2011-01547 (zoning- stairway to lk)

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, , • Grading Plan w/Soil Boring Locations (1 and 2)—Otto and Associates, dated 9/7/11 • Independent Testing Technologies, Inc Soil Boring B-1 (10/25/11 • Independent Testing Technologies, Inc Soil Boring B-2 (10/25/11) • Direct Shear Test (Sample A) — Date Reported 10/31/2011 • Direct Shear Test (Sample C) —Date Reported 10/31/2011 Site Observations: The existing slope is approximately --50' in overall length and drops in elevation � 35', resulting in a slope angle of approximately 35°. There is an existing stairway made of timber which winds down the slope, extending from the top of the slope to the bank near the lake. There are three existing timber walls. There is one wall near the top of the slope, one near the mid slope and a small wall near the base of slope. The walls were previously cut and installed into the bank to accommodate the installation of the timber stairway. The existing timber stairway and retaining walls are relatively stable but are experiencing deterioration. To my knowledge the existing retaining wall was installed over 20 years prior. It is also my understanding that there have been no slope failures in the vicinity of the Koch Residence, as per William Koch. Project History: It is my understating that a new staircase is to be constructed above grade on posts with concrete footings, extending down the existing slope as shown on the Stair System Plan created by Concept Landscaping. Concept Landscaping has proposed removing the existing timber wall and creating a new timber wall structure and burying the retaining wall structure with fill so it is no longer visible. The slope is then to be vegetated as shown on the Planting Plan created by Concept Landscaping. The City of Orono required the slope be analyzed and approved for global stability after the proposed new retaining wall was constructed and buried with fill to the finished grade shown on the Grading Plan prepared by Otto and Associates. Gray Engineering, LLC (Gray) was subsequently contracted to analyze the slope. To obtain the necessary information for the analysis, they self performed 3 bucket auger borings at undisclosed locations near the top of the slope. They extended two borings to a 10' depth, and 1 boring to a 6' depth. The material was predominantly classified as sandy lean clay by Gray. Gray then performed a global analysis for the project using GSLOPE Limit Equilibrium Slope Stability Analysis software, recognized and approved software within the geotechnical engineering community. Gray assumed the strength of the native site material to be cp = 28° and C = 50 psf. After inserting this and other strength of material parameters into the global stability analysis software, it yielded a safety factor of 1.03. Gray therefore concluded that this safety factor did not meet the required minimum and suggested that additional engineering of the retaining wall system would be required. The minimum safety factor far a global stability analysis is typically required to be a minimum of 1.3 throughout the retaining wall engineering and geotechnical engineering community Civil Design Professiona/s • 8609 Lyndale Ave. S.,Suite 200 B/oomington,MN 55420 • Office 952-303-5312 • www.cdp-us.com 2