Retaining Walls With Many Faces

Sept. 1, 2003
Ketchum, ID, Landscape Architect Ron Adams designs for some of the most expensive properties in the Pacific Northwest, including Sun Valley residences and commercial properties. He incorporates dry-stacked rock retaining walls in many of his designs. The thing about using walls in a design, says Adams, is that they are so obvious. “But we can make them ornamental as well as functional,” he adds. Dry-stacked-rock, cast-in-place, and mechanically stabilized earth retaining walls all serve a variety of functions other than simply holding soil where we want it to stay. Adams uses walls to change berm height, for example, or to cut a pathway through a berm. Additionally, he has used walls in his designs to define space, such as walling around an open area to define that space and its desired use. “I designed a flash wall in front of a house to change the grade and pull water away from the house,” he recalls. Last but certainly not least, walls are used for pure aesthetics or, as Adams puts it, “for the beauty of the wall itself.” He often designs walls with pockets for plants or incorporates benches into his walls. “It is very inviting to people, even if they don’t use [the bench]. It provides a friendly atmosphere.” Adams’s examples of how retaining walls may function beyond their expected uses give only a small glimpse of what can be done with retaining walls. Here we will examine several case studies showing how retaining walls have been used in designs for purposes other than what might seem obvious. Tiered Planter Wall in New York
Rectangular panels were used to create a tiered retaining wall alongside a New York freeway.As part of a New York State Department of Transportation project, general contractor Schultz Construction Inc. of Round Lake, NY, was hired to construct two large mechanically stabilized earth retaining walls. In addition to the highway walls, the plans called for two large planter walls that faced a public school and Route 979. Because of the “obviousness” of the walls to the public, aesthetics was a major factor in the design. Cost was the other. Schultz contacted engineers at The Reinforced Earth Company and asked them to develop the Planter Wall System. With the help of William A. Dailey Inc. of Vermont, The Reinforced Earth Company designed the wall to provide from two to five tier levels where plants would eventually hang over. The system used rectangular panels 3 m wide with heights that varied from 1.5 to 2.25 m and reinforced soil to provide a stepped-back series of the planting tiers. After topsoil was filled in on the upper portions of each tier, the beds were planted with ornamentals. The planter wall on Ulster Avenue provides more than a 1,000 lin. m of ornamental planting beds for citizens and visitors of New York to enjoy. Providing Ventilation for a Parking GarageWhen drafter Ken August of KeyWest Retaining Systems Inc. in Wilsonville, OR, became involved in planning a retaining wall at the Ritz-Carlton in Las Vegas, NV, he was faced with many difficult challenges. The original plans had called for a cast-in-place wall to be built outside a parking garage facility at the Ritz. One problem for the owners, however, became cost. Time and materials to install a cast-in-place wall turned out to be cost-prohibitive, but August was up to the challenge of creating another form of the same wall. The Ritz-Carlton Las Vegas entry and parking garage wall. “We were able to engineer a Lock+Load wall using 20-foot geogrids,” August recalls. “But the other challenge was that we had a 20-foot row to put up a 36-foot wall. And we succeeded.” The project engineer on the Ritz parking garage, John Dunbar of Culp and Tanner Inc. in Chico, CA, describes why the wall was so important as well as challenging. “There were two reasons. One, the parking garage had a ventilation problem. Without opening up the sides to allow air to flow, we would have had to go with a very costly inside ventilation system. And the other reason was we had soil pushing up on just one side of the garage, and we had to relieve some of that pressure. But from there we got into another challenge – how far back to set the wall to still be able to call the garage ‘open.'” The parking garage dimensions were 185 x 400 ft. with five vertical levels. Dunbar says the soil wall ran the entire length of the structure and upward about three levels. While both the drafter and project engineer worked to solve their challenges, the structural engineer, David A. Hall of Hall Structural Engineering Company from Portland, OR, was working through his own challenges. “We had extremely hard soil in Las Vegas,” Hall recalls. “It had a high internal angle of friction – about 38° – where typical should be 32° to 34° for most granular soils. Organic soils should be less than 28°, and clay has no friction angle.” In addition, from the parking garage to the wall there was only 6 ft. of space. Construction crews would normally have had a very hard time excavating with a backhoe, but Hall explains that by using the Lock+Load wall system, they compacted the backfill right up to the back of the stone. August says the wall was engineered using geogrids (Stratagrid from Strata Systems Inc.). “We went from Strata 700 on the first three panels of Lock+Load, then spaced them about every 32 inches,” he explains. For the first 10 ft. of wall height, Strata 700 was used; for the second 10 ft., Strata 600; for the third 10 ft., Strata 500; and for the last 10 ft., Strata 200. Based on Lock+Load recommendations, the wall has a natural 1:10 batter. With the owners wanting to open by March 2003, builders worked diligently on the 22,000 ft.2 of wall and had it completed in a month and a half. The owners and general contractor were impressed enough with the parking garage wall that they decided to use the same type of wall on the Ritz-Carlton main entrance, for other smaller uses around the property, and for a U-shaped emergency fire access. Transforming