, Google Earth images, visual examination and photography, and by computer modeling of the most likely potential rockfall events. SITE LOCATION The site lies at the northern edge of Vail Village, and the lot lies entirely within a designated zone of high rockfall hazard potential. The potential source of rockfall is the hillside to the north of the lot. The proposed addition would be on the northern, uphill side of the house, and would therefore be the most likely portion of the house to suffer first impact from a future rockfall event. The addition covers 29 feet of the 52 foot width of the house. Proposed addition and house are both of standard residential wood frame construction. Rockfall Potential, Nate Ford Addition, 1079 Sandstone Drive, Vail Page 2 of 11 The portion of the hill that is of principal concern as a potential source of falling rock is the portion that lies directly up the slope from the addition, perpendicular to elevation contours. It extends in a narrow zone (less than 30 feet wide) approximately 1100 feet horizontally to the northwest and approximately 535 feet vertically up the hill from the addition. It comprises slightly less than % of an acre, and encounters relatively few outcropping boulders. TOPOGRAPHY Topographic information was principally obtained from the USGS Vail West 7.5 minute topographic quadrangle map. A small amount of the topographic data was taken from a recent lot survey by Eagle Valley Surveying, Inc. A Site Plan with traced contours follows on next page. The overall mountain rises to about 10,325 feet elevation, but the portion directly uphill subpeaks at a ridge at 8825 elevation. The grade adjacent to the proposed dwelling addition would lie at an elevation of approximately 8290 feet. The average slope gradient from the house site at the valley bottom to the top of the zone of interest, about 535 feet up, is about 49%, or about 26°. POTENTIAL ROCKFALL PATH The most likely potential path for falling rocks to follow in arriving at the dwelling is defined by the perpendiculars to the slope contours, since the reaction of the slope against the falling boulders tends to push them out in that perpendicular direction. Contours from the USGS topographic map that were used to define the travel path have been superimposed on a satellite image to illustrate the rockfall area in an overhead plan view Site Plan, on the next page. The zone directly above the dwelling that provides direct perpendicular travel paths to the dwelling has been outlined in red. Irregularly shaped blocks may bounce in irregular ways as they impact the slope at odd angles, and local irregularities in the ground surface may cause the same effect. These irregularities allow blocks originating within the defined travel path to bounce elsewhere, and they also allow blocks originating elsewhere to bounce into the defined travel path. Nevertheless, most travel will be guided along the perpendicular direction, deviations will be relatively random, and boulders travelling into the defined path should approximately compensate for boulders travelling out and away from the path. Analysis of the direct travel path above the dwelling is therefore reasonable. 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E'�l �� 1r� �� SCAI,� I " - 2C;�� CQ�11"�1�� 1�J1��V'AL '0' c� �� ,�u�� c � � ��l�f� L��dvill�, Col or��o Rockfall Potential, Nate Ford Addition, 1079 Sandstone Drive, Vail Page 4 of 11 rockfall dynamics. The profile along this line is shown on a Slope Profile drawing, which has been appended as an oversize sheet on Page 11 following the report text. GEOLOGY The hillside at this site has been mapped by the US Geological Survey as a layered sequence of sedimentary rocks, with an overlying veneer of glacial till in some lower areas. Even within USGS publications, some of the data is conflicting, and no field geologic study has been undertaken to resolve any conflicts. Some of the spatial data, in particular, should therefore be taken to be approximate. The only bedrock unit of interest for rockfall at this site is the Pennsylvanian Minturn Formation, which underlies the entire inferred source area for potential falling rocks and dips north-northwest at about 18°. The Minturn is a sequence of dirty sandstones with lenses and layers of limestone and carbonate-rich sands. The sandstone, siltstone and mudstone layers generally weather to smaller fragments and contribute fewer boulders to the hillside, while the carbonates possess more integrity and form more of the boulders. The topmost unit of the Minturn here is the Jacque Mountain Limestone Member, a thin (40 to 50 foot thick) unit which coincidentally marks the ridgeline just above the extreme top of the potential rockfall zone, at approximate elevation 8825. It is one of the principal boulder- forming units on the hillside, but appears to be either completely above the subject rockfall zone or else so limited in the intersection of its basal units with the rockfall path that its contribution to the rockfall zone should be minor. The glacial till mantle in the lower half of the rockfall zone provides a soil mantle that retains moisture and supports a significant aspen forest. This forest has developed a thicker, softer soil with fewer