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Home My WebLink AboutDRB120205 - CR1 - 061112 SEGERBERG ARCHITECTS LETTER OF TRANSMITTAL 101 Eagle Road—Building 6 DATE 6-8-12 JOB NO. 35001.00 Avon, Colorado 81620 ATTENTION David Rhoades (970)476-4433 RE Zuckerman Residence DRB submittal Fax (970)476-4608 TO Town of Vail WE ARE SENDING YOU ®Attached ❑Under separate cover via the following items: ❑ Shop drawings ❑Prints ❑ Plans ❑ Samples ❑ Specifications ❑Copy of letter ❑Change order ❑ COPIES DATE NO. DESCRIPTION 1 5-30-12 Rock Fall Assessment 1 6-8-12 Revised site plan with Primary Unit's ridge adjustment over garage 1 6-9-12 Revised elevations with Primary Unit's ridge adjusted over garage ❑For approval ❑ Approved as submitted ❑ Resubmit copies for approval ®For your use ❑ Approved as noted ❑ Submit copies for distribution ❑As requested ❑ Returned for corrections ❑ Return corrected prints ❑For review and comment ❑ ❑FOR BIDS DUE ❑PRINTS RETURNED AFTER LOAN TO US REMARKS The rock fall assessment is new and has been discussed with Bill.These sheets are to replace previously submitted sheets. Foe review on June 22,2012. COPY TO:file SIGNED: Sue Humm for Kurt Segerberg If enclosures are not as noted,kindly notify us at once I—q fn Hepworth-Pawlak Geotechnical,Inc. /�� ech 5020 :970- Road 8 (v` T Glenwood Springs, Colorado 81601 Phone:970-945-7988 HEPWORTH-PAWLAK GEOTECHNICAL Fax:970-945-8454 email: hpgeoGahpgeorech.com May 30, 2012 Laura Zuckerman 3807 Lupine Drive Vail, Colorado 81657 Job No. 112 134A Subject: Rockfall Assessment, Lot 3, Bighorn Subdivision Second Addition, 3807 Lupine Drive, Vail, Colorado Dear Ms. Zuckerman: As requested, we have preformed an assessment of the potential rockfall risk to the proposed redevelopment of Lot 3. The lot is located in the Gore Creek valley east of the Vail town center in the Bighorn area as shown on Figure 1. The purpose of our assessment was to review the potential rockfall risk based on a field reconnaissance, aerial photographs, published regional geology and rockfall information and our experience. Our assessment is of a reconnaissance nature. A field review of the project area was made on May 24, 2012. In addition we have looked at aerial photographs and reviewed published regional geology and rockfall hazards information. Our study is in accordance with our May 3, 2012 proposal accepted by you. Summary of Findings: Occasional rockfalls have occurred in the project area during about the past 13,000 years and rockfall appear to still be an active geologic process. The rockfall risk to the proposed new residence on Lot 3 can be characterized as having a low probability of occurrence but having potentially severe consequences. We estimate that the statistical recurrence probability of a rockfall in the vicinity of Lot 3 is longer than about 100 years but less than about 1,000 years. The statistical recurrence probability that a future rockfall will hit the proposed new residence is longer (less likely) than that indicated for the area as a whole. If a future rockfall were to hit the proposed new duplex residence, it is likely that it would result in severe damage to the residence and potential for severe harm to the building occupants. If this risk is not acceptable to you or government regulatory agencies then the feasibility of mitigation to reduce the risk should be considered. Proposed Redevelopment: It is our understanding that the existing single family residence on Lot 3 will be razed and replaced with a slightly larger new duplex residence as shown on Figure 2. The new duplex residence will be a multi-story, frame structure typical of the existing residences in the project neighborhood. Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989 Laura Zuckerman May 30, 2012 Page 2 Project Site Conditions: Lot 3 is located just to the southwest of Gore Creek on the valley floor of a u-shaped, previously glaciated, mountain valley, see Figure 3. Relief between the valley floor at the lot and the top of the very steep valley side to the southwest is about 2,000 feet. The very steep valley side has an average slope of around 56 percent and several nearly vertical limestone cliffs are present on the valley side. These cliffs in the past have produced occasional rockfalls that have reached the valley floor, see Figure 2. The average slope of the valley floor between the steep valley side and the Lot 3 is around 15 percent and the proposed building site is located about 450 feet to the northeast of the abrupt slope transition between the valley side and valley floor. Existing residences are located on most