a San Francisco HillsideSan Francisco Landscape Architect Jeffrey Miller was delighted with the challenge of transforming a rundown and neglected hillside into a small community gathering spot. The daunting 60-ft. drop over the length of the project area was going to require a special sense of space, color, and community togetherness. Left to right: Potrero Heights started with a master design plan, the hillside was initially graded into terraces, and the finshed product after installation is completed.Miller says his goal with the unused slope in the Potrero Heights area was to “make use of every square inch of the landscape space.” Of the final project, he notes, “The curving terrace walls, its highly textured and colored materials, as well as a refined sense of spatial hierarchy are a successful expression of the integration of a complex social program within the context of a challenging physical environment.” Landscaped terraces connect the private residences and offer private gardening spots. Miller developed a mini-amphitheater located next to an artist’s building for convenience. Several scaled-down community gathering spots offer seating built into the retaining walls and edible-fruit trees with ornamentals in planting beds. The area is inviting and peaceful. Adjacent to the site, native gardens bloom, with some tending by middle school students, teachers, and other community-minded neighbors. Because the project demanded large and small curves and undulating walls, Miller made use of the StoneWall Select retaining walls from ICD Corporation. One challenge was that the site’s soil, called serpentine, is considered a toxic substance when pulverized. The choice of how to handle the soil was quite obvious: Leave it alone. Miller said the decision to construct large-scale retaining walls was based on handling of the soils. “With the larger walls, we were able to design a project that created usable areas for gathering and gardening on this steep site,” Miller explains. “The StoneWall Select product was chosen because of its flexibility in forming the curving walls that we wanted; its hollow construction, which allowed for ease of handling in the difficult-to-access work area; and its positive drainage characteristics, as it makes extensive use of gravel fill in the block cells and behind the walls.” The main walls vary in height from 2 ft. around the amphitheater to 8 ft. in the wrapping garden terraces. Winding through the small community area, approximately 750 lin. ft. of walls brighten up the once-rundown area and provide vegetable and flower gardens for the community to enjoy. Naturally drought-tolerant plants and an extensive drip irrigation system support the gardens. Two Walls to Benefit the WetlandsJust 50 mi. northwest of Atlanta, GA, retaining walls were needed in a wetlands area to support an access road, preserve a stream, and accommodate a large culvert crossing. The walls also needed to confine a detention pond. Soft foundation soils made the job challenging. “We value-engineered the project to obtain the required bearing capacities without a complicated foundation system,” notes David Hesterlee, P.E., president of Contour Inc., the project installation contractor. The SierraScape Wire-Formed Retaining Wall System was used to construct the walls, which were 55 ft. high at the highest points and totaled 60,000 ft.2 For long-term stability, some of the Tensar geogrid embedments were as long as 50 ft. The system’s positive mechanical connection rods secured the geogrid to the wire forms for structural stability. “Those rods eliminated the need for BX Geogrid wrap at the face. That meant a quicker installation,” says Hesterlee. The wire forms were filled with an 18-in. layer of stone aggregate, geotextile, and sandy clay fill from the site. Contour crews of 10-20 installers completed up to 2,500 ft.2 per day. The speed of installation and the cost of this construction method compared to concrete-face retaining walls saved the developer 30-60%. “The developer saved significantly, given the size of this project,” notes Hesterlee. Contour and TET, the company that provided engineering services on the project, were called back later to install 10,000 ft.2 of Mesa Retaining Wall Systems along the development’s entrance. Sculpted Nail Walls Imitate Nature A fairly new construction technique called soil or rock nail walls was presented to the Kentucky Transportation Cabinet as an alternative to a designed cast-in-place wall along a roadway. An 8,300-ft.2 wall was constructed and faced with shotcrete, which was sculpted and stained to achieve a natural look. Pat Carr, president of The Judy Company, which performed the excavation work, notes the goal of the plan was to construct the wall in such a way that it would blend in with the natural surroundings and travelers would never know it was an artificial wall.Built in stages from the top down, the soil nail wall involved excavating 4- to 6-ft. vertical benches, then drilling holes into the soil or rock face – about 6-8 in. in diameter for soil and about 3-4 in. in diameter for rock. The holes are filled with grout, and steel bars or “nails” are inserted. When the grout sets, the nails are fitted with steel bearing plates, and vertical and horizontal drains are installed. Reinforcing steel is added to the face of the wall. A particular challenge in constructing the wall, according to Carr, was that a cemetery sits directly on top of the cut. Steve Jimenez of Boulderscape in Capistrano Beach, CA, agrees. “One problem was that they had to cut back into the hillside and then retain it,” Jimenez says. “And they wanted the wall to undulate.” Jimenez says Boulderscape first photographed the natural rock in the area, paying close attention to the leaching bands. Because the natural stone was limestone, a gray background was chosen, and the stains match the leaching minerals coming through the natural walls.After The Judy Company finished the nail wall, Boulderscape employees