The Kinnickinnic River, one of three concrete-lined waterways running through Milwaukee, WI, eventually empties into Lake Michigan. The river has had its share of problems, including pollutant contamination, excessive sediment buildup, and urban flooding.
As part of its flood mitigation project, the Milwaukee Metropolitan Sewerage District (MMSD) authorized the construction of a river-lining retaining wall consisting of more than 10,000 square feet of ReCon retaining wall block.
“The goal was to get rid of the water runoff quicker, so that it would reduce the amount of flooding upstream,” explains Jeff Miller, senior project manager with Giles Engineering Associates. “The problems that were being caused by the fast-running water were something that they wanted to cure. So the waterway was redesigned at this location, and part of it was the retaining wall.
“It’s in a river valley-although I hate to use the term “˜river valley,’ because it makes it sound like it’s very deep-but it’s on the order of 40 to 50 feet elevation difference from the side down to the waterway. There are slopes on each side. To make it work from a geometry standpoint and also a hydraulic waterflow standpoint, part of the project was to hold back some of the slopes with retaining walls.”
Stan Hamilton of ReCon adds, “Features of the ReCon system that were helpful on this project included the size and mass of the block, allowing for the construction of a gravity wall and eliminating the need for additional excavation for reinforcing geogrids. In addition, the wet-cast, air-entrained mix design from which the ReCon block is manufactured has excellent durability, which is helpful in resisting the adverse impact of freeze-thaw cycles.”
Miller notes that there were cut limestone blocks on the project site from earlier retaining wall construction, and MMSD had considered using those for the new retaining wall to be built. “But to use those in a fashion that the project designers wanted would be costly, and there might not be enough material because of the hydraulics of the waterway and the water levels,” he says.
“When I was running some calculations for stability, it didn’t work out real well. So other materials were offered as an alternate, and it turned out to be a combination of ReCon walls and modular block walls above the high-water level. The ReCon block was then selected for the site, among other reasons, for the freeze-thaw resistance that it had, and for the confidence it gave the project designers.”
There were a number of engineering challenges that Miller had to deal with, including heterogeneous soil conditions, steep slopes, overall stability concerns, and fluctuating water levels. “Regarding heterogeneous soil, that refers to both natural soil and fill soil that had been placed there many years ago,” he explains. “Along with this were leftover environmental-impacted soil conditions beneath the concrete lining of the waterway that needed some excavation.”
60,000 Square Feet of Walls
In southeast Mississippi, a stretch of I-59 runs through the town of Laurel. “Years ago, when they built the interstate, I-59, they had to put a pretty significant curve in it, to miss some housing, I believe,” says Brent Quick, district area engineer for the Mississippi Department of Transportation.
“Over the years, it got to the point where there were a lot of wrecks out there, and the Department of Transportation came through and bought property and straightened out the interstate, what they called the S-curve.”
The slopes along the reconstructed highway needed to be stabilized, but there was a problem. “We did not have the money or the time to go in and buy additional right of way,” Quick explains. “We had to stay within the footprint that we were in. That is why we went with a retaining wall.”
The DOT opted to use Redi-Rock products for the 12 retaining walls on the project, totaling nearly 60,000 square feet and ranging in height from 10 feet to about 28 feet. Part of the attraction of Redi-Rock was the ability to work within tight quarters. Two of the sites lacked sufficient right of way for conventional geogrid placement, and therefore large gravity walls were built.
The largest of these, approximately 10,000 square feet in size, was constructed with a base of Redi-Rock 60-inch blocks weighing 3,290 pounds apiece. The remainder of the wall consisted of Redi-Rock 41-inch blocks, each with a weight of 2,351 pounds.
According to Jason Sailers of Piedmont Precast, the manufacturer of the blocks used on this project, “Redi-Rock was able to limit the excavation, not only in areas where it was tight and they didn’t have room for geogrid reinforcement, but also on the geogrid walls as well. In that case, the grid lengths at the bottom of the wall turned out to be a little bit shorter.”
In addition, Sailers says, “We were able to use a select backfill that the contractor wanted to use in the geogrid reinforcement areas. He wanted to use a sand material, but what we ran into down there is that a lot of guys, at least here in Georgia, will use a granite stone, which is very affordable. But once we got down into southern Mississippi, granite is not available.
