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-builderis 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.
Atlanta: 54-Foot-High Wall
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-foot-long retaining wall along the entire west boundary. The wall varies from a few feet to as high as 54 feet, 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 feet 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 foot long and up to 54 foot 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-foot-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-foot-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-inch-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 foot high, the geogrid might extend horizontally into the embankment about 6 or 7 feet. Where the wall is 54 foot high, the geogrid might extend back as much as 48 feet. 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 feet 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-foot-high bottom wall and a 30-foot-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 offsite.