Erosion Control - March/April 2017
Changing Land Surfaces With Retaining Walls
Linda Robinson 2017-01-26 14:08:35
Retaining walls have become the modern-day structures of choice for us to manipulate the surfaces on which we choose to build our lives. From creating more buildable space on a site, to helping stabilize bridges, they are ubiquitous workhorses that are often easily overlooked. But sometimes they take their inspiration from unusual sources. A Bridge and Walking Trail for the Man in Black In 1968, Johnny Cash, the “Man in Black” recorded his 27th album live at Folsom State Prison in California. Fifteen years earlier, after watching the film Inside the Walls of Folsom Prison, he had written “Folsom Prison Blues.” That was in 1953, while Cash was in the Air Force, and it was a long time before he convinced record producer Bob Johnston to record the album live in the prison. While Cash took a break between the two sets that were played for the prisoners that day, his production crew took the opportunity to photograph the dark halls and gray walls of Folsom State Prison. Some of the surreal photos of Cash in his customary black attire, talking with prisoners and guards alike, were later published on the album. One historic photo of Cash standing in front of the walls of the prison, below the east-gate guard tower, became the inspiration for a $3.8 million hiking trail. The trail has a bicycle and pedestrian track, and a bridge spanning a roadway, complete with two large gray towers on either side. What else would such a trail be named, but the Johnny Cash Trail and Bike/Pedestrian Bridge? The city of Folsom opened the 2,250-foot multiuse path and overcrossing at the intersection of Folsom Lake Crossing Road and East Natoma Street last October, with phase II planned for completion in 2017. The design for the bridge originally called for concrete bridge abutments. However, two companies involved in the project, Holdrege and Kull (H&K) and Westcon Construction, negotiated with the city of Folsom for a design using mechanically stabilized earth (MSE) foundation abutments, as a more economical option. H&K is a firm of consulting engineers and geologists in Nevada City, CA, and Westcon is headquartered in Newcastle, CA. H&K had experience with Ultrablock and geogrid reinforcement on a previous pedestrian bridge in Auburn, CA, where designers had demonstrated that all the necessary wall components could be placed more efficiently, and that the design could conserve concrete and be aesthetically appealing. With 2-ton reclaimed concrete blocks, Ultrablock satisfied all of the necessary requirements for the retaining walls. Working with the city of Folsom, H&K provided the necessary designs for the MSE bridge abutments and consulted on bridge construction. Like its namesake, though, the Johnny Cash Pedestrian Bridge was fraught with challenges. In particular, the 25-foot-high and adjoining concrete bridge walls being spaced only 15 feet apart made stability a major concern. Dokken Engineering worked with the city of Folsom to design the paths that connect the East Lake Natoma and Folsom Lake Crossing trails. Tony Powers of Dokken explains that usually, the Ultrablock components wouldn’t necessarily require the use of geogrid. On the other hand, Chuck Kull with H&K notes that the design had a bit of a paradox, in that a wall as tall as this one would require geogrid for stabilization. These walls, however, were too close together to allow the required lengths of geogrid. Therefore, the geogrid layers would overlap and compromise the effectiveness. Powers explains that in a typical situation, the length of the geogrid is based on 80% of the height of the wall. In effect, the team resolved to shorten the wall. By backfilling the lower 10 feet of the wall with a cement-soil mixture, they reduced the amount of geogrid required. “The challenge of this project was to design the reinforcing fabric in such a way so that fabric from both sides of the bridge did not interfere with the internal stability of the walls. This was performed by offsetting the height of blocks on each side so that the fabric had sufficient soil between the reinforced layers,” says Kull. By staggering the blocks on the opposing walls, they were able to have more than a foot of soil fill material between layers of Strata geogrid. At the 25-foot wall, they began placing geogrid at blocks three and four and continued to blocks nine and 10, overlapping the geofabric from the opposite side as necessary. Finally, concrete leveling pads minimized the potential for disproportionate settling. The deck slab and copings were constructed as one unit, and essentially “pinned” into the top of the Ultrablock units “to provide a level of redundancy for seismic loads.” At the extreme top layer of the wall, Westcon constructed cone-shaped copings with precast concrete. These cones are one of the primary features that stand out to people approaching the bridge. They have made the site as inimitable as its namesake is infamous. Cash’s family made many comments at ribbon cutting about how much joy he would have taken from the entire trail and the legendary bridge. Additional retaining walls and trails are planned for the second phase of the