Tricking Gravity : The Art of Holding Up a Wall

Jan. 1, 2003
Oh, those walls! We take them for granted. We drive past them, under them, and over them and gaze at them absentmindedly while daydreaming. Yet most of us never think about the science behind their construction or makeup as long as they perform their function. The ancients knew the value and necessity of reinforcing soil. Papyrus was used in ancient Egypt not just to make paper but also to retain soil. Bamboo was used in part of China’s Great Wall. Retaining walls have diverse functions in the modern world, including structural walls for changes in grade, bridge abutment walls, waterfront structures, tunnel access walls, stream channelization, wing walls, and parking-area support.A major change in the use of retaining walls in the United States occurred after The Reinforced Earth Company in Vienna, VA, developed a way of building retaining walls with precast concrete panels in the late 1970s and early 1980s. The company has become a leader in highway applications because of the tremendous cost savings realized with precast panels versus cast-in-place concrete walls reinforced with steel rebar. Abutment Walls and Highway Considerations
Retaining walls on the WYE Stage 2 projectThe Reinforced Earth Company has four types of these walls all in one area on the WYE Interchange reconstruction project in Boise, ID. Stage 2 of the WYE is the second part of the project to reconstruct the junction of Interstates 84 and 184, more commonly referred to as the Connector in southwest Boise. Stage 2 was bid at $36 million and is estimated to be finished in late 2004. Ryan Ward, the project manager with Central Paving Inc. of Boise, says The Reinforced Earth Company walls and wall panels have worked well for this project. In all, Don Grabner of The Reinforced Earth says Stage 2 of the WYE project includes 2,066 panels of 73 different types. “We’ve used 42,717 square feet and 120,434 linear feet of reinforcing strips totaling 69.7 tons, with lengths from 8 to 25 linear feet, “Grabner reports. “The steel and miscellaneous materials were shipped to the contractor in February and March 2002. Panel production [precasting] started in February 2002, and panel shipments started in mid-March 2002.” The four Reinforced Earth walls include one bridge abutment on the Braid B area that incorporates 221 panels of 32 types and one that used 604 panels of 34 types. Two abutment walls were built on highway ramp areas, one using 730 panels of 45 types and the second using 511 panels of 41 types. Different Wall Constructions Not all wall structures are constructed the same. The poured-in-place reinforced concrete wall has long been a standard and is still widely used. But as managers and contractors searched for more economical ways to build walls, the demand for reinforced-soil segmental walls grew. Still, to stare at the artistic architecture in the walls, one can’t help but wonder about their strength, drainage, soil foundations, and “quite honestly” what keeps them from falling down.For segmental retaining walls (SRWs) not much higher than 4-5 ft., Minneapolis, MNÐbased Keystone Retaining Wall Systems’s block has become an industry standard, offering stability at 8 in. high and 18 in. wide. Each block is 22 in. deep to furnish the necessary stability to the nonreinforced-soil wall. But to remain competitive in the growing market, Keystone also developed a system of mechanically stabilized earth walls that can be built to greater heights with the same degree of safety. As the segmental block market developed, some disadvantages began to emerge. Metal strips used for soil reinforcement were susceptible to corrosion when they became wet or the soil behind the wall was saturated too long. According to David Ash, president of Vancouver, BC’s Lock+Load Retaining Walls Ltd., water is the primary reason for true wall failure. Don Armstrong of Anchor Wall Systems in Minnetonka, MN, adds that inadequate design, defective soil compaction behind the wall, and poor construction practices at the site are factors that contribute to failures of reinforced soil SRWs, although fewer than 1% of such walls actually fail. Soil is the fundamental component of stability of retaining walls. Whether it’s a compacted gravel base to be used for a foundation for the first layer of block or compacted fill material placed between the layers of geogrid, the integrity of the material is important. Stabilizing Slopes at a Fraction of the Cost of Cast-in-Place WallsLock+Load “stones” are used in a residential application as a simple gravity wall (no geogrid used). The stones are easily positioned using a hand truck. Landscape designers and engineers increasingly are asked to design within challenging space limitations. These sites often pose their own types of problems for the contractors. In some instances, they have to stabilize moderate to severe slopes to use the site. Rather than use the more costly cast-in-place concrete retaining walls, many contractors are using soil-reinforced retaining walls and depending on geogrid to provide the stability necessary for the wall to act as a single mass of soil. With either polyethylene or polyester geogrid layered at different elevations and backfilled with soil that is then compacted, the wall becomes comparable in strength to a cast-in-place product. The advantage for the customer is that it is less expensive. Lock+Load units have a distinction among these units because the “stone” is actually steel- and fiber-reinforced. In addition, says Ron Wormus of Lock+Load, the stones do not actually stack directly atop one another as some other wall units do. The wall banter provides flexibility and allows the blocks to move slightly with earth movement, and in some cases the offsets allow small areas for landscaping. In essence, it was economy that led the Ritz-Carlton Hotel in Las Vegas, NV, to choose Lock+Load Retaining Walls when it needed an 18,000-ft.2 wall area for a hotel parking facility. “Basically there are two types of products: precast and modular block. Precast products have a higher dollar amount associated with them, while modular block is less expensive,” explains Ash. “We provide a precast product with a modular block dollar amount attached to it. With Lock+Load systems, you could look at the stone as the rock and the sand and gravel as the mortar. So