About 20,000 square feet of blocks was used in the construction of this wall at a commercial development project in El Dorado Hills, Calif. It is about 20 feet high and according to the engineering specialist on the project, was one-third the cost of the nearly $1.5 million dollars proposed for a cast-in-place concrete wall.

These tiered segmental retaining walls were built at a residence in La Crosse, Wis. For the low-voltage lighting, the contractor ran the wiring as the blocks were installed. Then the lights were attached to the faces of the units. The blocks' solid cores reduced the risk of breaking the faces.

Even though the foundation of concrete building materials can be traced back to ancient Egypt, segmental retaining wall units made from concrete are a relatively new innovation, only landing on the market in the mid-1980s. Around the same time, geogrid soil reinforcing was introduced. Combining these technologies produced a way to build structurally strong, earth-retaining walls, economically and efficiently.

For these reasons, SRWs became popular for everything from residential landscaping to large-scale applications such as commercial erosion control. Because of this popularity, they are very familiar to landscape contractors. Even so, to assure the use of best practices, LC/DBM sought out the advice of industry experts.

SRWs offer a number of obvious advantages over other wall types. They are less expensive and can be installed more easily and quickly than hand-stacked stone walls. Compared to concrete cast-in-place walls, SRWs have more design flexibility.

About 20,000 square feet of blocks was used in the construction of this wall at a commercial development project in El Dorado Hills, Calif. It is about 20 feet high and according to the engineering specialist on the project, was one-third the cost of the nearly $1.5 million dollars proposed for a cast-in-place concrete wall.

These tiered segmental retaining walls were built at a residence in La Crosse, Wis. For the low-voltage lighting, the contractor ran the wiring as the blocks were installed. Then the lights were attached to the faces of the units. The blocks' solid cores reduced the risk of breaking the faces.

"Retaining wall systems can be used to create a variety of designs," says Ken Jopp, marketing manager for Versa-Lok, "from a 4-foot-tall garden wall to a 40-foot-tall serpentine retaining wall. You can create multi-angled curves and corners. This versatility is an advantage to contractors."

SRWs also have superior aesthetics including, as mentioned by Al Pfannenstein, a national advisor for Belgard, the option of having blended colors.

Josh Peterson, field superintendent for Villa Landscapes in Oakdale, Minn., adds, "Gabion walls lack the aesthetic of SRWs. With boulder walls you don't have a nice, finished even top, so rock and mulch spills over into the voids. Plus the taller the boulder wall you build, the more heavy equipment you need."

Both Pfannenstein and Matt Walz, a staff engineer at Rosetta Hardscapes, point out the benefit, from a quality control standpoint, of starting with a product that has already been engineered. Pfannenstein offers up a less obvious advantage. A cast-in-place wall in cold climates requires installation below the frost line to insure that it doesn't fail in the winter.

"In Minnesota, you have to dig down an extra 55 inches and fill it full of concrete," says Pfannenstein.

Since SRWs are drystacked, they are flexible, which allows the units to move and adjust relative to one another with the movement of the ground, and not show signs of distress. This is true not only in soil shifts caused by freezing and thawing, but also by earthquakes as SRWs have proved to be very forgiving in some of the biggest recent temblors, including the magnitude 8.8 quake in Chile in 2010.

Belgard's new MegaTandem is a wall block system whose size and mechanical structure allow for the building of gravity walls up to 10 feet high without reinforcing. It was designed for retaining wall projects where it is difficult or impossible to excavate for stabilization. The blocks feature a positive mechanical connection using reinforced polypropylene connecting members.

The Claremont Collection, one of Rosetta Hardscapes newest innovations, is designed for seatwalls or small retaining walls. Each unit is finished on both the front and back surfaces. Their Outcropping Collection can often rely on the massive size of each block (up to six feet in length and up to two feet in height) to hold back the earth without geosynthetic reinforcement.

The amount of loading put on a retaining wall depends on the slope, the soil conditions, and the weight of the structures and other masses behind it. When the load is greater than one wall can withstand, multiple walls can be built with terraces between them. These walls are built with Techo-Bloc monumental wall blocks, which are one of the company's larger products (15.75 inches tall, 20.5 inches deep, 31.5 inches long).

On the list of cost-effectiveness benefits, The National Concrete Masonry Association includes, "on-site soil can usually be used, eliminating costs associated with importing fill and/or removing excavated materials."

Groundwork
As for best practices, Pete Baloglou, the director of education and information, and sales director at Techo-Bloc, starts us out at the bottom - the foundation.

"In real estate, it's location, location, location," states Baloglou. "In SRWs, it's compaction, compaction, compaction: ensuring the foundation, soil reinforced zone, toe soils, etc., have increased density to ensure stability of the structure."

"Proper base preparation is a critical element in the construction of your retaining wall," agrees Walz. "Not only is it important to provide a stable foundation for the wall, but a properly prepared base will greatly increase the speed and efficiency of your wall installation."