project that’s scheduled to be completed in 2017. Beginning at Cimmarron Circle, the 1.25-mile trail includes an undercrossing at Prison Road and a 190-foot-long wooden bridge that will provide views of the American River and Lake Natoma. The “Man in Black” would appreciate the planned park, including the faux guard towers, 7-foot guitar picks, and a prison-cell- like sculpture made of giant guitar necks. The winding paths and rolling hills of oak trees offer views of the 1,200 acres that surround the 136-year old prison. Just so there are no misunderstandings, large yellow signs are posted throughout the park that read “California State Prison Property. No Trespassing.” Lowering a Road 12 Feet in Michigan In early July each year, the town of Traverse City, MI, hosts its weeklong National Cherry Festival. This area produces more tart cherries than anyplace else in the US—some 360 million pounds of cherries yearly. Most of the town’s economy is based on tourism, with the Cherry Festival bringing an estimated 500,000 visitors annually from around the world. Organizers once baked a cherry pie weighing more than 28,000 pounds, and cherry pies and preserves are in stiff competition at the festival each year. This town takes the National Cherry Festival seriously. As with any city sponsoring a major festival, Traverse City wants to ensure tourists are safe while they watch the Cherry Royale Parade and the Cherryland Band Classic. Whether they’re watching the fireworks or the long-awaited Blue Angels, festivalgoers are in good hands in this Coast Guard town. Tourists have no problem getting to Traverse City, as the local Cherry Capital Airport has regular flights from Chicago, Detroit, and Minneapolis. During summer months, airlines also provide regular flights to and from New York, Cleveland, and Denver. Smaller flights are available to parts of northern Michigan as well. Traverse City boasts state parks with some 250 campsites located just 3 miles east of downtown. The town has a thriving central business district in the downtown Front Street neighborhood and four major highway routes that move traffic through the town and outlying areas. Residents use South Airport Road to access east-west vicinities, other business locales, and Cherry Capital Airport. It’s considered one of the city’s busiest roads and, until recently, it included one of Traverse City’s most perilous intersections at Lafranier Road. A steep 12% grade at the base of Lafranier Road, combined with Traverse City’s coastal weather patterns that often lay sudden and unpredictable layers of precipitation on roadways, made stopping at the intersection during winter months difficult. To compound the problem, Lafranier Road had nothing to support the neighborhoods growing up in the area—no sidewalks, no ADA ramps, and no bicycle pathways. Nobody would argue that the street was in need of a major upgrade. Team Elmer’s, a family-owned concrete, crane, and excavation company in Traverse City, was tasked with refurbishing Lafranier Road, but with some special provisions. The Grand Traverse County Road Commission required that the 12% grade be lowered to 8%, which meant cutting out 12 feet of a 1.5-mile reach of the steep hill at the South Airport intersection. This meant Team Elmer’s would have a tight working space. The hill was in the way, which meant they had to either move it or cut it out. First, the water utilities had to be moved. The city receives a lot of snowfall and often has high snow accumulation. To protect them from freeze-thaw cycles, the utilities in this case had to be dropped 10 feet lower, to allow for good coverage after the road was moved. “All the utilities have a minimum coverage that the Department of Environmental Quality sets,” explains Jim Johnson, Grand Traverse County highway engineer. “Water and the force main are both six feet, and sewer is five, because we have a frost line here at four to four-and-a-half feet.” Team Elmer’s had additional challenges when it came to moving the water lines. The requirements included moving a 20-inch water main 10 feet lower and 15 feet away from the second operational line. Next, they could not turn off the water supply to the city in order to do the move. Team Elmer’s used a line stop to move the 20-inch water main. Line stops are used to temporarily stop the water from flowing down the main pipes during construction. They are often left in place once installed and can be used later if the utility company needs to temporarily suspend flow in the future. Al MacDonald, project manager with Team Elmer’s, says 20-inch line stops are actually rare, mainly due to their size. “We had a 20-inch water main and a utility line that had to be buried, so we had to lower that utility main. The line stop was a way to maintain flow to the city while we did that. And you have to be careful when there’s bends and elbows in the lines, because the force of the water is so strong.” The area of Lafranier Road that was cut down by 12 feet caused some distress when it came to adding the amenities for the neighborhoods. Team Elmer’s wasn’t going to have room to add a center turn lane, widen the road, and build the recommended pedestrian-ready sidewalks. Instead, the Grand Traverse County Road