if you want to build a large wall, you want to have good, strong mortar.” Working in Tight PlacesThe Lock+Load stones are shipped in two pieces “the counterfort footing and a rocklike facing panel “and are assembled on-site after excavation by hooking up the counterfort head under the galvanized connecting loop. The stone is then tipped back into a horizontal position, and construction workers pick them up with Lock+Load’s Stone Truck to set them into position. After all the stones are aligned, level, and plumb, they are ready for backfill and compaction. The Ritz-Carlton (Las Vegas) retaining wall was built within 6 ft. of the building structure.At the Ritz-Carlton, the wall had to be constructed within a maximum 20-ft. easement behind the wall face. “The wall has a maximum height of 36 feet with a short three-to-one back slope at its top, so this was a significant restriction for both design and erection of the wall,” Wormus explains. Instead of a belowground parking facility, the Ritz-Carlton project was designed to be an open excavation just below the ground floor. This design affords more natural lighting and ventilation to the parking structure and saves on cost. Once excavation was complete, outer walls were constructed for aesthetics and to prevent erosion of the cut faces in the parking structure. The 475-ft. wall was completed by Pacific Lock+Load Inc. of Portland, OR, with Perini and Sons as the general contractor. The Ritz-Carlton Hotel facility was completed in five weeks during summer 2002 at a cost of about $400,000. Draining the Water Problems When large masses of soil are retained behind a wall, appropriate steps must be taken to prevent water building up and placing too much pressure on the wall. Crushed aggregate filled up to 1 ft. directly behind the wall and extending to the soil embankment will often allow suitable drainage away from the wall. Most of the onsite backfill can be used and compacted with the geogrid in layers, providing it is permeable material and not hardened clay-base soil. More than 18,000 ft. 2 of Anchor Vertica blocks were used for this multitiered wall project along Botany Bay.In addition to filling the area behind the wall with loose aggregate to prevent hydrostatic pressure from building up, some wall designers incorporate a drain on the bottom of the compacted fill that runs horizontally along the back of the embankment and wicks moisture from the wall. A PVC drain system then delivers the excess moisture to an area well outside the edge of the retained soil and wall area. Planning for Unreachable JobsThe size of most SRW blocks allows contractors to build walls in otherwise inaccessible locations, as when Foresight Products was called in for a special residential retaining wall in Bloomington, MN. The main problem at the site was that the geographical conditions prohibited normal excavation for installation of geogrid usually used for the type of wall requested by the client. Two walls, one behind the other, were to be installed, and the only access was from the driveway in front of them. The alternative was an expensive, cast-in-place gravity wall or a sheet-pile wall. Manta Ray anchors were used as tiebacks so the wall could be installed as quickly and inexpensively as possible. Manta Ray and Stingray are Foresight Products earth anchors that can be proof tested at the site. The anchors used at the Minnesota project were Manta Ray MR-2-24G that were equipped with ¾-10 x 6Ðft. galvanized all-thread rods. The embedment depth after load locking was 9 ft. behind the heel of the second wall. The ultimate minimum proof load at the end of the job was 10,500 lb. The Manta Ray concept worked well for the Minnesota residence. Most of the anchors were driven to a 10- to 11-ft. depth in less than five minutes and, according to the onsite contractor and Foresight Products, the anchors were driven and locked in five hours. Anchor Diamond Pro blocks in an eye-pleasing banded arrangement at the Great River Medical CenterAnchor Landmark retaining wall system with mechanical connections Making It Look Good: Blending Curves and ColorsFor all of the science, equations, and formulas of retaining walls and masonry structures, there also are times when suppliers, landscapers, and engineers are ask to satisfy cosmetic concerns. Maccaferri, an engineering firm and distributor of Anchor Wall Systems headquartered in Williamsport, MD, found this situation when designing an 18,000-ft.2 soil-reinforced retaining wall to help preserve the beauty of Botany Bay in New South Wales, Australia. When the Rockford City Council began accepting bids for the wall, one challenge was that the wall had to match the gray sandy beach for which Botany Bay is known. Pioneer Building Products, an Australian producer of Anchor Wall products, found it could match the color using specialty aggregates for the blocks. The Healing Power of Aesthetics in ArchitectureWhen making improvements to the Great River Medical Center in West Burlington, IA, Steve Levitt, the center’s director of development, wanted to include walls in the outdoor courtyards and dining areas. His specification was that the walls blend well with the hospital and offices and add a pleasing atmosphere for visitors and patients at the 102-ac. medical campus.To coordinate the hospital with the wall and landscape construction, Levitt tapped into local resources. The project includes three Anchor Wall Systems retaining walls on the medical campus. A 14-ac. lake on the premises is now surrounded with a softly curved wall that creates a nice edge around the lake while discouraging migrating geese from venturing onto the main lawn. The hospital’s brick docking area also got an 18-ft. cinder-brown retaining wall. The third wall surrounds an outdoor courtyard and dining area for employees and visitors. White and pewter Anchor Diamond Pro blocks were banded to complement the precast exterior of the medical office facility. The wall acts as an abutment for a waterfall, creating a soothing place to visit. “When we considered what Anchor Wall Systems offered, we immediately liked the economics and the enhanced appearance that you get with the block versus a poured concrete wall,” Levitt explains. He notes, “The walls add an aesthetic appeal for a healing environment.” And from the responses so far, the walls were just what the doctors ordered.