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Photo: Tensar International

Retaining walls are typically built to stabilize an unstable or eroding slope, or to create more usable level land. Numerous options are available in terms of wall type, size, strength, and appearance. Two keys to avoiding retaining wall failure are proper soil stabilization and proper drainage behind the wall.

Left unchecked, soil or gravel backfill behind a wall will exert unrelenting pressure on the structure and may ultimately cause the wall to collapse. A common method of avoiding this problem is to use geogrid, a strong synthetic mesh laid down in layers between courses of a retaining wall and extending horizontally into the soil behind the wall. Geogrid is typically supplied in rolls of material, which can be cut to whatever size is required for a specific project.

To demonstrate the effectiveness of geogrid, Joseph Kowalski, P.E., of RetainingWallExpert.com, filled a small wooden box with gravel and had an average-sized man stand on the gravel. The contents immediately compacted, and the box broke on the spot.

Kowalski then filled a similar wooden box with the same gravel, but with layers of geogrid added. This time the weight of the man was easily supported. Then, with the use of a ramp, a car was positioned with one wheel resting on top of the gravel in the box. Again, the gravel stayed in place and the box remained intact.

Applied to compacted backfill between courses of a retaining wall, geogrid greatly stabilizes the backfill, reducing pressure on the wall. It is unseen once the construction of the wall is completed.

SRW Products supplies a variety of geogrids. “They are composed of high-molecular-weight, high-tenacity multifilament polyester yarns,” the company states, “that are woven into a stable network placed under tension. The high-strength polyester yarns are coated with a PVC material.” The SRW geogrids come with a 0.75-inch by 0.75-inch aperture size and a long-term design strength ranging from a little over 1,000 pounds per foot to well over 5,000 pounds per foot.

Huesker is another major supplier of geogrids, including Stabilenka, used for reinforced earth structures; Fortrac, often used with retaining walls; and Fortrac 3D. A 21-meter-high mechanically stabilized earth wall was recently constructed at Germany’s Mercedes Benz Arena, an extension of the Forumla 1 race circuit. Fortrac geogrids of various weights were used for reinforcement. The geogrids are available in a variety of widths and lengths to reduce overlapping.

An alternative to geogrids—when there is not enough space on the site to excavate and extend the geogrid into the backfill, for example—is the use of heavy gravity walls. Redi-Rock is one manufacturer of such material. “Redi-Rock walls are used in situations when you have a relatively high wall, maybe 6 to 10 feet high, but you don’t have very much room for geogrid,” Kowalski says. “These are really big concrete blocks that form a gravity wall. They’re made of wet-cast concrete, which means they have air-entrained concrete. They’re solid concrete blocks between 28 inches and 60 inches from the face of the concrete to the rear. They weigh about 2,500 pounds per block; they form a big, heavy gravity structure that won’t tip over. They do a really good job of dressing up sites where you want a nice aesthetic appeal but you don’t have a lot of room for geogrid.”

Another common source of trouble is the accumulation of water behind a retaining wall. This can cause the wall to bow forward and ultimately collapse entirely.

“Retaining walls are designed to handle some water that gets behind them,” Kowalski explains. “They are not designed to be able to handle the amount of water generated by the roof of a building or a home. That’s too much water, and the retaining walls will fall over if those downspouts are deposited and discharge directly behind a wall.”

The material used for retaining wall backfill is also important. Heavy, clay soils will retain water for a long time, whereas light, sandy soil and small rocks or gravel allow water to drain easily. In addition, drainage systems are commonly built into or behind the wall to further reduce water pressure.

Photo: Lock + Load At the water tank site in Brookings, OR, geogrids were used behind the wall and in the fill below to help withstand seismic activity.

The Two MarketPointe site has different elevations that had to be managed with retaining walls. In selecting retaining wall materials, appearance was an important factor in addition to performance. “Ideally, we wanted to achieve a finished look that would present a higher aesthetic quality than plain concrete blocks or poured concrete,” says project manager Kevin Kangas. “The Millenia system met the project’s aesthetic requirements.”