“But we were able to engineer these walls to benefit the contractors with what backfill they wanted. They came to us and said, “˜We would like to use this backfill, can you design under these parameters?’ and we were able to do it.”
Piedmont Precast found that it had to dramatically increase its output to meet the project’s demands. “We added some calcium to our concrete, to decrease the curing time, so we could get the blocks fully cured, up to their proper psi,” says Sailers. “It takes 28 days to cure without calcium. With calcium, we were achieving 4,000 psi within about 12 days. They were able to put the blocks in, rather than waiting an additional 16 days for curing.”
Shotcrete Sculpting
Springfield, MA, is the home of Baystate Medical Center, a state-of-the-art facility that bills itself as the “Hospital of the Future.” Brock Gaspar, construction manager for Schnabel Foundation Co., explains that Baystate was building an addition to the hospital and wanted an attractive retaining wall as part of the project.
“The reason for the permanent retaining wall was to make space for parking next to the building there,” he says. “The parking lot was for upper-level hospital employees-doctors and nurses.
“We went with a sculpted shotcrete face to make it look nice. It’s one of those things where it was going to be seen for the long term, and they wanted a good final touch on that wall. We brought in Boulderscape to do that work.
“Behind that shotcrete facing, there is a soldier beam and lagging wall, with permanent tiebacks. This consisted of horizontally drilled holes, going back 40 to 50 feet and anchored behind the wall to hold the wall up. It’s a small-diameter hole, 4 to 5 inches, filled with grout, and then there’s a reinforcing bar that carries the load from the wall. It’s in the soil behind the wall.
“We put some insulation and drain boards in to help any water purge through the soldier beam and lagging. There are drains at the bottom of the wall. Afterward, we shotcreted the wall, with a 9- to 10-inch shotcrete facing. Once the shotcrete facing is on, the same day they shoot it, they stain it and sculpt it.
“Those guys out there at Boulderscape are very skilled at what they do,” he adds. “They’re laborers by trade, but they’re almost classified as artisans. They really do good, detailed work.”
It turns out that these artisans were called upon to make significant changes on the fly.
“Originally, the wall was supposed to be a little bit different style,” Gaspar says. “It was supposed to have more of an architectural block finish to it. So they went out there the first day and had the owner’s reps come out, and they did a trial panel right on the wall.
“They didn’t really like what they saw, so the guys out in the field said “˜Let’s try to give it more of a natural finish.’ So they went out there and sculpted the shotcrete while it was still wet, in a fashion they thought would mirror the rock quality in the local western Massachusetts area.
“The owners really liked that, so they ended up going with that new, natural-looking rock face wall, rather than a block-finish wall. That was a last-minute change the owner wanted to do, pretty much right on the spot. Then they continued right down the wall, completing it in the fashion that the owner wanted.”
Avoiding Right-of-Way Encroachment
A CVS Pharmacy was being built adjacent to a busy state highway in Lebanon, PA, and the parking lot required the construction of a
This wall’s proximity to the highway allowed no room for geogrid reinforcement.
retaining wall. The problem was that a standard segmental wall would have required a geogrid reinforcement extending well into the PennDOT right of way, which was not an option. The highway had to remain fully open during construction.
“They had to go down instead of back, so that they wouldn’t encroach,” explains Bart Shippee of Shippee Engineering, referring to the original wall design from another engineering firm. “They came up with a traditional soldier pile and lagging with precast concrete panels for the type of retaining wall.
“It’s a concrete system, where large pilings are drilled down to the bedrock. Then there are I-beams or H-beams that are inserted into the cavity that’s created, and the cavity is filled with concrete. These I-beams stick up, and then you slide precast concrete panels over them.
“The beams are installed vertically in the ground, perhaps around every 10 feet, on center approximately. Then you slip these precast panels, which might be 3 feet by 10 feet, into place, and these are stacked up. That would create the retaining wall.
“That was quite a costly solution, and when the owner started pricing the original design, he contacted me to see if we could value-engineer him some other solutions. So we looked at a couple of options for him.
“We developed what we thought was the most cost-effective solution by using the Anchorplex system, from Anchor Wall Systems, which is pretty slick. It consists of attaching segmental retaining wall units to a concrete-like mass that’s poured behind the wall.