Commission opted to build retaining walls using Redi-Rock blocks. After excavating approximately 11,737 cubic yards of soil and sand, crews installed the walls consisting of 1,300 Redi-Rock Ledgestone wet-cast concrete blocks. Wet-cast concrete offers a greater level of detail with great durability and longevity. Walls using Ledgestone have a random, old-world stacked-stone appearance but incorporate Redi-Rock’s knob-and-groove building design. Marvin Verwys of MDC, a Redi-Rock block supplier in Charlevoix, MI, describes the project. “Each block in that system has its own function. They were 60-inches-deep and weighed 3,300 pounds each. And each wall unit extended about 300 linear feet. Team Elmer’s put the retaining wall up extremely fast—super fast, and they did a great job on it.” Beginning the project in April 2015, Team Elmer’s had a completion date of July 2, 2015, just in time for Cherry Festival traffic. Midway through, changes were made that required lowering a manhole structure, moving a fire hydrant, and incorporating a few other challenges. In spite of the two-week extension that was required, Team Elmer’s finished the project on the morning of July 2. The ribbon-cutting ceremony for Lafranier Road’s opening was held in time for holiday traffic and the Traverse City National Cherry Festival that week. Rockwall, TX, Gains Ground When Kroger’s opened its first grocery store in 1883 in Ohio, its founder Bernard Kroger had one philosophy: “Be particular. Never sell anything that you would not want yourself.” That dictum and a $372 savings account began what is now the third-largest retailer in the world. In 2015, the $109.83-billion super giant was, by revenue, the largest chain in the country, and the second-largest general retailer behind Walmart. With headquarters still in Cincinnati, the chain now has stores in 34 states, recently adding one in Rockwall, TX. Rockwall, population 37,490, is a suburb of Dallas and part of the Dallas and Fort Worth metropolis. The Rockwall area has an archaeological site that dates back 13,500 years called the Brushey Creek Clovis site. Early occupations included Caddo and Creek Indians. But the name came when the settlers dug wells in the area and found large underground rock displays, described as “sandstone rock walls with jointed wall-like features.” The US Soil Conservation Service lists Rockwall as having fine, silt soil with 0 to 5% limestone and chert gravels with calcium carbonate equivalents equal to 40 to 70%, derived from mudstone. These were the foundation for the area that became known as Rockwall. “As is often the case, the development site had a slope to it,” says Stan Hamilton, president of ReCon Wall Systems in St. Louis Park, MN. “To accommodate the new store and the surrounding parking requirements, the site needed to be level. That meant that on one side of the site, excavation of the existing slope would be required to lower the elevation, while on the other side of the site, new soil needed to be added to raise the site elevation. A stormwater detention pond was a requirement. This resulted in the need for retaining walls.” Todd Sternfeld, chief executive officer and owner of Superior Concrete in Euless, TX, which manufactured the blocks for the project, explains that the utility company had expressed some concern about where the “fill” side was. In addition, he notes, “If a pavement is close to a retaining wall, we have to be real cognizant of that. We cut down about 14 or 15 feet on that east side, on the back of where the store would be. But it’s hard to reuse that [excavated material]. We use select fill. It has a certain density. And it has to be pre-tested for adhesion, and there’s some wetting that has to be done.” Winkelmann & Associates of Dallas, TX, was the civil engineering firm on the project. As Maria Bonilla reviewed designs, she went searching for specific products that would meet the customer’s needs. She was initially looking for a geogrid wall and a taller—over 17 feet—gravity wall for the “fill” and “cut” sides of the lot. There were some challenges, though. Utility and property lines would be butting up to the backside of the wall, too close for the footprint a geogrid wall would need. And the products chosen need to have a degree of long-term durability with respect to water, because the walls would support detention ponds. When Superior Concrete and Winkelmann and Associates got together on the Kroger’s project, they found exactly what was needed—a tall wall with a small footprint. One of ReCon’s retaining walls manufactured by Superior is a rather tall gravity wall built with large wet-cast retaining wall blocks. Completing the requirement for a small footprint, Superior also manufactured and installed 5,000 square feet of concrete privacy fence that was mounted directly on top of the retaining wall. Using ReCon’s 24-inch-deep Superior Cobblestone block, the “fill” walls reached a maximum height of 16 feet. Atop a leveling pad with the drain tile, Recon’s reinforced walls typically layer the 24-inch blocks and geogrid and backfill with increasingly wider layers of drainage aggregate. “Because of the mass of the ReCon blocks, you don’t need to use as much geogrid, depending on conditions, such as