SRWs Create Landscape Design Interest

By Alan Starling

 

North Central University sits in the heart of downtown Minneapolis, so space for students to congregate is limited. The amphitheater style setting provided an ideal area for enjoying the outdoors in a courtyard area between a cafeteria and dormitory.

Ever since significant use of segmental retaining wall units (SRWs) began in 1984, unique applications have been implemented by innovative wall contractors. One has only to look back 3000 years to the ziggurats of ancient Babylonia to discover interesting soil reinforced design. While technically not SRWs, the ziggurats, including the Tower of Babel, were constructed using technology similar to today?EUR??,,????'???s soil-reinforced SRW design by utilizing woven reed mats between layers of soil.

An accepted design methodology coupled with multitudes of color, texture and shapes provides the contractor with the confidence and palette to go ?EUR??,,????'??outside?EUR??,,????'?? of traditional wall design. SRW units are being used to increase useable space while enhancing the landscape. Planter boxes, columns, water features and seat walls are all examples of interesting SRW designs. More structural, nontraditional uses of SRWs include amphitheaters, living or plantable walls, parapet walls and land bridge walls.

According to Sam Miller, P.E., president of Scott Miller, Consulting Engineer, Little Rock, Arkansas, SRWs lend themselves to graceful radiuses, 90-degree corners, stairs and columns. ?EUR??,,????'??I have designed segmental retaining wall to include steps, planters, walls, columns, retaining walls and parapet walls all incorporated into the same project.?EUR??,,????'??

Water features incorporated into retaining wall structures are growing in demand due to aesthetic beauty and the therapeutic effects.

One of the hottest things in the green industry are water gardens. SRWs can be combined with concrete paving stones and natural stone to create ponds and waterfalls, complete with lush vegetation and aquatic life. Tumbled-style SRW blocks in warm, earth tone hues can help complement the natural look. By virtue of being relatively inert, concrete blocks are environmentally friendly and will not introduce toxic chemicals into the environment or break down and decay over time. In areas where freeze/thaw is an issue, blocks must meet testing criteria of ASTM C1262. Important design considerations include following correct line and grade to ensure that water moves properly and is contained, ensuring that proper liners are used behind the blocks, as SRWs are permeable systems. You must take measures to avoid scour and erosion behind the walls, especially in areas of cascading or flowing water.

Raised planter boxes, especially ones designed with multi-piece block systems like the Anchor Highland Stone produced by Pavestone Company, are another interesting example of SRW design. Owing to the flexibility of the SRW units, 90-degree corners, sweeping radiuses and tiers can all be built into a single structure. These can safely be built to heights of four feet with no geosynthetic reinforcement. As these types of structures tend to contain saturated soils, drainage rock and filter fabric are essential for long-term performance and to retain the soil medium required for plantings. Additionally, many manufactures also produce paving stones in colors and textures that can accentuate the SRW.

SRW?EUR??,,????'???s tame a 10-foot grade change from house to outdoor living.

Terraced wall applications are another area where contractors can show their creativity. Terraces help break up the continuity of a single, tall wall, provide plantable areas, and, with the addition of benches or tables, can add useable, livable space between tiers. The individual walls can parallel each other, bend in graceful radiuses, move away from each other or join back together in a single, taller wall. Planter boxes can be designed into the walls and stairs, and blend in well with terraced SRWs. The primary design consideration to remember with terraced walls is the 2 to 1 rule: walls must be spaced twice the height of the taller wall apart from each other to function properly as separate walls. Walls spaced closer together must be designed as a single tall wall. Terraced walls should always be designed by a licensed engineer and should be evaluated for global stability. Additionally, providing proper drainage is essential.

Residential yards can be a great proving ground for unusual SRW designs. Jill Alcantara with Summerset Pools, Plano, Texas, working in conjunction with Mike Henjum and Randy Miller of Concrete Paver Systems, Dallas, Texas, renovated tired rail road tie walls into a backyard showplace for a Coppell, Texas family. Faced with decaying and failing railroad tie walls and wood decking, a challenging 10-foot grade change from house to pool and numerous large pecan trees, Mrs. Alcantara had to stabilize the grade change and create a safe, comfortable stair assembly to the pool area. SRWs replaced a portion of the tie walls, an innovative double-sided wall supported portions of the new concrete paver and wood deck, providing protection for the 36-inch caliper trees. An L-shaped stairway was constructed of straight split SRW units with a sizeable landing between sets of steps, large enough to be furnishable. Complementary colored concrete paving stones were adhered on top of the straight split blocks, functioning as the tread on the steps; wrought iron handrails and fencing were added. Additionally, a freestanding column topped by a large gas lantern was built with SRW units and incorporated into the stair assembly. The SRW units chosen for this project were buff and terra cotta and were installed in a monochromatic blend to achieve the look the homeowner desired. Mrs. Alcantara notes, ?EUR??,,????'??This was a challenging project and accomplished only because of cooperation between the pool and wall contractors. The SRW system we chose was builder-friendly.?EUR??,,????'??

The artist designed this light rail transit station, near a city lake, to invoke a feeling of cascading water down a hillside.

Another interesting SRW design is to incorporate plants directly into the wall fascia. Philip Lamb, the design artist of White Rock Lake Station, Dallas Rapid Transit, solved the retaining issues on site by utilizing an SRW. The wall faces the highway in a cascade of earth-tone blocks with pockets for plants between the blocks. A green oasis of plants greets passengers, creating a rich, natural environment and the feeling of cascading, flowing water. Walls designed with live plantings should be capable of handling saturated soil conditions and need to be built with filter fabric to prevent soil from washing out the wall face. Irrigation systems also need to be designed integral to the wall and plant species need to be carefully chosen to ensure that they can tolerate the higher temperatures that can be associated with concrete blocks.

Land bridges can be built for driveway crossings or to cross small streams and gullies to connect different tracts of land. A common scenario is to supply businesses with additionally parking. Constructed with SRW blocks, they offer a more economical solution than casting a concrete structure and offer aesthetics untouched by plain gray concrete. In wet applications, the construction site must be dewatered and sufficient blocks must be embedded to prevent scouring or undermining of the walls. Additionally, some form of erosion protection must be provided at the base of the walls. A culvert sized to allow normal water flow is built through the wall and filter fabric should extend at least as high on the wall as the 100-year flood height. Additionally, rock riprap should be placed on the downstream exit of the culvert to prevent scour holes from developing.

Amphitheater seating with SRWs is also becoming increasingly popular. SRWs can tame difficult transitions in grade while providing stable, all-weather seating. Schools, municipalities and park systems are attracted by these systems that turn otherwise unusable space into focal points. Straight-faced seat walls are the norm, but multi-piece tumbled style SRWs can be very attractive. Global stability is of primary concern and must be addressed by a qualified geotechnical or civil engineer.

SRWs need not be limited only to mundane changes in grade but can be utilized in unlimited ways. They provide the economics, aesthetics, design flexibility and structural stability. Compared to other retaining wall choices, SRWs are often the most logical solution for the contractor.