Schenley Gardens: Multitiered Walls
The Schenley Gardens project, completed last year in Manassas, VA, involved a steep hill between a hotel and a retirement home. The owner wanted more useful space for guests.
Segmental Wall Specialists of Manassas designed a five-tiered space, with each tier supported by a 10-foot-high, 400-foot-long curved retaining wall. The grassy, patioed horizontal tiers vary from 10 to 35 ft. wide. Guests use the spaces for sitting and socializing, and they can readily move from one tier to another via connecting staircases.
Jim Weber, president of Segmental Wall Specialists, says his firm designs and constructs rock, steel pile, reinforced-concrete, and segmental-wall systems. He and the client first considered using reinforced concrete walls covered with a brick veneer. Weber explains that they decided to use the segmental wall system because it could be built for less than half the cost and in half the time of the concrete and veneer option because there is no formwork to build, there is no waiting for the concrete to cure, and backfilling can be done as the segmental blocks are laid. Segmental blocks come in a wide range of shapes, textures, and colors, and they make it easy to construct a wall along a graceful curve because they are tapered from front to back.
Segmental Wall Specialists used Versa-Lok blocks on this project because position easily along tight curves. Versa-Lok also produces blocks that can be used to create a parapet atop the wall. The blocks have textured front and back faces.
Design and construction flexibility was important because the owner was not sure of the exact location and curvature of the wall he wanted. Segmental Wall Specialists laid out the first few courses along a curved garden hose. The owner was unsatisfied, so the company relaid it in a new location.
Segmental-wall construction also offers easier maintenance and greater durability than a poured-in-place wall. A segmental wall is not subject to heavy lateral forces because the geogrid strengthens the soil, making it laterally stable. The 1- to 2-foot-thick aggregate drainage zone immediately behind the segmental blocks allows water to flow from the backfill through the aggregate blanket and through the nonmortared spaces between the blocks, ensuring that no wall-damaging hydrostatic pressures build up and no water lingers in the embankment to cause freeze-thaw problems in winter.
In contrast, concrete walls are inflexible against high lateral forces. Under freeze-thaw action, the concrete wall is more apt to crack, whereas a segmental wall can move slightly to relieve these forces because the foundation is aggregate, not concrete. The brick veneers used on concrete walls, Weber says, usually have a life of only 20–30 years. Water often finds its way beneath the veneer, where it can freeze and cause bricks to spall.
Soil Reinforcement and Backfill Material
Weber uses geogrid rather than geofabric on all his segmental-wall projects. Typically geogrid is laid in parallel planes spaced 2 feet apart vertically. The backfill placed atop each geogrid layer is compacted in 8-inch-thick lifts using a trench roller or by hand.
Weber is reluctant to use geofabrics because he suspects they sometimes hold water in the embankment, adding to lateral loads and creating freeze-thaw problems. From time to time, Segmental Wall Specialists is called to repair a failed wall. On some occasions, when a filter fabric positioned in a vertical plane between the drainage blanket and backfilled zone is removed, water gushes out. It would be better, Weber believes, in those situations when there is risk of water accumulation, to use 0.75-inch-diameter clean aggregate for the entire backfilled reinforced zone than to place a filter fabric vertically between the backfilled area and aggregate blanket. Weber is not entirely against filter fabrics: He often places a layer of fabric on top of an aggregate backfill. The fabric prevents the topsoil from migrating into the clean backfill.
In many cases, Weber tries to use onsite soil for backfilling–even soils rich in silt or clay–rather than hauling in material. These soils are more slippery than sandy soils or aggregate, however, and do not grip the geogrid with as much friction. The sandier the soil and the more angular its particles, the better the friction. When silt and clay soils are used for backfill, the engineer has to design for them, perhaps using a longer length of geogrid (e.g., 8 ft. rather than 6 ft.).
Weber believes that most block systems on the market provide adequate connection strength. In many cases, the geogrid is merely sandwiched between two segmental block courses and angular gravel is placed on top of the geogrid immediately behind the block, adequately connecting block and grid.
“The so-called ‘positive connection’ between block and geogrid that some manufacturers are touting is not all that important,” he remarks. “The National Concrete Masonry Association is not pushing the concept; only certain manufacturers. They are trying to get it written into specifications. We use all the segmental block systems on the market and don’t see that the systems with a positive connection between block and geogrid are any better.”