boulders, and the trees and undergrowth probably also impede boulders from gaining momentum. The forest extends for a way uphill from the glacial till as well. COMPUTER MODELING OF POTENTIAL FOR DAMAGE FROM ROCKFALL EVENTS The Colorado Rockfall Simulation Program was used to model the potential for occurrence of rockfall damage on this site. This program was first developed in 1984 through 1989 as funded research at the Colorado School of Mines to deal with the numerous rockfall design problems facing CDOT. Since then it has been expanded and updated, validated and verified by field testing, and used from California and Oregon to Switzerland. The program has been extensively reviewed and tested and calibrated to field data, and is used by mines and public road agencies. The principal input for this site is the profile of the hillside that was created from the topographic map. Randomly generated rockfall events are initiated by the program on the profiled hillside and their progress falling down the hillside is calculated by the program. The Rockfall Potential, Nate Ford Addition, 1079 Sandstone Drive, Vail Page 5 of 11 slope was divided into 19 segments based on varying steepness and hardness of the rock or soil or degree of vegetative cover. Other program input parameters include starting zone, rock size, rock shape, rock density, surface roughness, tangential coefficient of friction, and normal coefficient of restitution. Output results include velocities, bounce heights, kinetic energies, and where the rocks stop rolling. Specific points may be selected for analysis of output data. Rock sizes of 2 to 4 foot diameter were selected for analysis, based on rocks observed at the site. Most of the boulder-forming units have boulders of that size available. Rock density was set to 165 Ibs/cu ft, which is an intermediate value for a carbonate rich layer. The remaining factors have been mostly empirically derived from comparison to actual rockfall behaviors at other sites. Surface roughness factor reflects a mixture of both slope roughness and boulder irregularity. The worst case, a roughly spherical boulder is assumed. Surface roughness here was set to 3 to 6 inches for the 2 foot boulders and 6 to 12 inches for the 4 foot boulders, to account for irregularities in the slopes and in the shapes of the boulders. The surface roughness parameter was set to the lower value for the lower portion of the path where a boulder may have lost some of its asperities. Tangential friction and normal restitution were set to a reasonable range of values, as typical of bedrock, rocky soil or softer soil slopes at other sites. Each bounce of a boulder tends to slow it down or stop it, unless the slope is sufficiently steep to overcome the soil damping and accelerate it further along on its way. Boulders will fairly quickly roll to a stop on flat ground. A series of runs were done, for upper and lower boulder source segments, to check the potential for hazards from the various rock zones along the profile. This showed that 2 foot diameter boulders from the top boulder zone at 8800+feet were unlikely to travel to slope bottom. About 4% of the larger boulders in the top zone would be expected reach the addition, with the great majority being stopped by the damping from forest soils. After struggling through the forest, the few survivors quickly build speed again in the bare lower slope area. The lower source was capable of delivering any well-rounded boulders to the slope bottom and on into the dwelling. These lower source boulders could build speed quickly and could be just as damaging as ones from above. The program randomly initiates a thousand boulder movements within an outcrop source zone and then tabulates the ensuing results for all of the boulders into a statistical representation of probable behavior. The upper source runs were analyzed at 3 points: the upper edge of the forested area, the lower exit from the forested area, and the north wall of the addition. It was found for all runs Rockfall Potential, Nate Ford Addition, 1079 Sandstone Drive, Vail Page 6 of 11 that all boulders that were successfully initiated (about 85%) accelerated rapidly through the upper bare area and into the forest, then were decelerated by the damping effect of soft forest soils. None of the smaller boulders escaped the forest area, but the very few larger boulders that did make it all the way through the forest were rapidly re-accelerated down to the house. The median calculated characteristics of potential rockfall boulders reaching the slope at bottom of forest and reaching the north wall of the addition are tabulated below: Elevation Diameter Percent Percent Mean Mean Mean of of Rock Arriving Arriving Velocity Bounce Height Kinetic Energy Ori�in (feet) Bottom at House (fps) (feet) (foot-pounds) 8825 2 0 0 -- -- -- 8825 3 4 4 43 1 91,000 8825 4 5 5 40 1 185,000 8375 3 -- 85 38 1 74,000 The calculated energy levels in the boulders that do reach the house are probably sufficient to penetrate standard wood frame walls or knock them down and structurally damage the house. If a rock were to hit at a critical load support location, much of the structure might collapse. The house walls would probably not significantly alter the