Retaining walls have become a familiar sight on property used to develop new housing or commercial buildings, as ideal land for buildings becomes increasingly scarce in some markets. These structures are often used either to stabilize an unstable slope or to increase the utility of land that otherwise would not be suitable for building. For example, in some situations retaining walls allow the use of dirt fill to level off an uneven slope to allow construction of a building, parking lot, or even a street.
As demonstrated by several recent projects, retaining walls can play a major role in erosion control for public works, as well as in smaller-scale residential and commercial land improvement. Fast-track construction and appearance are two major advantages over the alternatives in many situations.
Stabilizing Banks-Attractively
A major tourist attraction in Tulsa, OK, is Oral Roberts University and several buildings on campus that are designed with modern architecture. A potentially attractive feature of the landscape is Fred Creek, a 4.5-mile tributary of the Arkansas River that winds its way through the campus. “Creek” is actually a polite term for a large body of water with a powerful flow rate that has significantly eroded its banks near the campus in recent years. In 2006, the city of Tulsa applied for a permit through the US Army Corps of Engineers (USACE) to place dredged or fill material in Waters of the United States and prepared an environmental assessment to help the USACE meet its obligations as the lead federal agency under the National Environmental Policy Act of 1969 and the Clean Water Act.
Bill Robison, senior special projects engineer of stormwater planning for the City of Tulsa Public Works, reports that the city planned to improve the creek’s conveyance to contain a 100-year flood, and erosion control was a major part of the plan. “We have extremely high velocities in this reach of channel and sandy soils,” Robison points out. “We’re looking at velocities as high as 12 feet per second in some areas, and in sandy soils you can imagine the amount of erosion occurring.” According to Robison, the creek was threatening the structural integrity of sanitary sewer mains crossing the creek, water mains under the creek, utilities serving the university, hot and cold water lines, communication lines, and the abutments of several bridges that cross the creek on the campus. Indeed, the creek’s 100-year floodplain includes a significant portion of the ORU campus. In previous years, the parking lots at the Mabee Center entertainment and sports arena and the City of Faith medical center, as well as two streets on the campus, had flooded.
The city and university have tried to control erosion from the creek by stabilizing the banks with riprap. This approach has not only required a great deal of maintenance but also created a less-than-ideal appearance. The city ultimately decided to control the erosion by having several retaining walls constructed along a 3-mile stretch of Fred Creek and its tributaries. The first two phases are built on an easement within the ORU campus extending about a mile. Crossland Heavy Construction of Tulsa began phase one in October 2007 and had nearly completed work by the end of 2008. Horizon Construction Co. of Tulsa began phase two in September 2008 and is scheduled to complete work in September 2009.
To manage the creek’s flow, a hydrologist was hired, and to ensure an attractive erosion control structure appearance, a landscape architect, Alaback Design Associates of Tulsa, was hired. The eventual choice of a segmental retaining wall design was based on aesthetic as well as functional priorities, says Robison. Up to four more phases will follow the first two as funding is obtained, he adds.
The planning team, which included engineering firm Tetra Tech of Tulsa and Alaback Design Associates, worked closely with ORU officials to ensure that the retaining wall design made allowances for future campus construction projects per the university’s master plan. Where possible, the creek is being realigned to improve conveyance and minimize erosion. “There were a few places where they were looking to expand their parking lots and add buildings, and so we actually worked proactively with that and shifted the channel in some places to allow them to build,” says Mike Peters, project manager with Alaback Design Associates. “We didn’t want to spend a lot of money on this and then in five or even 10 years they would want to put a building in.”
A section on channel creek design in an environmental assessment that Tetra Tech prepared in 2006 specified the use of “terraced stone walls…consisting of stacked rectangular boulders.” Robison points out that the walls’ high visibility warranted the use of natural boulders-or something like them.
“The ORU campus is kind of a tourist attraction and a highlight of Tulsa-it’s got really unique architecture throughout the campus,” he says. “It draws a lot of visitors, which is why there was an extensive landscaping budget within this as well. In addition to the functionality and trying to control the erosion and protect structures and enhance the hydraulic capacity of the creek, we were doing something to add to the attractiveness of the university, not detract from it.” Significantly, the budget for the first two phases was about $15 million, which, in addition to the retaining walls, includes the replacement of several bridges and utility relocations. In December 2008, Robison projected the cost of the remaining phases at about $15 million.
“It’s a very pedestrian setting; this creek winds through the heart of the campus, so it’s very visible in a lot of areas and has a lot of pedestrian activity around it,” adds Peters. “The university has a lot of trails that follow it, and there are buildings that flank it, so it’s very, very visible.”
Using actual natural boulders in retaining walls with typical heights of 6 feet entailed a major structural drawback, Robison says. “The inherent problem with that was, with the voids between the stones and with these high velocities, we were concerned about the jetting action that would occur in the bends in the channel just eating out the substrate behind the boulders,” he says.
Peters echoes the fact that a better alternative to natural boulders was available for this project. “Initially we did consider natural stone, because we were looking at habitat restoration and our intent was the natural feeling of it. We’ve done other projects with natural stone, but it’s much more difficult because the rock is not uniform and you end up with a lot of voids and, also, it doesn’t seal as well.”
