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Over the past decade, segmental retaining walls (SRWs) have surpassed treated timber and cast-in-place concrete structures to become the popular choice in residential and commercial erosion control and soil retention projects. More environmentally friendly than treated timber, and more economical to use than poured concrete, SRWs also offer ease of installation and a wide range of options in design, color, and appearance. There are numerous SRW systems on the market. Some systems have units that are hollow on the inside, while other systems offer solid block. When using the hollow system, you most likely will need to order special units to make corners and curves, and you must fill each block with crushed gravel to make the wall structurally safe. On the other hand, with a solid SRW system you avoid having special units for curves and corners - the blocks can be modified on site as needed - and the structure tends to be more durable than its hollow counterparts. The more premium systems use high-strength pins inserted into engineered slots and pinholes to make alignment foolproof and to interlock the units. "This added assurance that the wall is properly aligned is a big plus," says Rick Johnson, operations manager of Willow Creek Concrete Products, Inc., a large manufacturer and supplier of SRWs in Minnesota. SRWs as Shoreline Protection SRWs are engineered to withstand the elements in a variety of water applications - lakefronts, rivers, storm water channels, and flood control installations. They offer unsurpassed durability before, during, and after construction. The solid characteristics make these systems highly resistant to damage from rapid water flow, spalling, and impact from floating debris. SRW systems, such as solid, pinned systems, are dry stacked and do not use mortar for installation. Mortarless construction permits water to pass through joints and helps prevent excess hydrostatic pressure from forming behind the walls. With SRW systems, footings below the frost level are generally not required therefore eliminating the need for deep excavations and the use of heavy construction equipment. An interlocking system ensures that the solid, pinned units save further on time, labor, and materials during installation, while providing strength and durability. A SRW system was used in the reclamation of the Minneapolis flour-milling district, an area on the National Register for Historic Places. The project developers chose the VERSA-LOK Weathered( Standard Retaining Wall System to complement the century old mill ruins and bridge. The Weathered units offered the look and feel of timeworn natural stone, which fit perfectly with the historic feel of the project. After the initial excavation, the installation crew from Martin Lake Contracting, Inc., of Stacy, Minn., began wall construction by preparing a leveling pad using compacted, crushed gravel topped with a thin layer of sand. However, the crew quickly noticed the job was going to be anything but ordinary. The site is connected to the Mississippi River, and an adjacent lock and dam so each time the locks opened up, the tailrace (a channel that moves water away from a water wheel or turbine) would flood. Because of the fluxation in water levels, the installers laid down a soil separation fabric and an additional layer of gravel to ensure the base remained solid and firm. Then the base course of segmental retaining wall units was laid down. At the same time, riprap was also placed in the bed of the tailrace and in front of the walls to further prevent erosion and washouts. Building the tailrace wall was a challenge, laying down one course of retaining wall units at a time and interlocking them with the nylon/fiberglass pins. "The pinning system gave us more flexibility in keeping the wall straight," says Brad Koecher, crew leader with Martin Lake Contractors. "Plus, there were no hollow cores to fill, and that saved us time and made the work easier." Upstream, no more than 300 feet, lay a river inlet where water entered from the Mississippi River on its way to the tailrace 20 to 30 feet below. The concrete river inlet needed 16-foot-high SRWs to support the soil to the left and right of the inlet. Both the tailrace and river inlet walls were completed and unveiled to the public in the first phase of the Mill Ruins Park project, which is to include pedestrian/bicycle paths and the excavation of additional ruins to enhance the historic value of the area. SRWs in Flood Control Situations Whenever an SRW is placed in a permanent or periodic water environment, unique construction aspects should be addressed, including soil reinforcement, foundation requirements, internal drainage provisions, ice forces, and other specific issues. But this is true of any erosion control measure when used in a water application. To combat annual spring floods, the city of Rochester, Minn., presented the U.S. Army Corps of Engineers with a unique challenge: deepen and widen a waterway channel that was part of the city's $100 million dollar flood control project. This particular channel bisected the municipal golf course, which was in danger of losing three greens and two tee boxes to the eroding banks of the Zumbro River. The city-owned course has always been an important source of revenue to the community, and this stage was integral in the overall flood-water remedy. According to Neil Schwanz, an engineer with the Corps, a number of options were considered, such as concrete cantilever retaining walls, steel sheet piling, or SRWs. After researching the alternatives, engineers concluded that SRWs would cost the city $60,000 less than cantilevered walls and allow for quicker, easier installation. So the Corps and general contractor, Ames Construction of Burnsville, Minn., went to work installing almost 20,000 square feet of the standard retaining wall system. Because the walls are placed on nonrigid, granular leveling pads, they are flexible and can tolerate minor earth movement without damage. This feature makes them well suited to all climates, but especially suited to the freeze/thaw cycles experienced in Minnesota. The Corps, Ames Construction, and the city of Rochester were very pleased with the results. Joe Mueller, installation crew foreman with Ames Construction, noted, "Not only did the VERSA-LOK SRW save the city money, it also blended well with the surrounding golf course and Zumbro River landscapes." Tall Wall Applications using SRWs SRWs are routinely chosen as preferred alternatives to traditional retaining wall types by private clients, state departments of transportation, and the U.S. Army Corps of Engineers. With proper soil reinforcement, systems have been used to build walls in excess of 40 feet in height. These walls require no concrete frost footings. Units are placed on shallow leveling pads approximately six inches thick and 24 inches wide. Because pressures are not transferred to a footing, these tall walls may be constructed on soils with bearing capacities less than those required for other retaining wall types. As part of an economic development strategy to create more jobs, the town of Wilton, Maine, decided to support development of an old mill building in their downtown. In order to hire workers for the project, the town needed to build an adjacent parking lot to accompany the additional traffic, the only problem was that the land wasn't exactly at level grade. The parking lot was designed to complement the historic renovations in the downtown area. It was to include granite curbing, decorative lighting, trees, shrubs, and a retaining wall to negotiate the grade changes and hold back the earth from the forested hilltop above. City officials wanted a retaining wall that would enhance the natural surroundings and fit into the historic scheme and design for the reclaimed mill, while fitting the city's tight budgets. A massive retaining wall system was installed. The Mosaic(r) system is the only solid, pinned SRW system on the market offering a random-pattern appearance that lends a look reminiscent of rustic stone walls. With the Weathered texture option, the Mosaic wall offered the feel of centuries old, handcrafted natural stonewalls still found throughout the historic New England and Mid-Atlantic areas. It is imperative to note that any SRW that is to support heavy loads or exceed four feet in height requires special soil reinforcement and may also require professionally designed plans. It is important to consult a qualified engineer if unsure about any construction, site, or soil conditions. Solid segmental retaining walls offer an attractive and economical alternative in any erosion control and soil retention project. These flexible and durable walls are available in textures and colors to complement any environment, and provide a lifetime of virtually maintenance-free performance. 4

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June 18, 2019, 6:41 pm PDT

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