of the nearby lots in the neighborhood of Lot 3.. A mixed conifer and aspen forest is present on the very steep valley side. Aspen trees predominate on the valley floor. Geologic Setting: The Gore Creek valley in the Vail area was glaciated by valley glaciers several times during the Quaternary. The most recent glaciation of the Gore Creek valley was the late Pleistocene-age, Pinedale glaciation that occurred during Marine Oxygen-Isotope Stage 2. The Pinedale-age, Gore Creek valley glacier originated in five cirques located near the crest of the Gore Range to the east of Vail. The valley glacier system had branches with total lengths of between 10 and 16 miles. The terminal moraine of the system was in West Vail about 6.5 miles down valley from Lot 3 (Scott and Others, 2003). The Pinedale valley glaciers in the southern and middle Rocky Mountains began to retreat from their terminal moraines starting at around 16,800 years before the present and it took about 7,200 years for the Pinedale cirques to become ice free around 9,600 years before the present (Benson and Others, 2005; Price, 2004; and Porter and Others,1983). The average retreat rate for the Gore Creek valley glacier system is estimated to have been around 0.18 miles per 100 years. This indicates that the Pinedale valley glacier had retreated up valley of Lot 3 by around 13,000 years before the present. The middle Pennsylvanian-age Minturn Formation is the formation rocks that forms the very steep Gore Creek valley side to the southwest of Lot 3 (Kellogg and Others, 2003). The Minturn is an interstratified arkosic sandstone, siltstone, pebbly sandstone and conglomerate with some marine limestone beds. The bedding usually strikes to the northwest (parallel to the valley side) and dips between 4 and 16 degrees to the southwest into the valley side. The limestone beds commonly form nearly vertical cliffs and the limestone outcrops along the cliffs have occasionally produced rockfall during about the past 13,000 years since this reach of the valley was deglaciated. Evidence of post-glacial rockfalls are the large, limestone blocks on the valley floor below the cliffs, see Figure 2. Field Observations: Large limestone blocks that are the result of occasional past rockfall are present in the neighborhood of Lot 3. Large rockfall blocks observed in the field and on aerial photographs are shown on Figure 2. At least seventeen rockfall blocks are present in the neighborhood of Lot 3. The rockfall blocks are typically large. Observations in this neighborhood and at other areas to the southeast indicate that the Job No. 112 134A GaFAech Laura Zuckerman May 30, 2012 Page 3 blocks have an average maximum dimension of about 11 feet and estimated weight of around 24 tons. Rockfall Risk Assessment: Occasional rockfalls have occurred in the project area during about the past 13,000 years and rockfall appear to still be an active geologic process. Future rockfalls should be expected in the area. The Town of Vail's official rockfall hazard map shows that Lot 3 and other lots in the neighborhood are in a severe rockfall hazard area(Town of Vail, 2000). The Town of Vail's rockfall map is based largely on a regional rockfall study by Schmueser and Associates (1984). Without long term observations it is not possible to develop a statistical recurrence probability for a future rockfall at the proposed new duplex residence on Lot 3 with a high level of confidence. Based on this review, we characterize the rockfall risk to the proposed new duplex residence on Lot 3 as having a low probability of occurrence but having potentially severe consequences. Considering the number of rockfall blocks in the neighborhood of Lot 3, we estimate that the statistical recurrence probability of a rockfall in the vicinity of Lot 3 is longer than about 100 years but less than about 1,000 years. The statistical recurrence probability that a future rockfall will hit the proposed new duplex residence or other existing residences in the neighborhood is longer (less likely) than that indicated for the area as a whole. If a future rockfall were to hit the proposed new duplex residence or other existing residences in the neighborhood, it is likely that it would result in severe damage to the residence and potential for severe harm to the building occupants. If this risk is not acceptable to you or government regulatory agencies then the feasibility of mitigation to reduce the risk should be considered. Rockfall Mitigation: Additional studies using numerical modeling will be needed to assess the feasibility of rockfall mitigation to reduce the risk