“But it differs in several key ways from regular concrete in that we eliminate all the sand and all the fine aggregate. We intentionally create a very honeycomb-like structure that’s basically a clean gravel and cement paste. So it’s naturally free-draining and naturally very strong, once the concrete sets. That’s what we used to create our mass behind the wall, and that allowed us to really shorten up the excavation behind the wall.”
Because of the PennDOT right-of-way limitation, Shippee had very little room to maneuver. “It was quite tight,” he notes. “We were able to design a wall height of about 7 feet, 8 inches, and we only had to go back about two and a half feet behind the block units themselves. That’s not very much excavation at all. Geogrid would require a minimum of at least another 50% longer than that, which would have pushed us past the right of way.”
The road stayed open during construction. “Part of the permitting process required that we did not impede the flow of traffic. We came pretty much right up to the old sidewalk. The sidewalk stayed, and the curb and the roadway stayed. It was a safe distance away, so the traffic was not at risk. So we didn’t have to stop traffic on that state road.”
Because the owner had asked for alternatives, Shippee had also presented the option of a reverse cantilever cast-in-place wall. But as he explains, “It’s a no-brainer once you know your construction costs. Cast-in-place is always fairly expensive with anything.” He says that going with the Anchorplex system instead saved the owner approximately $160,000.
Campus-wide Retaining Walls
The University of Wisconsin-Stout is actually located in Menomonie, WI. “We’re the only UW campus that’s not named after a town,” says Mike Smith, building and grounds supervisor for the school for the past five years. “The “˜Stout’ name comes from a gentleman who was a lumber baron, who actually started the university, back in the 1800s.”
The campus, spread across 122 acres, contains a variety of retaining walls, largely to control drainage problems. Smith explains that he has had particularly good experiences with Versa-Lok products.
“The big thing for us is that I’ve used them in the past. I’ve used a few different styles. I like the pinning method, I like the weight of the stones, and I like the density. They’re solid-backed, and I’ve had really good luck with them. They have stayed in place, and I like the ability to compact aggregate behind them. They just work well for me.”
One recent project proved especially challenging.
“We built a wall behind one of our buildings in which the hill was on about a 40-degree slope,” says Smith. “It was unretained before; it was just a slope, with a catch basin at the bottom. The hill was about 20 feet high, and it drained an area up above the hill, probably about 20,000 square feet. It all went down this 40-degree-slope hill, to the catch basin on the bottom.
“It was planted in hostas and weeds. There was also a staircase that ran down this hillside to an emergency exit. But the staircase was too close to the building; it ran down the side of a wall. There was about 8 feet between the wall and the stairs, which made it not impossible to retain that, but very difficult, requiring lots of cuts and lots of funky steps.
“The first time I walked down that hill, where the water had run a path, I fell into a 4-foot-deep gully. The catch basin at the bottom had a dome cover, and it had about 3 inches of sediment built up onto the dome. It was all from that hillside.
“The other unique issue is that there was a huge oak tree on this hillside, and it was pushing 100 years old, but you could not see the root flare. So at some phase of construction, at some point in time-and this building had been there since the late 1960s-they buried the trunk of this big oak tree.”
The stairs were moved, and steps were built from Versa-Lok blocks. “We slid them over so we could actually put a wall up to the building, to stop the hill from sliding, and then we had to curve it around the oak tree. We excavated out around the tree, because we wanted to find the root flare, so it would survive.
“We also use a ton of compost, just straight compost for topsoil. In behind this wall, our top 8 inches of soil is 100% compost. It absorbs a lot more water.
“Because we only had the one storm sewer at the bottom of this retaining wall, we piped in two smaller catch basins up above it, with lawn area around it, so we could actually filter any sediment that was coming through the runoff, then pipe that down to the storm sewer at the bottom. So we’re discharging into the storm sewer, down at the bottom, but most of it is going into the ground instead of running off.”
There are a variety of retaining walls on campus using the Versa-Lok system. The smallest ones, less than about 24 inches in height, generally don’t require any support mechanism.
“Anything that’s over about 3 feet,” Smith explains, “we’ll put 12 inches of aggregate behind it, straight down to the drain tile at the bottom. Then there is a discharge grate we actually integrate right into the wall. So the tile connects to this discharge grate, and any excess water will weep out the front.