Six retaining walls, encompassing a total of 2,325 square feet and reaching up to 10 feet in height, were supplied by Minneapolis-based Millenia Wall Solutions. The walls consisted of lightweight segmental retaining wall units, weighing just 5 pounds per square foot. After the units were placed and secured, necessary weight was achieved by adding crushed stone fill, which was brought to the wall with a front-end loader.

The retaining wall units are composed of recycled polymers, which do not absorb water or chemicals, an important consideration in the Minnesota climate. One of the retaining walls at Two MarketPointe is adjacent to a curved driveway leading down to a loading dock. In Minnesota winters, road salt might well erode the wall if it were constructed with concrete.

As explained by the wall manufacturer, “Millenia’s proprietary imaging technology enables the company to take exact impressions of stone faces from rock formations and quarried stone from which it fabricates the molds used to produce its retaining wall faces and caps. When the polymeric material is injected into the molds, it flows and fills completely to express all the fine details and subtle variations of real stone. Special paint and additives impart the look and texture of natural stone.”

To complement the grey fascia of the office building, developer Ryan selected SandStone SRW units in a slate-grey color. Kangas notes that because the resulting wall achieved a very natural look, “A landscape architect could not tell that the retaining walls are not real stone until he got right up close to them.”

St. Cloud Hospital
St. Cloud Hospital in St. Cloud, MN, is the site of a $225 million expansion project to add a new hospital wing as well as a larger warehouse, distribution center, and loading dock. Five hundred new parking spaces are also being added.

Photo: Stone Strong Systems For the Margaretville Bridge project, cavity-fill retaining wall blocks lined the creek bed and provided bridge deck support at the abutments.

Photo: Stone Strong Systems Redi-Rock Most retaining walls are built using a one-half bond pattern. The wall for the St. Cloud Hospital project, however, was constructed with a one-third bond pattern.

The majority of the wall was built using Redi-Rock 28-inch blocks and is reinforced with geogrid in every course. About 200 square feet of the wall was constructed using Redi-Rock 41-inch gravity blocks to minimize excavation at the hospital’s property line, due to the presence of underground utilities close to the building. The gravity blocks allowed the wall in this section to be built without geogrid, which kept the wall construction from disturbing the neighboring property and saved the neighbor’s mature trees. Normally, the Redi-Rock walls have a 4-degree angle of batter to them, but special blocks were made that stack completely vertically, producing a zero-batter wall.

Hardrives Inc. supplied the blocks for the project. The company’s Dominic Lundebrek described the site: “There’s a big hillside that goes down to the river, and a loading dock and a truck entrance to the hospital are at the bottom of the hillside. Right now they’re accessing it from the south end on a little, narrow trail. But the hospital expansion will block that access road. So they needed to build a new road from the north end to access the bottom of the hill, and that’s the main thing these retaining walls did— allow the construction of roadways going down the bottom of the hill. The retaining wall stands about 33 feet high at the tallest point, on the riverbank.”

At some points, the wall is less than 100 yards from the riverfront. Lundebrek adds, “It was a pretty steep hill there to start with, at the river bluffs. It was too steep to punch a road down, so the retaining wall kept the soil back and allowed us to carve a trail down to the bottom.”

Located so close to the Mississippi, the project had environmental issues as well. “It was a pretty tight spot,” he explains, “and they were concerned about any runoff or concrete. We had to be real careful about everything, making sure everything was contained and nothing could get into the river.”

For backfill, mostly onsite material was used, but some difficulties were encountered. “There was some heavy, dark, silty clay soil when they got down to the bottom, only a few feet from the top of the river level,” Lundebrek says. "They had to excavate that and put some good stuff back in. The backfill for the wall was what they had excavated for the roadways, but most of it they had to screen because there was larger rock in there; they had to get all the large rocks out so they didn’t cut through the geogrid. They had to screen everything. I believe 2 or 3 inches was the largest rock they would allow because of the geogrid.”

The drainage system used 4-inch drain tile with access points punched through the base of the wall approximately every 50 feet where a drain was put in.

