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HomeMy WebLinkAboutBluff Alteration Review 2013 H . , G .._.. . June 27,2013 Project Number:13-347 Jacob Tupy J.B.Woodfitter P.O. Box 1028 Prior Lake,MN 55372 RE: Bluff Alteration Review,4853 Beach Street NE,Prior Lake,MN Dear Mr.Tupy: We understand a landscaping contractor disturbed the bluff area at 4853 Beach Street NE in Prior Lake, Minnesota while working on the adjacent property at 4865 Beach Street. We further understand the disturbances to the bluff were performed without permit(s) and were unauthorized in accordance with City of Prior Lake (City) ordinances and/or Zoning Codes. The City issued a "stop work order" on the property and requested that the disturbed areas be restored to their previous condition and requested that a slope stability analysis be performed by a registered engineer in the State of Minnesota. JB Woodfitter retained Haugo GeoTechnical Services to perform the slope stability analysis and provide recommendations for bluff restoration. This letter provides a brief description of the observed bluff disturbance(s), presents the results of the global stability analysis, and presents recommendations for restoration of the disturbed areas on the Beach Street property. Project and Site Description The site is located on the north shore of Prior Lake, along the south side of Beach Street, between Bluebird Trail and Rosewood Road,and carries the mailing address of 4853 Beach Street NE in Prior Lake, Minnesota. The site consists of a third of an acre residential lakeshore lot with the legal description of Lot 1, Block 1 of the Berggren Beach addition. A single family house existed on the property until early June, 2013 when it was demolished to make way for a new house. The site is currently vacant and the ground surface is generally exposed soil,as the excavation for the proposed new house is schedule to begin. The site sloped down from north to south, with elevations of the site ranging from about 957 to 900 mean sea level (MSL), towards Prior Lake. The southern third of the lot is designated a "bluff" with elevation ranging 936 to 900 MSL,including the shoreline of Prior Lake. To aid in our evaluation we were provided with a Certificate of Survey, prepared by Probe Engineering Company, Inc., dated June 20th, 2013. The survey included survey information, the proposed house location,the "Bluffline" (elevation 936 MSL) and topographic information. We were 13590 GROVE DIVE #2760 MAPLE GROVE, MN 55311 also provided with a copy of the Unauthorized Bluff Alteration letter, prepared by the City of Prior Lake, dated June 18th, 2013. Attached to the letter was a Certificate of Survey prepared by Valley Surveying Co.,dated June 25th,2012. Site Observations We conducted a site visit on June 20th, 2013. Based on our observations during the site visit, it appears a ramp was cut into the top of the bluff,near the center of the property from east to west,to a depth of about 4 to 5 feet. To the south of the ramp we observed some boulders and large cobbles that appeared to be placed as a temporary retaining wall to retain a mix of topsoil and the underlying lean clay soils. Global Stability Analysis We retained Gray Engineering to perform a global stability analysis of the slope in the disturbed area. Based on photos and information obtained from the site visit the disturbed area was modeled and analyzed. Results of the global stability analysis indicated a Factor of Safety (FOS) of approximately 1.37. Values greater than 1 indicate a stable slope. A copy of the results of the global stability analysis is attached. Discussion Based on the results of the global stability analysis it appears that construction activities conducted on the property have not resulted in an "unstable" slope. Our analysis has shown an FOS well in excess of 1 indicating the slope is stable in its current condition. In accordance with the "stop work order" the disturbed areas will need to be restored to their original condition. Based on our observations during the, site visit, the ramp area will need to backfilled and the soil fill and boulders will need to be removed from the bluff, south of the ramp. Finally,the bluff should be reseeded per the City's"stop work order". The following paragraphs provide recommendations backfilling and restoring the disturbed areas. Recommendations We recommend that the previously placed fill,boulders and cobbles and any other soft or loose soil placed during construction activities, if any, be removed from the disturbed area. We recommend that fill or backfill required to restore former site grades be "benched" or "keyed" into the underlying native soil to reduce the potential for slip planes to develop between the fill and underlying soil. We recommend "benching" or excavating into the slope at 1 feet vertical intervals to key the fill into the slope. We recommend each bench be a minimum of 3 feet wide. 1 35'X1 GROVE DRIVE #2780 MAPLE GROVE, MN 55311 Additional fill, if any, required to attain site grades may consist of any debris-free, non organic mineral soil. The on-site native lean clay soils stockpiled near the recently demolish house area appear to be suitable for fill placement. Organic soils such as topsoil, organic silt or organic clay, if encountered, should not be reused as fill "structural fill" to restore the ramp area. It is our opinion that topsoil or other organic soils are suitable for use within the upper 6 inches of the restored area for use as growing medium for grasses and other vegetation. We recommend that the restoration backfill placed to attain site grades be compacted to a minimum of 95 percent of its standard Proctor density (ASTM D 698). Fill should be placed within 3 percentage points above and 1 percentage point below its optimum moisture content as determined by the standard Proctor. All fill should be placed in thin lifts and be compacted with a vibratory compactor operating in vibratory mode. We recommend following the City of Prior Lake guidelines and specifications for seeding and turf restoration. Similarly, we recommend following all requirement for erosion control protection measures during the restoration. General HGTS does not practice in the field of surveying;therefore the locations and elevations referenced in this letter should be considered approximate. Haugo GeoTechnical Services LLC has used the degree of skill and care ordinarily exercised under similar circumstance by members of the profession currently practicing in this locality. No warranty expressed or implied is made. Thank you for the opportunity to assist you on this project. If you have any questions or need additional information please contact Joe Westphal at 612-269-4027 or Paul Gionfriddo at 763-954- 1101. Sincerely, HAUGO GEOTECHNICAL SERVICES,LLC Paul S.Gionfriddo,P.E. Senior Engineer 1 35'i 0 GROVE DRIVE 2713, MAPLE GROVE* MN 55311 Professional Certification I hereby certify that this plan,specification or report was prepared by me or under my direct supervision and that I am a duly licensed professional engineer under the laws of the State of Minnesota. N L WF '.4671. VS-%, LICENSED T PROFESSIONAL Joe Westphal,P.E. S ENGINEER I Project Engineer N' 49833 License Number 49833 %9���..,.� June 27,2013 ! ©F MINK Attachments: Certificate of Survey (2013) Global Stability Analysis 1 3570 GROVE DRIVE #278, MAPLE GROVE, MN 5531 1 0 0 0 0 0 0 0 Z CO .0 CO D)) Oa) 0)) 0) 0) O) 60) m0) C O Y CO m , IIIIM 111111111 H11 1111 1H1 11 . o O LII NNN (O C C) C N m U - J 0 C) m — C .:1 a)C J C) C W >, cC O 0 (O m CO m II OII O LL x _ C M U _ J J ) O C 8 N C — .- w a A o Ow N — a if w r m N m E O 0 rn • grn 0- a M 0) Y J _ O m Co U o 0 fCC rn C O a 1- CD m a O N O M O a N N O_ 0 CO CO N O a CO O O O O O OO