Developed in the mid-1980s, reinforced-soil segmental retaining walls are increasingly used in shopping centers, in industrial and office parks, and along highways and waterfronts. In many cases they are supplanting the traditional workhorse solution: the reinforced-concrete retaining wall.
The designer-builder is often besieged with numerous technical questions: What sort of foundation? Geogrid or geotextile for soil reinforcement? What spacing between reinforcing planes? How far back to extend the reinforcement into the embankment? How to tie the reinforcement to the wall? What type of backfill to use in the reinforcement zone? How to compact it properly?
Several in-depth case histories will answer these questions.
Lance Paradis, a civil engineer with Soil Reinforcement Design Inc. in Woodstock, GA, helped design a major segmental wall at a large site being developed for a shopping center 15 mi. north of downtown Atlanta. Known as the Forum, the site is on the west side of north-south State Route 141.
Originally the site sloped downward significantly westward from the highway. The developer wanted a flat site so the buildings would be on the same level with the parking facilities. Flattening the site, Paradis explains, meant hauling in much structural fill (a sandy silt) and constructing a 2,000-ft.-long retaining wall along the entire west boundary. The wall varies from a few feet to as high as 54 ft., pushing the height limits of segmental-wall design. It is the largest retaining wall ever built in the Atlanta region.
Paradis says the site civil engineer considered three options: geogrid reinforced slope, cast-in-place concrete wall, and reinforced-soil segmental concrete-block wall. To save money, the site engineer considered using a steep slope along the western end of the property. A stable slope must be no steeper than 2:1 (H:V). A steeper slope, given the site’s soils, would be unstable: The soil would slide or slough off.
Even a moderate slope would consume too much valuable real estate. Accordingly, the site engineer considered making the slope as steep as 1:3. This is technically feasible if the slope is properly reinforced with either geogrids or geofabrics, which Soil Reinforcement Design uses equally among its reinforced-slope projects. Based on other reinforced slopes that he has designed in the eastern United States, Paradis says geogrid and geofabric are equally effective in reinforcing a slope but that fabric is more cost-effective. The geogrid or fabric must be installed in horizontal planes, plane n + 1 being 1-3 ft. above plane n, the precise distance depending on the particular design.
Paradis preferred a reinforced slope because it’s much cheaper than any kind of retaining wall. But for the height necessary for this project, even a steep reinforced slope would use up too much valuable real estate.
The site engineer next considered the option of constructing a cast-in-place reinforced concrete retaining wall 2,000 ft. long and up to 54 ft. high. It would require massive amounts of reinforcing steel, concrete, and pricey labor. This idea sank under the weight of the cost. That left the option of a less expensive, mortarless, soil-reinforced segmental concrete-block retaining wall.
Immediately west of the wall, the site developers constructed a 400-ft.-long detention pond for runoff from the paved parking areas. The water would gradually seep into the ground. The presence of the basin, says Paradis, altered the segmental-wall design. The pond can handle a 100-year flood. The overflow from that event would seep through the segmental wall and soak the backfill. If the pond’s drainage rate was faster than the drainage rate of the backfill, the added weight would place excessive stress on the wall system. To deal with this possibility, Paradis placed extra geogrid soil reinforcement along the 400-ft.-long stretch of the wall. Engineers placed filter fabric vertically within the wall system to prevent waterborne particles in the wetted backfill region from being transported into and clogging the aggregate drainage blanket. A transverse view of the wall reveals the segmental concrete block, 12-in.-deep aggregate drainage blanket, filter fabric, and geogrid-reinforced backfill region.
Filter fabric was not used within the entire wall, declares Paradis, because there should be very little seepage into the backfill other than the stretch of wall near the pond. The surface above the backfill region will be paved, and water atop the pavement will quickly drain and be diverted to the basin.
Where the wall is only 10 ft. high, the geogrid might extend horizontally into the embankment about 6 or 7 ft. Where the wall is 54 ft. high, the geogrid might extend back as much as 48 ft. Generally speaking, the geogrid extends back into the embankment from 0.6 to 1.0 times the height of the wall at that point. Near the detention pond, the geogrid extends back 1.0 times the height to accommodate the hydraulic loads should the wall be inundated.
Paradis states that a wall higher than 60 ft. would have to bear excessive lateral loads, and the bottom blocks would have to support the weight of all the blocks above, pushing the limits of their compressive strength. The blocks used for this project, Keystone Retaining Wall’s Standard 128-lb. block, are among the biggest available. Anchor Wall’s Landmark block can also handle these compressive forces. The only solution for higher soil-reinforced walls is to design them in tiers: for example, a 40-ft.-high bottom wall and a 30-ft.-high second wall set back some distance.
On this shopping-center project, the geogrid is held in position by being sandwiched between two courses of block. The backfill used in the earth-reinforced zone is a sandy silt, some of it hauled in from off-site.