The total size of the retaining wall was approximately 25,000 square feet. An unusual feature is the uncommon bond pattern used. While most retaining walls have a one-half bond pattern, meaning that each course of block is offset by half the length of the block below, the St. Cloud Hospital walls have a one-third bond pattern, so that each course of block is offset by one-third. This required installation crews to custom-cut a portion of the blocks in each course to create a unique look for the hospital’s walls.

Margaretville Bridge
The East Branch of the Delaware River, together with a number of tributaries, runs through the village of Margaretville in upstate New York. One of those tributaries is Bull Run Creek, and flooding from major storms had caused severe damage to its six-lane Walnut Street bridge.

“We were replacing flood-damaged stone walls that were on either side of the stream,” explains Andy Bell, president of A.S. Bell Engineering. “Bull Run Creek runs down through the village, and it crosses under three or four structures before it gets to the Delaware River. We had replaced one structure the previous year where they had a three-sided box culvert, and then we just continued the project on either side with a Stone Strong retaining wall.”

He adds, “While it’s not a long bridge, it’s geometrically challenging along its entire length as it winds and curves on a downhill slope over the creek with two skewed angles. So we needed to construct bridge walls that curve concentrically around the creek’s bend—and we needed to do it quickly before the next snowfall. Steel beams and a concrete poured deck would simply take too long.”

The Stone Strong Systems cavity-fill retaining wall blocks, provided by LHV Precast, served two purposes. They lined the creek bed and provided bridge deck support at the abutments. “We actually capped the walls with concrete and made a bridge seat out of them, and put precast slabs on them for a bridge,” says Bell.

Because the channel winds through the neighborhoods and was getting close to the existing houses, there was no room for the use of geogrid. Furthermore, as Bell explains, “There was no need to use a geogrid. The Stone Strong walls are such that for that height wall you don’t need geogrid—they’re self-standing. For the most part, the blocks are filled with stone. Under the bridge seat, we filled them with concrete.”

A separate drainage system was unnecessary because the stone-filled hollow cores of the retaining wall blocks act as the unit’s drainage system. The concrete-filled portions supporting the bridge deck were narrow enough not to cause drainage concerns.

Asked to identify the biggest challenge of this project, Bell says, “Mostly geometry! The two walls on either side of the creek were on concentric circles, and the street itself was on almost a 45-degree skew with the stream at that point. It didn’t make a nice 90-degree angle under the bridge; it was actually on an arc. There are three panels of the precast bridge slabs that we put down at one dimension of skew, and downstream three are on a different skew angle, so it was definitely a challenge.”

Another issue was the tight working space, with houses nearby. “We closed the road off,” Bell says. “It’s a relatively narrow street. Getting the crane in there to set the pieces and wires was a little tough. The beauty of these blocks is you can set them with a large excavator, so it wasn’t until we got to the bridge slabs that we needed the crane. You can just offload the blocks right from the truck and set them right down with a large excavator right into the stream and backfill them. We actually put a temporary dam upstream and downstream of the site and pumped the water around the site. We didn’t actually work in the stream.”

Canton Marketplace
Canton Marketplace, in Canton, GA, is a mixed-use site being developed for condos, retail space, and possibly single-family residences. As Joe Bailey of Tensar International explains, “It’s a very large project—about 75,000 to 80,000 square feet for the retaining wall—and there are some interesting features to the project. The height of the walls reaches over 50 feet in spots, and they have a broken section in which to plant trees for architectural interest. Some sections have trellises attached so that ivy can grow up on them.”

Tensar supplied geogrid for the project, while Shelby Block provided the retaining wall blocks. Contour Mesa Retaining Wall Systems handled the engineering and construction work.

Whereas a geogrid might normally extend just a few feet back into the retaining wall backfill, for Canton Marketplace it was extraordinarily long. “It’s probably a 40- or 50-foot grid length—there’s a lot of grid in this project,” Bailey says. But no anchoring was required. “They don’t need to be anchored; they don’t need to distribute stresses that way. They interlock with the soil and they distribute stresses back through the soil.”