To address both the structural and aesthetic requirements of the project, the city opted for the Redi-Rock segmental retaining wall system, which utilizes wet-cast concrete units weighing more than 1 ton each. With a face area measuring 5.75 square feet, the unit size is intended to eliminate the need for geogrid, even in taller structures. To address the aesthetic requirements, the team recommended the use of a limestone face instead of the other available texture of cobblestone.
Steve Browne, who works for segmental concrete wall unit manufacturer SI Precast Concrete in Ozark, MO, notes that the banks have typical slopes of 4:1 and the system’s knob-and-groove connections suit the need for setting off courses from one another by about 1.5 inches without the need for geogrid. “We try and avoid geogrid whenever possible, just because if you have to come back in the future and run a utility or repair a utility line and they cut your geogrid, your design is then flawed,” says Robison.
“One of the reasons that Redi-Rock was selected was the massive rock face. You don’t have the voids between the stones that you would get with the natural boulders, and they actually used a sandstone color stain that’s applied after the blocks are in place, so you’ve got a more uniform face,” Robison continues. “We’ve had problems with the smaller modular blocks like eight-by-sixteens. The smaller blocks just tend to blow out when you’re looking at anything like 5-feet-per-second velocities. It’s also a lot less expensive than what the natural stone would be. The other thing is the labor costs-when you’re laying stone boulders, you’ve got to fit them together like a jigsaw puzzle, whereas these are the same basic shape. You just grab the next one in line and stick it in place.”
“It still has a natural appearance, and, from a constructability standpoint, once you get the bottom course in, it’s pretty simple at that point to lay the blocks in,” Peters adds. “We also liked the ability to stain it; we basically specified a tan color to make it look like real stone, and I think that makes it look more natural. Also, Redi-Rock makes what’s considered a half-block, so you have these longer-width blocks, and there are also shorter ones that we’ve specified to mix in. So we have a combination of block sizes, which we think makes it look more natural and less uniform.” The use of so many half-units created a challenge for SI Precast, which normally supplies about 15% half units for a given retaining wall project. For the Fred Creek retaining walls, he says, about 30% of the units were half-units.
Peters argues that, even though the walls should have a pleasing appearance, they do not necessarily blend into the ORU campus. “I really think, because of the scale of this, that we were essentially creating a new look,” he says. “It’s a big enough project that it’s establishing its own character. As landscape architects, we were charged with restoring the habitat or trying to vegetate the sides, and so we wanted a natural setting. We felt like the stone had to work with that. By having a natural color and varying the sizes, we were able to give a natural look to it. In some areas, we’ve got significant elevation changes, so we’ve created planting terraces. We think that really helps in terms of not having a high, imposing wall. We can plant it and make it look a lot more natural; it’s also safer to have not so much height.”
Urgent Situation, Tight Schedule
An even more urgent situation in the Eastern United States recently benefited from the quick constructability and structural integrity of a segmental retaining wall in stabilizing creek banks. A portion of an automobile body repair shop in Putnam Valley, NY, had been condemned because the bank of Peekskill Hollow Brook had severely eroded underneath the shop’s foundation by the summer of 2008. This created the potential for the entire building to collapse into the brook, which eventually empties into the Hudson River. Matt Noviello, a consulting engineer and attorney retained by the neighboring city of Peekskill-which took responsibility for controlling the erosion because the brook empties into a reservoir providing most of its drinking water-reports that several factors necessitated the use of a retaining wall system that could be constructed quickly.
Noviello notes that not only was the body shop threatened, but also the soil on the property of George’s Super Station was suspected to be contaminated from underground fuel storage tanks from a gas station previously located there, a situation that would threaten Peekskill’s water supply. Seasonal factors that would have adversely affected retaining wall construction amplified the need for a fast-track schedule. First, construction affecting trout-populated streams such as this one is required to be completed by the end of September of a given year so as to not affect spawning. Second, officials were concerned that the Atlantic Ocean hurricane season might soon affect the weather farther inland, an occasional occurrence.
“I was representing the city of Peekskill, and we were concerned that, since the existing retaining wall was in a state of failure and had partially fallen over, if that soil got loose and eroded into the brook and put contaminated hydrocarbons into the brook, the drinking water could be contaminated for years. The cost to clean it up could be $10 million, $20 million, $40 million, and there was no alternative source of water. That’s [Peekskill’s] primary source of water and they have, at most, reserves for only about six days. Even if they tap into New York City’s water supply, they can only get 20% or 30% of their water. We had to protect that source of water.”
Noviello recommended the construction of a retaining wall consisting of the Stone Strong concrete segmental retaining wall system along a 120-foot section of the brook that flows just a few feet away from George’s Super Station. The system utilizes wall units, in this case supplied by LHV Precast Inc. in Kingston, NY, with an uncommonly massive size-3 by 8 feet-said to be the largest available for a segmental retaining wall. This system is also designed to eliminate the need for geogrid or other mechanical tiebacks in walls that are 12 to 15 feet tall. According to Stone Strong, walls as tall as 40 feet can be constructed with proper engineering. The units also have internal voids to allow filling with granular material or concrete, which creates an interconnected wall system to eliminate the need for additional excavation and external backfilling, allowing for faster construction.