to the proposed new duplex residence on Lot 3. Outcrop stabilization will not be a practical mitigation because of the extent and location of the potential outcrop start zones, see Figure 3. Further studies may show that on-lot mitigation will be feasible with a rockfall catching fence or a mechanically stabilized earth (MSE)wall. Since several other residences in the neighborhood are also exposed to a rockfall risk consideration could be given to mitigating the rockfall risk with a community wide rockfall catching fence or MSE wall. Limitations: This study was conducted according to generally accepted engineering geology principles and practices in this area, at this time. We make no warranty either express or implied. The information submitted in this report is of a reconnaissance nature and is based on a field review, aerial photograph interpretations, published regional information and our experience. This report has been prepared exclusively for our client and is an assessment of the potential rockfall risks to the proposed new duplex residence on Lot 3. If the rockfall risk described in this report is unacceptable then additional studies will be needed to evaluate the feasibility of mitigation. Site specific assessments of other geologic conditions that may present a risk to the proposed redevelopment of Lot Job No. 112 134A GAZ*tech Laura Zuckerman May 30, 2012 Page 4 3 are not within the scope of this study. We are not responsible for technical interpretations by others of our information. If you have questions or need further assistance, please let us know. Respectfully Submitted, HEPWORTH - PAWLAK GEOTECHNICAL, INC. Ralph Mock Engineering Geologist Reviewed by: i J � Steven L. Pawlak, P.E a 15222 RGM/ksw eA0 S/L ZA: 'P S'pNAL cc: Segerberg Meyhew Arc j{ �0 Kurt Segerberg (kse eg rberg@smarchs.com) Attachments: Figure 1 -Project Site Location Figure 2 - Rockfall Blocks in Vicinity of Lot 3 Figure 3 - Potential Rockfall Start Areas REFERENCES Benson, L. and Others, 2005,New Data for Late Pleistocene Pinedale Alpine Glaciations from Southwestern Colorado: Quaternary Science Review V. 25, p. 49-65. Kellogg, K. S. and Others, 2003, Geology Map of the Vail East Quadrangle, Eagle County, Colorado: U. S. Geological Survey, Map MF-2375, Version 1.0. Pierce K. L, 2004,Pleistocene Glaciations in the Rocky Mountains in The Quaternary Period in the United States, Gillespie A. R., Porter, S. C and Atwater Editors: Elsevier, New York, p. 63-78. Job No. 112 134A G99tech Laura Zuckerman May 30, 2012 Page 5 Porter S. C. and Others, 1983,Late Wisconsin Mountain Glaciers in the Western United States in Late-Quaternary Environments of the United States, Wright, H. E. Jr., and Porter, S. C. editors: University of Minnesota Press, Minneapolis. Schmueser and Associates, 1984,Rockfall Study- Town of Vail: Prepared for the Town of Vail (November, 29, 1984). Scott R. B. and Others, 2002, Geology Map of the Vail West Quadrangle, Eagle County, Colorado: U. S. Geological Survey, Miscellaneous Field Studies Map 2369, Version 1.0. Town of Vail, 2000, Official Rockfall Hazard Map, Town of Vail: Prepared by the Town of Vail, Vail, Colorado (Adopted by the Town Council on October 17, 2000). Job No. 112 134A GecPtech 7 .J1 Lot 3 or Fig e WN) A:\ !w 0 1500 ft. Scale: I in.= 1500 ft- Contour Interval=40 ft. May 2012 112 134A G&rtech','," Lot 3 Bighorn Subdivision Second Addition Figure 1 Project Site Location I GS • • tii0 ti�, 9y Lot 5 ti0 Lot4 6 a° ei0 y0 a •Co 4'N m •p0 >° /\ C;P "90 P o Lot3 f • c �X 3 F C�A�P C •� �� ooa Lot 6 0 Lot 7 m Lot 2 O • �� Lot 7 • Lot 8 • goa Q� Lot 6 Lot 10 6 Lot 13 OiL P Lot 11 Lo 12 Lot 14 Explanation: • Rockfall Blocks E;:" Existino Buildings Base of Steep Valley Side LL-_ Proposed New Residence Lot 3 0 900 ft. I I I Scale: 1 in.=900 ft. Contour Interval: 40 ft. May 2012 112 134A G@(PytgCh Lot 3 Bighorn Subdivision Second Addition Figure 2 He�woerH.Pnwu.K CCOrecHmcnL Rockfall Blocks in Vicinity of Lot 3 s f Vail Boundary pelvi e� Lot 3 o� 1 r ire) 1/ O 6; 9200 1� �0 o• 9400 ro 9600 5� ro ID 9800 5 8 9000 ro 02� 9zPo s"a Z a`0oo 9600 � g66� Q ro 42 \ 0�0 ^O r rOLO r 4 / Explanation: ® Rock Outcrop Cliffs: ® Horizontal Bedding: Prominent limestone outcrop cliffs that are potential start areas for rockfall. 39 Strike and Dip: Contact: Approximate boundary of Strike and dip of bedding in degrees. prominent rock outcrops. 0 900 ft. I I I Scale:1 in.=900 ft. Modified from Kellogg and Others(2003) Contour Interval: 40 ft. May 2012 112 134A ~ Lot 3 Bighorn Subdivision Second Addition Figure 3 HEPWORrH�PAWLAK GEOTECHNICAL Potential Rockfall Start Areas