progress of a boulder unless it were to hit a major structural impediment. SITE SPECIFIC OBSERVATIONS The slope above the addition is convex in overall horizontal plan. This geometry reduces the width of the potential rockfall path as it is traced up the hillside, from 29 feet wide at the bottom to zero width at some point near the top. Boulders that roll down the hillside outside of that path will tend to roll elsewhere. The path traverses areas of sparse vegetation and relatively thin soil cover at top and bottom, but the middle area, comprising about 47% of its length, is forested to varying degrees. The lower portion of the forested area has a thicker organic soil layer developed on glacial moraine, and many of its aspen reach diameters of 9 or 10 inches. While the larger aspen are too sparse to impede most rockfalls, the thicker, softer soil in which they thrive, along with the locally slightly shallower slope angle, are effective dampers to rockfall progress. The path maintains an overall average gradient of about 49%, which is an angle of about 26 degrees, a slope that is sufficiently steep for carrying and accelerating rockfalls if the soils are firm or thin or absent and the surface is relatively uniform. It is not, however, sufficiently steep to propagate rockfalls if the soil is thick and relatively soft. Forest vegetation is a further decelerating factor. At this relatively shallow slope angle, with fairly smooth slopes and short available acceleration distances that are sparse of forest soil and vegetation, the boulders Rockfall Potential, Nate Ford Addition, 1079 Sandstone Drive, Vail Page 7 of 11 would tend to stay close to the ground and roll as much as bounce in their descents. Few bounces are expected to exceed 2 or 3 feet in height. This particular potential rockfall path has two principal rockfall sources, and the available width of each source outcrop is small. The horizontal outcrop width between rocks suitable for rolling downhill is generally much greater than the path width. Slope areas vertically in-between the two sources do not appear to produce significant boulder-forming outcrops. The top source (at 8825 elevation) may not be capable of providing any boulders to the rockfall path at all, since map analysis indicates the rockfall path goes to zero width at its intersection with that upper rock source bed. The combination of almost no upper source material and a very low success rate for rocks reaching the bottom makes this a relatively low-risk source. The bottom outcrop source (at elevation 8375) is only about 200 feet uphill from the proposed addition. Although any roughly spherical boulder that originates at the lower source would almost certainly reach the house, a careful look at that source layer has identified only one boulder that seems potentially threatening at present. There are quite a few slabby boulders that are lying at the ground surface in this lower zone, but they lack the mobility to roll and bounce. There could possibly be some small potential for slabby or tabular rocks to break free during the winter season and slide along an icy snow layer and down into the addition. A low berm or wall could be an effective structural mitigation against this possibility. One specific 3 foot boulder in the lower source outcrop that is within the potential rockfall path does seem somewhat threatening (photo next page). It has weathered to a crudely spherical shape and requires relatively little additional weathering around its base to break free. It could remain stable for a century or it could become unstable next season. If it were to bounce downhill it might either roll into the addition, or it might instead break apart along its several deeply-weathered fractures into several slabs that won't roll or bounce. It does represent a potential threat, and it seems to be the principal predictable threat on the hillside. It is also readily accessible for proactive mitigation, which is recommended. PREDICTION OF FUTURE ROCKFALL EVENTS Some change in state is required to initiate a rockfall event. Rockfall events are usually triggered either by a mechanical stimulus of some sort, or by gradual loss of strength or restraint at the support point of a boulder. Loss of strength due to weathering is a continual process that progresses at a geological rate, over thousands of years, as the hillside gradually erodes and falls away. One characteristic of these dirty sandstones of the Minturn Formation is that the boulders themselves weather smaller with time, and that only the more resistant layers contribute boulders which can outlive the slope erosion process and therefore need to tumble downhill. This is a very slow, very sporadic, and unpredictable process. 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"� ��9r r;l�`��'; °�� I��''i��%:r. ti, ,�yt,�� ✓� � _ � ',� � �,�1 � r ��1��°��f•$ �•r � • � �� +r �� r i'� I�1��'' . �' -""c�� . �g ,X �f� ,�,.,��'�`It�`�'J, ;�,, °;� ��' �f�t� ��r:.�`�+' .'� ^� ��' �A+ � ,��+ ,,� r d r ( � � �.,,'��'7''`'�1 �}��._. �� . ,/' � ,i ��, � �. �`� ,� .:; ..� �� ;a'-�'��°��"��,, '��. ;� ,��` , ,w��,ti��J p'�"�r l�'; „s� '�I ��( , " '-�",�„' � - `��r� ,�`�,� �,;�, '� '",�.';, ��4�y � 1�� dF'— . ir� �.l �i.. *i� �� ..