Soil Reinforcement Design was also recently called in to repair a 28-ft.-high, 200-ft.-long retaining wall in York, PA. A 50-ft. section near the midpoint failed a mere two months after construction. The main cause of failure, Paradis reports, was poor management of surface-water flows, resulting in excessive seepage into the backfill.
To repair the wall, Soil Reinforcement Design drove 180 galvanized-steel earth anchors through the existing wall for about 30 ft. into the soil. Then another segmental wall was constructed in front of the old one, tying the new wall to the earth anchors.
Paradis acknowledges that there are a fair number of segmental-wall failures, usually caused by one or more of these factors:
- Inadequate Design. The geogrids might be too short or the spacing between geogrid planes too great.
- Poor Onsite Water Management. Water buildup in the reinforced-soil embankment behind the wall adds excessive loads that cause slope and wall failure.
- Improper Construction. Incorrect geogrid orientation or improperly compacted backfill can cause failure.
- Other Factors. One possibility might be the soil’s inadequate bearing capacity.
Norm Amend, a designer and builder of retaining walls with Veco Inc. in Roswell, GA, says his firm is currently involved in the design and construction of four retaining walls at a shopping center under construction in Braselton, GA, 30 mi. northeast of Atlanta. The site engineer called for two longer walls (260 and 302 ft., with a maximum height of 25 ft.) and two shorter walls (60 ft. and 200 ft. and up to 10 ft. high).
Amend says there were only two options for the retaining wall: poured-in-place concrete and reinforced-soil segmental concrete block. He states that they have stopped costing out the poured-in-place concrete alternative. “The only option today for a retaining wall is the earth-reinforced segmental wall because of the substantial cost advantages over a reinforced-concrete wall. Contractors have confidence in these segmental walls-even for walls 40 feet or more in height.”
Amend considers segmental walls a natural for fill areas because the placing and backfilling of the geogrid or geofabric can be done while that part of the site is being filled. If the wall is being constructed in a cut area, overexcavation is needed beyond the line for the retaining wall to make room for the geosynthetics for the project.
The taller walls on this site have some curves and right-angle turns to avoid encroaching on adjacent wetlands. The 90º turns made construction difficult and required more blocks, notes Amend, but did not significantly affect the soil-reinforcement layout. This project was delayed by bad weather. Using segmental construction expedited the project, getting it back on schedule.
The walls were constructed using Anchor Landmark blocks and polyester geofabric-similar in weight to a heavy canvas-for soil reinforcement. Amend says geofabric was chosen for soil reinforcement because it was only 60-70% of the cost of the geogrid. The cost advantage is a major reason why Veco often opts for fabric over geogrid on many other projects.
The company does use geogrid on high-wall projects (greater than 25-45 ft.) where Amend believes a positive connection between geogrid and wall is especially important. Two block systems that offer this kind of connection are Anchor Landmark and Tensar Mesa Block.
One of the difficulties of segmental-wall construction is that it often cannot be built right on the property line because the wall subcontractor must be able to excavate the embankment far enough to be able to lay down numerous layers of geosynthetic and backfill. Generally, notes Amend, the soil-reinforcing zone extends behind the wall from 0.7 to 1.3 times the wall height. This means that a 30-ft.-high wall will have to be built 30 ft. from the property line, unless the developer can secure an easement from the adjacent property owner to allow excavation on his site.
Another difficulty is that building on a steep hillside might cause a landslide. A third problem is when there is not enough distance between the wall and an existing building for the reinforced-soil zone.
To remedy such situations, Veco uses a technique called intrusive reinforcement: building a retaining wall from the top downward. A few feet of soil are excavated first from the hillside or embankment. Then a series of steel helical anchors are screwed into the embankment, pitched downward slightly from the horizontal. Next, a long beam, or waler, is bolted horizontally to these anchors. The process is then repeated downward.
The waler wall provides temporary shoring for hillside support during construction and provides a permanent retaining wall. Once this wall reaches the main grading level of the site, Veco constructs a segmental concrete block wall 4 ft. in front. The segmental wall is fastened to the anchor-waler wall by geogrid looping between the two structures. The segmental wall here is merely a veneer, with the embankment retained permanently by the waler wall. This allows a retaining wall to be built very close to the property line, assuming an easement can be secured for the soil anchors crossing the property line beneath the ground.
Veco recently used this technique on a project at the K-12 George Walton Academy in Monroe, GA. To expand the bleachers at the school football field, an existing sloped embankment had to be replaced with a vertical retaining wall. The difficulty was that this existing embankment was immediately adjacent the academy’s gymnasium. The excavation would undermine the gym foundation.
Amend says Veco considered using H-piles and lagging, with a veneer over the lagging; steel sheet piling; and intrusive reinforcement, with a segmental wall placed in front. The H-pile approach was too expensive, and steel sheet piling would be too difficult to drive into the rocky, hard ground, so Amend concluded that the intrusive reinforcement would be technically and economically feasible. The anchors penetrate deep into the embankment and beneath the gym’s foundation.
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-ft.-high, 400-ft.-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-ft.-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.
