Repairing a Failed Wall
Soil Reinforcement Design was also recently called in to repair a 28-foot-high, 200-foot-long retaining wall in York, PA. A 50-foot 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 feet 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.
Braselton: Four Walls
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 miles 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 90-degree 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.
No Space for Soil Reinforcement
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-foot-high wall will have to be built 30 feet 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 feet 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.