�.Fb�vy� .: q�d��6'"`• I% ; �A . p� ��. �fif �r+ �� li ��n{� � MYl�:�w .�.. ^�.:K 'Pl'1�'�u',_ L._-. e�.;�,sd/.1,��,(b_=��• r 1 ►. '� �.,�� ti i '� ..�.�1 This boulder appears most likely to pose a rocl<fall threat. Note fragmented nature due to deeply weathered bedding and joints, and note irregularity of shape. Location noted on Plan. Rockfall Potential, Nate Ford Addition, 1079 Sandstone Drive, Vail Page 9 of 11 Heavy rains may contribute to instability in several ways. They may reduce the effective strength of the supporting material by increasing the water pressures in the ground, they may erode the underlying supporting material, they may build up water pressures that tend to push a boulder out from the slope, and they may wash material into a crevice that works to wedge a boulder outwards from the slope. Periods during and after heavy rains are notable on highways throughout the state for the rockfalls that litter the highways. Periods of heavy, monsoon-type rain are therefore statistically much more prone to rockfall events, but are seldom predictive of specific rockfall events. Snowmelt seasons contribute to rockfall initiation in two ways. One is through the dai ly meltwater influx that affects the boulders as indicated in the previous paragraph. Another is through the nightly/daily freeze/thaw that incrementally pushes boulders out from the slope, since ice is 10% lower in density than water and expands 10% as it freezes. Snowmelt season is another statistically important rockfall season, but once again it is seldom predictive of specific events. Seismic acceleration is the initiating factor in many rockfall events. Magnitude 4 is the level at which many people begin to personally experience the sensation of shaking during an earthquake, and that also corresponds approximately to the lowest levels at which seismic- induced rockfalls have been documented. The region surrounding the site has been mostly free of recent faulting or recent significant earthquake activity. Although there have been many smaller ones, there has not been a recorded earthquake of magnitude 4 or larger within 50 miles of the site during the 40 year period of sensitive seismograph measurements in Colorado. In summary, there is no way to predict the occurrence of any specific rockfall event in the future. The only certainties are: 1) that the highest potential for individual rockfalls will occur during spring snowmelt and summer monsoon rains, 2) that rockfall could be initiated at this site during a rare major seismic event, and 3) that eventually, sooner or later over the long term, all of the boulders on this hillside will either weather away or fall or slide downhill. The expectation is that some rockfall will occur, but that it will probably be extremely infrequent on a human time scale. CONCLUSION This review was limited to establishing whether a significant degree of rockfall risk exists. Rockfall is distinct from other slope processes, such as avalanche, debris flow, landslide, creep, etc., which processes were not included in this study. In my opinion, except for the single noted boulder, there is a low expectation for a damaging rockfall to occur at this site during the life of the addition. However, this site does fit the minimum criteria observed at rockfall sites worldwide, and computer calculation of the kinetic energy involved indicates that damage could be significant if a rockfall were to occur, and particularly so if structurally critical support points or people were to be impacted. This definite Rockfall Potential, Nate Ford Addition, 1079 Sandstone Drive, Vail Page 10 of 11 potential should be understood and accepted by owners in their use of this property, and designed for accordingly if this degree of risk is not acceptable. It is not possible to predict if or when any specific rockfall might occur, and it is not possible to definitely rule out the possibility of a damaging rockfall. This is the usual situation for rockfall zones worldwide. Relative to other dangers of life that are commonly accepted, such as automobile accidents or skiing accidents, the probability of rockfall damage to this addition and its occupants is low. CERTIFICATION I certify that this site may be developed as proposed for a residential addition without corrective engineering or engineered construction or other mitigation or alterations if the owner is willing to accept the relatively low probability of damaging rockfall occurring at some unknown time during the life of the addition. The most likely potential rockfall boulder may be immediately mitigated to further lower the chances of damage, and this is recommended. I certify that the site is an area that is susceptible to rockfall, but that construction of the addition as proposed will not increase the hazard to other property or structures, or to public buildings, rights-of-way, roads, streets, easements, utilities, or facilities or other properties of any kind, as long as safe and proper construction practices are used. Thank you for the opportunity to work on this project. Please call if you have any questions. Sincerely, �:�� G,s �� Q. ��9 ��, ,� ~ �� i� � Willia Klaub rp E 34'��6`Color Box 909 �'c� �/�30�20�3 �� Leadville CO 8 6`�; ,, ��' 719-486-5780 �U�Q��`�`��