“For the taller walls, we put grid behind them, and filter fabric, and aggregate, and tile.”
Smith noted that he and two others in his department have horticulture degrees. “We live by two rules: function and aesthetics,” he says. “It’s got to work, and it’s got to look good.”
Limited Space, Limited Time
It is only fitting that the building housing the Allan Block corporate headquarters should display Allan Block products. So when the owner of the building decided to upgrade a retaining wall on the property, he selected the AB Fieldstone block.
“We went with the crushed face of the Fieldstone, as opposed to the split face of the previous block that was there,” explains Rich Lovdal, an Allan Block engineer.
Because of limited access behind the original wall, the owner chose to keep the old wall in place, with the new wall directly attached.
“They actually had both a tight space to work in as well as a limited time frame,” according to Lovdal. “Because it was essentially winter construction, they wanted to get it done quickly. They used no-fines concrete to cast between the old face and the Allan Block Fieldstone. That went into the cores of the Fieldstone, as well as behind and in that void. It adhered really well to the existing face, so the old face and the new face almost became monolithic.”
Because the same engineer that designed the previous wall did the adjustment to the design for the Fieldstone blocks, the engineering work went smoothly. The wall ranges from a height of 12 inches to 19 feet, with a total size of approximately 4,000 square feet.
“It’s basically in the parking lot,” Lovdal says, “and the contractors had saw-cut the pavement out about a foot and a half from the existing wall” to accommodate a leveling pad for the new wall. “When they completed the wall, they came in and poured a patch right to the face of the wall.”
He continues, “The pavement makes it watertight in front, and the water sheds away. The top was capped off as normal, with a little landscape fabric above the rock area, with black dirt and a vegetation mat. It’s already vegetated, with nice green grass.”
Lovdal comments, “The Allan Block Fieldstone is a two-piece unit. One of the aspects of this project is that we used the short-anchoring unit, which makes the depth of the block about 13 inches. There is a long-anchoring unit that can be dropped in; the spacing is the same. The long-anchoring unit makes the depth of the structure just under 2 feet. With that, you can build taller gravity walls without the use of geogrid.
“The units can actually be produced with locally available recycled material. It’s a nice green-friendly product that can be used for virtually any application.”
Manta Rays in the Backyard
An Atlanta condo association had a rather dated, unattractive, and overgrown two-tier retaining wall extending across the backyards of a number of its condo units. “It had very, very limited access, with only about 6 feet from the back of the units at widest area,” says George Colson, owner of R Walls Inc., who was called in to build a replacement wall. “It consisted of a 7-foot-tall wall with another 7- to 8-foot wall on top of it.
“The engineer on this project had already selected the anchors. He had actually put down that we could use either soil nails or the Manta Ray. On the upper terrace, we encapsulated the cross-tie wall, and we used Manta Ray anchors.
“We cut holes through the wall; in some areas we used MR-1s, and in some areas where the soil was really stiff and strong enough, we used MR-2s. Those anchors were pulled and tested to an 18 kip load, and then we tied a No. 4 rebar (reinforcing bar) over the face of that.
“This was then covered with shotcrete, with the Manta Rays extending through the shotcrete. On the outside of that, the engineer had us put a Versa-Lok modular block retaining wall system, with a foot of gravel behind that.
“It really had a beautiful finish on it. Once we had the upper wall encapsulated, and we had the shotcrete on top of it, the plan was to tear out the crosstie wall below that, and step back within 2 feet of the face of the upper terrace. But when we got in there, we talked with the engineer and the property owners and we made a change. We went ahead and drove all the anchors to the bottom, but we tore out the lower terrace all the way back to the face of the upper terrace. The Manta Ray anchors and the shotcrete were holding up the upper area, shoring that up.”
The result was a single 14- to 16-foot tall wall rather than a terrace. “It gave the property owners in the condo association about 7 to 8 feet more backyard. Instead of having 5 or 6 feet of backyard, now they have at least 12 feet of backyard. It gave them a lot of sunlight, and we landscaped everything.
“With the Manta Ray, I think the advantage is that they’re faster, there’s less mess, and you pull and test every anchor. With soil nails, most times they just pull a selection, a few of the anchors, and do a different kind of a test on them. With the Manta Rays, you get a good load test, and you do it every time.”