08-18-2025 - Agenda Packet Planning Commission - PC packet

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To: City Staff From: Kenneth W. Horns, P.E. Date: August 6, 2025 Re: Retaining Wall at 1200 Wildhurst Trail I’ve been asked to review the retaining wall constructed at 1200 Wildhurst Trail. My analysis below is based on over 40 years of civil engineering experience on a variety of site development and infrastructure projects. The existing retaining wall at 1200 Wildhurst Trail in Orono, Minnesota is a two-tiered segmental block retaining wall system that uses stacked dry cast blocks and a drainage system to resist earth pressures behind the wall. The wall system is a Winston Straight Retaining Wall System manufactured by County Materials, and is commonly used in residential landscape applications. The wall is situated near the bottom of a turfgrass slope in the back yard of a residential property, approximately 58 feet from Forest Lake Bay of Lake Minnetonka. The area immediately above the top wall tier, as well as the area between the walls, is covered by rock mulch, shrubs, and small trees. The turfgrass slope above the retaining wall is approximately 3.5H:1V (Horizontal : Vertical), and the turfgrass area below the wall generally flat. See Figure 1. Both walls are composed of five block courses (8-inch height each), plus a partial reveal of the buried base block (about 3-inches at most), resulting in a maximum wall height of 43 inches for both walls, excluding the cap block which does not provide support or earth retention. The wall heights taper to shorter heights at both ends. The top/west wall ranges in height from less than 12 inches at the south end, to a maximum height of 43 inches near the center, then again less than 12 inches at the north end. The bottom/east wall ranges in height from 17 inches at the south end, to a maximum height of 43 inches near the center, then less than 12 inches at the north end. The distance between the bottom/east wall to the top/west wall varies from 6.5 feet to 11.0 feet. I have reviewed the landscape contractor’s description of the installation, which includes details regarding the wall product used, the aggregate materials installed for the base below the wall and the drainage section behind the wall including drain tile and geotextile, and the compaction procedures performed on the base materials. A section drawing has been created based on the description provided by the landscape contractor. See Figure 2. In my professional opinion, the materials and installation procedures employed during construction of this retaining wall system are consistent with industry standards for construction of gravity (unreinforced) segmental block retaining walls. of this height and size. When the homeowner purchased the property in 2015, the backyard consisted of approximately 110 feet of a 3.5H:1V slope that slopes from the patio near the house to the east, then approximately 30 feet of a 139 Engineer's Report