Controlling movement of soil behind a retaining wall is crucial. “You’re assuming a wedge of soil is going to move up the face of the wall,” Bailey says. “Sometimes you have a very poor foundation and you can have stability issues. If you have a poor foundation, you can also have sliding issues. But with a proper foundation and with the geogrid, you capture a certain amount of soil. Between the frictional force it creates with the foundation and the sheer weight of it, the weight will resist overturning, and that frictional force will resist the sliding.”

For this project, the retaining wall is near vertical: “Just a tiny bit of batter,” according to Bailey. “I’ve seen the project, and it’s very impressive. It’s a good project, and it hasn’t moved. Structurally, it seems sound.”

Brookings Water Tank
When the town of Brookings, OR, decided to put in a new water tank, it had an unusual design requirement.

“The southern Oregon coast is in a similar seismic zone as San Francisco and those areas along the San Andreas Fault. It’s a pretty similar earthquake magnitude risk,” explains William Galli, senior principal with The Galli Group, the design geotechnical engineer for the project. “The project is a 1.6-million-gallon reservoir tank that they sited on a hillside. They wanted it close to the zone it would serve, and they wanted it to be at a certain elevation in order to meet pressure demands of the different zones in their city system. They found this lot—it’s not a very large lot—and it was sloping, and the project had to be designed for a 2,500-year seismic event, which comes out to about a magnitude 8.5 earthquake. It created a really interesting situation.”

Furthermore, he says, “There was an irregular rock layer an average of about 8 to 10 feet below the surface, and the soil was like a silky sand. We had them excavate all of that out from beneath the tank and for a distance below the slope of the tank and replace it with a big drainage system so they wouldn’t get any groundwater. If the groundwater comes up, it lowers the strength of the retaining wall. They put in a complete dewatering system underneath and then backfilled under the tank with 4-inch angular rock. There was a high friction angle with the rock. This continued a distance down the slope, and this was all interlocked into the rock underneath. Where the rock had an adverse slope—sloping in a downslope direction—we had them cut keys into it, so that the compacted rock above would interlock with that rock. We didn’t want any predefined slip points. Then we had the retaining wall designed so that it is well embedded down into this new rock layer that we created.”

To get the elevation needed, Galli explains, “The downslope edge of the tank needed to be about 9 feet in the air above the grade. They wanted a 12-foot-wide access road, so they ended up with an 11-foot-high retaining wall around the downslope edge.”

For backfill, angular crushed rock was used.  Galli describes the special drainage system: “We put a drainage system behind the retaining wall, although a lot of it was handled by the subdrainage system that was underneath the rock. We added some more drainage behind the walls as a precaution. They get really heavy rains on the coast—sometimes they can have 8 inches in one night. So we wanted to make sure there was no buildup of groundwater at all. The retaining wall itself was designed by an engineering firm working with Lock+Load. We stipulated that they use the geogrids behind the wall. We also used several layers of those down into the fill below to help tie the rockfill behind the wall with the rockfill in front of the wall. We wanted everything to resist as a unit. Our analysis indicated that if the wall stayed intact, it was going to withstand the load.”

It was the responsibility of The Galli Group to handle all the seismic design recommendations and the loads that the structural engineers had to work into their design.

“We did the construction control,” Galli says, “and we had people onsite when they excavated down into the rock to verify that they got the keys right and to verify the placement and compaction of all the backfill and the geogrids. A lot of people are surprised to hear what high seismic risk we have along the coast. There’s a Cascadia subduction fault off the coast where two plates meet, and one is subducting under the continental plate. That creates the potential for a large seismic event.”

Galli also describes the access issues he faced. “It was a very tight site; there was very little staging area, because the tank took up about 90% of the site. At least it seemed like it—maybe not quite that much. There was a two-lane road coming in, and it was residential, so we had to be careful about residences right around the tank. We had to be careful about noise and related issues. Also, down below the tank, across the road, was a slope that fell away very steeply about 40 feet below to a bunch of houses. So one of the big issues is that it was a high-hazard area, because if the tank failed, the people in those houses below are going to take the impact of the water. There was the relation between hazard and risk—if you have high hazard, you have to have very low risk.”

Especially when your water tank has to survive an 8.5-magnitude earthquake.