distance from buildings or fire lanes. You don’t have to go as far back and use as much,” says Sternfeld. “It’s just such a robust system.” The “cut walls” were designed as gravity walls to get that minimum footprint from the very beginning. The depth of the base block on a gravity wall generally needs to be from 35 to 50% of the height of the wall, compared to a geogrid length of 60 to 80% of the height of the wall. The retaining wall on this side of the site reached a minimum height of 17 feet, 4 inches, and an 84-inch-deep block was required at the base. “These blocks weigh from 1,250 to 5,000 pounds,” says Sternfeld. Rather than using a cast-in-place barrier on top of the retaining wall where a vehicle barrier was required, Superior installed ReCon’s double-sided Guardrail block, which saved money and matched the overall look of the wall. Tale of Two Railroads, Tale of Two Streams If any town needed flood mitigation, it was Johnson City, TN, (population 60,000). Floods had historically plagued the downtown area. A multi-phase master development plan for the city included Founder’s Park, which would incorporate flood control measures and upgrade a stream system, while providing an area that was ripe for economic and cultural development. Two large streams, Brush Creek and King Creek, converge at an intersection in downtown Johnson City. Historically, these two streams have been diverted into culverts at various locations. Of course, the 16-square-mile watershed was draining to the creeks long before Johnson City and its inhabitants arrived on the scene. Then came the culverts built to transfer the creeks underground, and buildings over the culverts. Larger culverts were necessary, until they were no longer large enough to convey the heavy storm events. “Just a five-year storm would totally flood Johnson City,” says Andy Best, Johnson City’s stormwater manager. He explains that originally the streams flowed through a series of culverts, “but, in the 1970s, the Tennessee Valley Authority put in a large—a very large—box culvert.” The diverted creek ran under a warehouse, made two 90-degree turns, and then fell about 6 to 10 feet in elevation. “We wanted to make the condition smoother and better. “Historically, Johnson City had two major railroads that ran through it,” says Best. “They used steam engines that had to have water, and Brush Creek could supply that water. The railroad runs through downtown and close to the park area.” In 2007, after numerous financially unrealistic recommendations over the years, a downtown task force was established, made up of property owners, representatives of the downtown development group, and two Johnson City commissioners. In examining the ongoing problems, the task force determined that although flood mitigation was critical, the cost could be justified only if the solution included other improvements as well. LDA Engineering was selected to help facilitate meetings and provide engineering design and consulting. In 2012, the city commission approved the first project, Founder’s Park. Thomas Construction Company removed 700 feet of the Brush Creek triple-barrel box culvert under the old warehouse. The project called for an environmentally enhanced channel and construction of a 5-acre greenway along the creek banks, featuring a 200-seat amphitheater. “Founder’s Park is five acres,” says Best. “And, we have already torn down another area and put in another 1-acre pond to add on to the park.” In the design, LDA Engineering worked with Henry Benefield of Benefield Richters, a design-build company, to develop the amphitheater and retaining walls that would satisfy the challenges posed by Brush Creek. They knew that the river would, at times, submerge the walls during significant rain events. LDA Engineering knew that Keystone Retaining Wall Systems’ retaining walls would withstand both the flow of the water and rapid water drawdown as well. The retaining wall that was built at the site has a tiered arrangement that allows for stream access when the river is at low flow and retains stability during peak flows. LDA Engineering specified Keystone’s Country Manor wall system, which met the technical requirements and also fits in with the general appearance of the downtown area. The tallest wall is 10-feet-high and the longest one 933-feet-long; the total length of the tiered walls is 2,278 feet. “The retaining wall runs the entire length of the project,” says Best. “The original bedrock was good rock, so they put the footing right on the rock.” The 200-seat amphitheatre’s walls were constructed with the same Country Manor blocks. Two 8-foot-wide Contech pedestrian truss bridges span the creek: a 25-foot-long Connector-style bridge and a 35-foot-long Continental bridge. The decking on both bridges is made from Trex, a recycled wood and plastic material. Founder’s Park now provides improved drainage and a green space for locals to gather. Best says the work will continue: “Since then, we’ve bought some property upstream and will install more retaining walls there. And we still need to do some additional upstream detention ponds.” EC Linda Robinson is a journalist specializing in agriculture and land-use planning.
Published by Forester Media. View All Articles.