Weber uses geogrid rather than geofabric on all his segmental-wall projects. Typically geogrid is laid in parallel planes spaced 2 ft. apart vertically. The backfill placed atop each geogrid layer is compacted in 8-in.-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-in.-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.”
Dick Stulgis, vice president of the environmental and underground consulting engineering firm Haley & Aldrich in Manchester, NH, says his firm mainly designs reinforced-soil segmental walls wherever a retaining wall is needed.
At a major supermarket site near Manchester, a 30-ft.-high, 500-ft.-long retaining wall was needed near the rear of the property. Haley & Aldrich considered a conventional reinforced-concrete retaining wall, a steep geogrid-reinforced slope with vegetative cover, or a geogrid-reinforced segmental wall.
The concrete wall, Stulgis says, was much too expensive and therefore rejected. Although the reinforced slope cost 20% less than the segmental wall, the owner rejected that option.
Stulgis maintains that the segmental retaining wall is less costly than a concrete wall from 4 to 50-plus ft. high, and the percentage of savings over the reinforced-concrete wall increases as the wall height increases. He uses either geogrid or geofabric. Although fabric is less expensive, Stulgis believes geogrid manufacturers have convinced many owners that the connection strength is better. He notes that the connection strength between soil reinforcement and segmental block is important, especially on high walls. The segmental wall is similar to the curtain wall on a building: The building’s frame carries the heavy loads, but the curtain wall still has to be firmly attached.
Stulgis has had to build a retaining wall where neither a cantilevered concrete wall nor a segmental wall works. Recently, Haley & Aldrich was hired by the New Hampshire Department of Transportation to widen a stretch of Kancamagus Highway in the White Mountains National Forest near Albany, NH. The project entailed cutting into the toe of a steep mountainside, possibly causing a slide. In this case, Haley & Aldrich used soil nails and shotcrete, building the wall from the top down. They excavated down 5 ft., drilled soil nails into the embankment, grouted into the annular space between soil nail and soil to fix the nails permanently, and then sprayed the exposed vertical face with a 4- to 12-in. layer of shotcrete. The workers repeated this process in 5-ft. increments until the entire 20-ft.-high wall was completed.
Stulgis believes that segmental walls are too often designed by unqualified people. Frequently they zero in on the design of the wall and reinforcing zone only, paying little attention to the actual soil conditions at the site. Such a myopic subsystem approach is a formula for failure. The wall designer, says Stulgis, must consider both soil conditions and water flows. The best precaution is for the owner or his agent to retain a qualified, successful geotechnical engineer to design the wall system, he emphasizes.
Neil Schwanz, a geotechnical engineer with the St. Paul District of the US Army Corps of Engineers, says his district has constructed a dozen segmental retaining walls along the banks of rivers and reservoirs in the past decade. Because of that experience and because it designs structures for a lifetime of 100 years or more, the corps has not yet made a full commitment to segmental walls. It still has more confidence in using cast-in-place concrete and anchored steel sheet piles for walls along riverbanks and lakeshores, Schwarz states. With more favorable field experience, he believes, the corps will build many segmental walls in the future, especially given their 50-70% cost savings over the alternatives.
One test installation lies along the Zumbro River as it passes eastward through Rochester, MN, 60 mi. south of Minneapolis, where the river periodically floods. The corps’ solution was to increase the cross-sectional area of the river by excavating its bottom and banks, Schwanz explains. Where buildings were close to the river, the banks had to be graded to a steep slope or else replaced with vertical retaining walls.
The corps has decades of experience with cantilevered concrete walls along waterfronts. Its disadvantages include having to excavate into the embankment to construct the base-slab cantilever, waiting for concrete to cure, high cost, and cracking caused by freeze-thaw.
The corps also has extensive experience with anchored steel sheet-pile retaining walls usually constructed along waterfronts where there is insufficient room to install a cantilevered concrete or reinforced-soil segmental wall. Once the steel sheet piles are in place, 30-ft.-long steel anchors are driven through and grouted to bolster the piles against lateral earth pressures. Where necessary, these anchors are driven beneath the foundations of nearby buildings. The corps is confident such an installation will last at least 100 years-even when the piles have no protective coating. A major advantage to this kind of wall is that it can be built where space is limited. Major disadvantages include high cost and poor aesthetics.
In 1993, the corps constructed a 700-ft. stretch of reinforced-soil segmental wall along the waterfront in Rochester, MN. To date, Schwanz observes, the wall shows no sign of deterioration. Such walls have promise for the corps because of lower cost, speed of construction, and appearance.
Schwanz notes that the corps designers assumed water would penetrate the wall and saturate the reinforced-soil zone. They used geogrid placed in horizontal planes 2 ft. apart. Schwanz says geogrid was designed specifically for a soil-reinforcing application but geofabric was not. There is concern that geotextile fabric could stretch or creep over time. He argues that it is important to use a clean sand backfill containing no silts or clays because of the saturation threat