He advises that the excavation for the base should expose fresh, undisturbed soil or rock, and that all disturbed soils that fall in along the base of the wall should be removed. "The NCMA design standard requires SRWs to be constructed on a six inch compacted course-graded aggregate material," says Pfannenstein. "If you are dealing with poor foundation soils, instead of putting in six inches as leveling pad, you might put in 12 to 18 inches to build up a firmer foundation for the wall to set on. You reduce the potential for any differential settlements – where a wall dips down where the foundation was not strong enough to support the weight of it."

He adds that there are times when the soil is so poor, such as spongy clay soil, that it might have to be replaced.

Building Up
With a properly prepared foundation, the blocks can begin to be placed. Walz's advice includes "wall construction should start at a fixed point such as a building wall, 90 degree corner, or at the lowest elevation of the wall. Check all blocks for level and alignment as they are placed. If you take the time to set the bottom row properly, installation of the upper rows of blocks is much easier and more efficient."

A tip from Richard Bodie, the vp of commercial sales for Pavestone LLC, is to run a string line along the back of the foundation pad to help align the blocks in the base course.

Another tip from Walz is to place and compact open-graded crushed stone in front of the bottom row of blocks to help hold them in place, and put washed drain stone or open-graded crushed stone in any voids between the blocks.

One way to structurally reinforce walls is with a geosynthetic web (as opposed to structural backfill). The recommended minimum length of the geogrid used is 60 to 70 percent of the wall's height. For drainage, walls require at least 12 inches of free-draining aggregate.

A gravity wall depends on its weight, the batter, and the friction between these Pavestone Anchor™ Highland Stone® segmental blocks to resist the load behind it while allowing common bond lines of varying heights.

Backing Up
The backfill is of critical importance. The minimum recommendation calls for 12 inches of clean open graded stone behind SRW systems. However, Baloglou makes a case for considering upping that to 36 inches. Benefits include the time and money saved by not having to measure and then compact the column behind the backfill, greater freeze/thaw resistance and better filtration and drainage.

On the topic of filtration, Baloglou offers, "Filtration has historically been handled naturally with the use of aggregates. With the advent of geotextiles, aggregate has been replaced by specified geotechnical fabrics.

"Using an industry approved fabric to segregate the soil reinforced zone or retained fill, from the column of clean stone behind the wall has become the unspoken rule. Recently, a fear of that fabric clogging and the subsequent hydrostatic pressure and lateral load build up has arisen. Three-fourth-inch, clean stone has billions of receptor points, and at a three foot depth would take exponentially longer to clog than a fabric."

He does allow, however, that geotextiles are important as a separator between the foundation soils and base material, and between the low permeability clay fill and the column of clean stone.

Adequate drainage is a necessity as Walz points out that, "the number one cause of retaining wall failure is hydrostatic pressure buildup behind a wall as water drains after a storm or from natural ground water seepage."

As a first line of defense, Baloglou advises that every effort be taken to keep surface water from approaching the wall, such as redirecting water away via a swale or berm. The risks from ground water also have to be addressed. Here again he cites a reason to increase the backfill to three feet in depth, as it will act as both chimney and blanket drains keeping sediment and water from reaching the SRW units.

Baloglou does acknowledge that a cost/benefit analysis should be done before deciding on boosting the backfill. Weighed against the benefits already mentioned are the costs, which include the added stone, as inexpensive as it can be, digging and hauling away the extra soil.

Bodie advises that for drainage, a drain tile be placed as low as possible behind the wall, so water drains down and away from the wall into a storm drain, or to an area lower than the wall.

Jopp adds, "Because the wall is built without mortar, water can weep through the joints to reduce hydrostatic pressure as well."

Calling in Support
"Unreinforced gravity walls typically can be built up to four feet high, depending on soil and other site conditions," says Jopp. "For taller walls, geogrid soil reinforcement is required to stabilize soil. With proper engineering and soil reinforcement, segmental retaining walls can be built up to 50 feet and higher."

Geogrid material installed at varying heights in a wall provide tensile strength to hold the reinforced soil together, so it behaves as one coherent mass, large enough and strong enough to resist destabilizing loads.

Bodie advises that when installing geogrid, to pull the reinforcement taut and pin the back edge in place with stakes.

"If there are any wrinkles," says Pfannenstein, "then there is any movement, it may allow the wall to move."

Versa-Lok's guidelines point out that adjacent sections of reinforcement should be placed immediately next to each other without overlap, but without gapping also.

To cap off the best practices advice, Versa-Lok suggests that installing caps is the one time that adhesives are appropriate, especially if the caps are overhanging the units below. But the recommendation is to not use rigid adhesives or mortar since the natural movement that the walls are designed for could cause the rigid adhesive bond to fail.

In spite of its relatively recent introduction to the list of features that landscape contractors can offer, segmental retaining walls, especially those used for erosion control, have proved to be, and will continue to be of great value to professionals and their clients alike.