attached a welded wire mesh to the face, and crews applied the shotcrete over the nail wall. Jimenez says the sculptor began work while the shotcrete was still wet, sculpting natural stones and boulders in the wall. Using the photos to guide them, Boulderscape crews decided where to place bands for leaching and other color bands of stain. “We used the photos to draw out markers for placement and outcroppings along with the stratification,” Jimenez explains. “After it’s dried and cured for about one week, we stain the walls to blend with the environment, with the goal being that motorists and visitors won’t realize it is a manmade wall.” A drain field installed behind the wall moves water away from the base. What appears on the outer facing to be streaks of leaching minerals is, in reality, only stained color bands. A Shotcrete Wall in El Salvador In yet another case involving extremely difficult soils, Gerardo Osegueda Gine of C.P.K. Consultores in El Salvador studied several designs and procedures for anchoring soil after he was hired to develop and install a retaining wall to protect residential properties. A design change had lowered the elevation of a nearby portion of the site, causing an increase in the height of the cut in his area. Shotcrete is sculpted into natural forms and colors to better blend with the surrounding landscape.“A decision was made to lower the level of the street by 7 meters from the project level,” Gine describes, “causing an increase in the height of the cut from the levels that were already occupied by residents.” After studying several products and procedures, Gine and his team decided to use Foresight Manta Ray anchor tiebacks. Gine also researched the performance of other soil nail walls along with American Association of State Highway and Transportation Officials information on various design principles. “The soil conditions were identified by performing an SPT [static penetrometric test] 4 meters into the slope,” Gine explains. “It determined a soil internal friction angle ranging from 32° to 34°. And the type of soil found was silty sand with volcanic ash.” Because the cut had been made previously, Gine indicated that the crew only needed to level off the surface of the remaining embankment and apply a light layer of a water cement sealant. At that time, workers also used the hydraulic driving unit to install the MR-2 anchors. “The soil was very hard; therefore, it was difficult to drive the anchors,” Gine recalls. “By [making] predrilled holes almost to full depth of the anchors using a 4-inch-diameter hydraulic auger, C.P.K. Consultores managed to install up to 16 anchors per day using the 18-horsepower hydraulic power pack and hammer.” After covering the area with reinforced wire mesh, crews cast a 5-cm shotcrete coating and post-tensioned the Manta Ray anchors to 12,000 lb. A final shotcrete and grass seed mixture was applied to the wall so it will later grow to resemble a grassy hillside or wall to fit in with the residential environment. Precast Blocks Help Rehabilitate Famous Venice Canal System After being closed to the public for more than 30 years, the famous canals in Venice, CA, were finally rehabilitated. The Venice Canals are part of the Venice Canals Historic District and as such are listed on the National Register of Historic Places. After 30 years of misuse, however, the canals were extremely deteriorated. The Venice Canal system was becoming deteriorated by misuse. After the installation of precast cement blocks, the canal system will now last years.“Most of the existing sidewalks along the canal were severely deteriorated or absent,” says Buu Q. Luu, P.E., construction manager “The canal bottom was filled with trash, debris, and many unwanted objects. Complete canal rehabilitation was needed, and a 55° slope at the embankment was the solution selected by the city engineer.” To facilitate water circulation into and out of the canal, the bottom was dredged and reconfigured. During excavation of the canal bottom, the crews had some problems getting the materials out because the soil in the bottom was too wet and took a long time to dry out. Disposal was a slight problem because of salt content, Luu explains. The canal banks were graded, and Loffelstein precast concrete blocks – predecessors of the Verdura retaining wall system – were installed for added strength. City engineers decided on a 55° slope to provide stability and structural permanence. After new concrete sidewalks were constructed, native wetland vegetation was planted in the cells of the blocks along the canal. “The total construction cost for the entire rehabilitation of the Venice Canal system was approximately $3.5 million,” Luu reports. “This was partially funded by the fronting property owners and the City of Los Angeles.” This portion of the Venice Canal project had some special challenges, says Luu. Crews had to maintain at least some water in the canals from September to April for the California least tern bird season. Additionally, Luu explains, no canal excavation was allowed during the least tern breeding season. “The solution was to construct half of the canal system at a time while maintaining water in the other half, utilizing a bypass water system within the canals.” Despite the many licenses and permits required for the project, as well as continually informing the community of specific construction areas that were blocked off, the project was completed as planned. “The historical Venice Canal waterway system was finally rehabilitated with a project that satisfied many important objectives, including improving water quality, wetland habitat, safety, stability, aesthetics, public access, and preserving historic context,” Luu concludes. “The city’s work was recognized and appreciated by the community and the Venice Canal Association. The city is proud to have delivered this project to the community for their enjoyment, safety, and welfare since this is a unique community in the City of Los Angeles.”