“You don’t need a footing, and you don’t need forms,” Noviello points out. “With most other “˜T’-walls, you have to dig way back, because the stem of the T is what keeps the wall from tipping over. A T-wall has soil on the tail, and a poured concrete cantilevered wall has the footing sticking back into the embankment and then the weight of the soil in the embankment. You don’t need any of that with this wall, which is a mass wall: The weight of the wall keeps it from tipping over. In a case like this, we couldn’t dig back, because that’s where the contaminated soil and [fuel] tanks were. These walls are 44 inches deep from face to back, and that’s all we had.”
The first two courses of the wall had to be constructed almost entirely under Peekskill Hollow Brook. The body of water itself presented some unusual challenges that Noviello, Aphrodite Construction Co. of Putnam Valley, and the rest of the building team overcame, largely through the use of the retaining wall system. “For a traditional wall, you have to excavate it, dewater it, put forms up, and put rebar up,” Noviello notes. “All of this is nearly impossible in a stream like this because it has a sandy, gravelly bed. No matter what kind of dam you put up, the water’s going to leak underneath your dam, so you can never get it dry. We put up this wall in about 3 feet of water-you can work in muddy water and still get the job done.”
Jason Haeusgen, project manager for Aphrodite Construction, argues that keeping sediment inside a makeshift dam prior to construction of the first two courses constituted the toughest part of the project. “That was the difficult part,” he says. “We filled about 500 or 600 sandbags and put down a base underneath the water about two [sandbags] wide and two high just to get it fairly level, and then we set in the cofferdams. I wrapped them with plastic on the outside to try to keep any turbulence from coming in, and on the inside, I wrapped them with heavy filter fabric to keep the sediment inside the dam. We were contained the whole time during the construction process with minimal disturbance; we weren’t able to pump out [the cofferdams], so that was our best option.” Another reason the creek bed had to be dammed, Haeusgen adds, was that stirring up a lot of sediment would have clogged the pumps in Peekskill’s water system. “We had to make [the city] aware every day that we were working there; they never had to shut down their pumps,” he adds.
“The idea was to put up the dam so that as you stir up the water and create mud and turbidity, it doesn’t escape-it stays contained,” says Noviello. “After that, you dig the hole for your footing and you put some gravel in there and guess as to the elevation of the gravel. Then you set the block in there and level it after the block is in, which is unique to the system.” The footing used a combination of aggregate and concrete, the latter placed into the internal cavities after leveling.
With the first course erected, productivity soared, Noviello and Haeusgen note. Using a sling and hook on the excavator’s bucket, the units were set in place for the next course and aligned before concrete was placed into the cavities. The tops of the units in all the courses except the top course had inverted rebar hoops, which fit into grooves in the bottoms of units in the course above in tongue-and-groove fashion, facilitating alignment.
Noviello points out that it was much less expensive to use the excavator to position the units compared with a crane, although the excavator did not have the lifting capacity to extend its boom to its full reach and lift each 6,200-pound unit. “Let’s say you’re putting up a poured concrete wall; you have to let the concrete cure before you put a 70,000- or 80,000-pound machine on it,” Noviello says. “But with this, you set those blocks, and that same day you can put a machine on it. I’ve had to do that before, so I knew it could be done.”
“This system was brand new to us,” says Haeusgen, adding that Aphrodite Construction has constructed segmental retaining walls and poured concrete walls for residential projects. “Once you get the bottom row in, everything goes as quickly as you can pass the blocks onto each other, so the system is very efficient. As soon as you get that base level, you can go as fast as the machines can bring the blocks. Then you fill the inside with concrete, and it locks everything in.” The first two courses were nearly submerged in the water, he points out. “We have elevation rods, so we could basically follow them. Unfortunately, someone has to go in the water, but once you get that done and you get the gravel in, it’s easy from there.” Aphrodite Construction completed about 600 square feet of the 1,800-square-foot structure per day and took less than a week to complete the job.
For this structure, Aphrodite did install geotextile filter fabric behind the concrete units to ensure that sediment would not creep into the water, although soil testing later indicated that the soil was not contaminated. Both the top and bottom of the fabric were covered with aggregate, which also acts as a filter for stormwater that migrates downward through the wall.
Both Noviello and Haeusgen like the appearance of the wall, which was not stained or integrally colored but has a granite-like texture. “This is probably one of the prettiest walls when it’s completed,” Noviello says, adding that he lobbied for the use of an attractive wall system because the structure is located right off of the main road that comes over a bridge and into Putnam Valley. “It looks like hand-hewn granite-there’s a lot of granite around here, but very little of it is hand-hewn.” Adds Haeusgen: “It’s a nice-looking product as opposed to a poured wall; it has some character to it. In a community where it’s right on the edge of the water, it looked a little better than what was there before.”
