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HomeMy WebLinkAboutDRB140055 Rockfall Mitigation ReportG1=q rte(::h HEPWORTH- PAWLAK GEOTECHNICAL April 16, 2014 Scott S. Turnipseed, AIA Attn: Alicia Davis Hepworrh- Pawlak Georechnical, Inc. 5020 County Road 154 Glenwood Springs, Colorado 81601 Phone:470- 945 -7985 Fax: 970 -945 -8454 email: hpgeo@hpgeorech.cvm P.O. Box 3388 Eagle, Colorado 81631 (aicia cr sstaia.com� Job No. 113 449A Subject: Review of Rockfall Risk and Mitigation, Proposed Stockmar Duplex, Lot 14, Bighorn Subdivision, 4096 Columbine Drive, Vail, Colorado Dear Ms. Davis: As requested, we have reviewed the conditions in the area and observed the subject site on April 15, 2014 to assess the rockfall risk to the residence and adjacent properties as a result of the proposed construction. The findings of our review are presented in this report. The services were supplemental to our agreement for professional services to Scott S. Turnipseed, AIA, dated December 3, 2013. We previously performed a subsoil study at the site which included a general discussion of potential geologic hazards affecting the site and submitted our findings in a report dated December 31, 2013, Job No. 113 449A. Project Data: The existing residential building on the lot will be razed and a new duplex residence will be constructed. We have been provided a building site location, grading and drainage plan for the construction, Sheet Al. 1, prepared by Scott S. Tumipseed, AIA dated March 10, 2014. The new duplex structure will occupy a significantly larger portion of the lot than the existing residential building. The Town of Vail requires the rockfall review because the property is identified as being potentially impacted by rockfall in their geologic hazard maps (Town of Vail, 2000). The development plan does not include rockfall mitigation at this time. Geologic Conditions: Town of Vail rockfall hazards map indicates that Lot 14 is located in the lower part of a potential "High Severity Rockfall" impact which extends down to Columbine Drive on the northeast side of the lot. Rock blocks associated with past rockfalls were not observed on the site but evidence of past rockfall, if present, may have been removed by prior construction grading. Snow cover at the time of our field review was roughly 3 feet and may have concealed rockfall blocks on -site. Based on aerial photography (Google Earth) and our site visit, a large rockfall block was visible approximately 175 feet west of the lot, and a large rockfall block was observed approximately 150 feet south of the project site in Gore Creek. Based on our site visit and review of aerial photography, large rockfall blocks were not observed to have crossed Gore Creek in the vicinity of the project site. Considering the limited number of rockfall blocks in the area of Lot 14, the length of horizontal runout from the steep slope to the lot Parker 303 -841 -7119 • Colorado Springs 719 - 633 -5562 • Silverthorne 970- 468 -1989 Scott S. Turnipseed, AIA April 16, 2014 Page 2 (approximately 200 feet), the intervening basin occupied by Gore Creek and being located at the lower extent of the rockfall hazard zone, in our opinion, we believe the rockfall severity to be low to moderate, but not nonexistent. Although we judge the rockfall hazard to the project site as low to moderate, rockfall by its nature is unpredictable and difficult to quantify. If a future rockfall were to hit the proposed construction, it is likely that it would result in damage to the structure and possibly harm the building occupants. If this rockfall risk potential is not acceptable to the building owners or government regulatory agencies then additional site specific rockfall study will be needed to evaluate the potential rockfall risk and feasibility of possible mitigation methods to reduce the risk. Findings and Recommendations: Based on our review of the mapped geologic conditions, review of aerial photography and the observed site conditions, in our opinion, construction of the proposed new duplex structure will not increase rockfall risks to adjacent properties or structures including public buildings, roads, street right -of -ways or utility easements or facilities. Based on the increased size of the new construction compared to the existing structure, in our opinion, the rockfall risk to the new construction could be slightly higher due to the larger building footprint but still low to moderate considering the site conditions. If you have any questions or need further assistance, please call our office. Sincerely, HEPWORTH — PAWLAK GEOTECHNICAL, INC. J es A. Parker, P.E., P.G. Reviewed by: Steven L. Pawlak, JAP /ksw Reference: 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). JOD INO. 1IJ 449A Gt�CPt@ch