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Cover Story

Segmented Retaining Walls

Geosynthetic Materials

Geogrids are plastics formed into a very open, grid-like configuration, i.e., they have large apertures. Geogrids are either stretched in one or two directions for improved physical properties or made on weaving machinery by unique methods. By themselves, there are at least 25 application areas, however, they function almost exclusively as reinforcement materials.

Geotextiles consist of synthetic fibers rather than natural ones so biodegradation is not a problem. These synthetic fibers are made into a flexible, porous fabric by standard weaving machinery or are matted together in a random, or non-woven, manner. They are porous to water flow across their manufactured plane and also within their plane, but to a widely varying degree.

Segmented retaining walls are of mortarless construction, and that might explain why few mason contractors seem interested in constructing them. That's a shame because this is a growing market and one that fits nicely with the mason and stonemason's trade.

Segmented or segmental retain walls (SRWs) are gravity structures — they depend on the weight of the block, not on mortar, to maintain their integrity and stability. These dry stacked blocks are used for large and often angled or sloping retaining walls and also for landscape elements where there is little soil pressure behind them — around trees and plantings, rising no more than one or two courses tall.

The National Concrete Masonry Association (NCMA) represents manufacturers of the blocks and offers masons a variety of informational booklets, technical notes, and instructions on SRW, both online (at www.ncma.org) and on paper.

Segmented Retaining WallsLance Carter, manager of Engineered Landscape Products at NCMA explains, "The companies that produce the SRW units or SRW system licensors might have people come out to the site and get a contractor started when they're doing their first installation, to help them understand the process. Another avenue is our association. We have a number of resources from design manuals to inspection guides to installation guides on how the industry and association recommends installing the retaining wall systems — everything from preparing the foundation to guidelines on preparing the leveling pad for the wall itself."

NCMA defines two types of SRW: Conventional (gravity) and soil-reinforced. Both are durable, can be constructed in locations with difficult access, allow tight curves or complex architectural layouts, and provide design flexibility.

Conventional SRW
Conventional- or gravity-segmented retaining walls block the movement of the soil behind them strictly by the weight of their blocks. They can be constructed of single or multiple depths of block and the maximum wall height of a single depth wall is directly proportional to its weight, width, batter, soil condition, and site geometry. By using multiple depths of units or by using tiered construction methods, the height can be increased significantly.

Carter notes that, while masons capable to place the units in these walls, they have to rethink some of the methods they are used to using. "Getting that first course level is going to be a little bit different than what the mason contractor is used to — where even if his leveling pad isn't perfectly level he has the ability to use some of his mortar, his bedding material, to level off the block. In segmental retaining walls mortar is not used, they're dry stacked, so some of those inconsistencies are caught up through shims, small pieces of material placed between blocks as they go up. The block producers and system licensors recommend different materials for their products and what to use as shims."

Instead of a pored concrete pad, the leveling pad for an SRW is often gravel. "An SRW is placed on a compacted gravel bed that's generally 6 to 8 inches in thickness and about 6 inches wider on each side than the depth of the block. So if it's a 12-inch deep unit, they make a 24-inch pad," says Carter. In lieu of gravel, the contractor can elect to place a 6-inch minimum thick unreinforced concrete leveling pad (3,000 psi concrete). Contractors often have different opinions as to which material, concrete or gravel, expedites construction, but both are exceptable materials. Segmented Retaining Walls

SRWs are generally installed with a slight setback between units, creating a batter into the soil behind the wall. The wall batter compensates for any minor lateral movement of the block face due to earth pressures. This prevents the wall from appearing to rotate. Conventional SRWs will often incorporate increased wall batter to improve wall system stability.

Increasing the unit width or weight provides greater stability, larger frictional resistance, and larger resisting moments because the stability of the system depends primarily on the mass and shear capacity of the SRW units. All SRW units provide a means of transferring lateral forces from one course to the next. Shear capacity provides lateral stability in mortarless systems such as these. The methods used to create shear capacity include shear pins or keys, leading lips, trailing lips, clips, pins or compacted columns of aggregate in open cores of the blocks.

Soil-reinforced SRW
Soil-reinforced retaining walls are composite systems consisting of SRW units combined with a mass of retained soil, integrated with the wall by horizontal layers of reinforcement, typically a geosynthetic material (see glossary). This reinforcement increases the effective width and weight of the gravity mass, thereby increasing the stability of the wall. These materials are usually high-tensile strength sheet material that comes in rolls of varying widths and can be geogrids or geotextiles in form. However, most in use today are geogrids.

The graphic illustrates how a soil-reinforced wall is engineered with the geosynthetic material. The geosynthetic material is placed between the units along a course and extended back into the retained soil to create a composite gravity mass structure. The weight of the block, various pins and indentations, or a combination of these features retains the geosynthetic material to the wall structure. By integrating the soil with the weight of the blocks themselves, the tensions between wall and soil are used to the advantage of the builder, instead of the wall fighting against the pressure of the soil.

The mechanical wall system comprised of the SRW units and reinforced soil mass offers the required resistance to external forces associated with taller walls, surcharged structures, or difficult soil conditions. In fact, these systems are often referred to as MSE — mechanically stabilized earth — walls.

Designs and definitions
Soil-reinforced SRWs are engineered to control external stability by adjusting the geosynthetic material length — the distance it projects into the retained soil. Increasing the length increases the resistance to overturning, base sliding, and bearing failures. Sometimes the length of the uppermost layer is locally extended in order to provide adequate anchorage or pullout capacity for all the geosynthetic layers. Quality and strength of the material, along with its interaction with the soil, may affect the length of the material specified.
Segmented Retaining Walls
Image courtesy of National Concrete Masonry Association

Typically, the spacing of the material (vertically) decreases with depth below the top of the wall because earth pressures increase linearly with depth. The spacing should be limited to prevent bulging of the wall face between geosynthetic connection points and to prevent exceeding the shear capacity between SRW units.

Although the engineer or architect will normally do the calculations and incorporate them in the specifications, it's a good idea for the mason contractor to understand the concepts and design factors of these walls. As the wall goes up, the contractor will be involved in inspections and will handle the adjustments that are necessary to ensure the stability and durability of the end product. Soil conditions can also change as the site is cleared or the depth plumbed, meaning onsite decisions will need to be made to correct for abnormalities.

Drainage is essential. Normally, drainage is provided by well-graded aggregates between the wall and the retained soil. A properly designed drainage system relieves hydrostatic pressure in the soil, prevents the soil from washing through the face of the wall, provides a stiff leveling pad to support a column of stacked facing units, and provides a working surface during the construction phase. Surface water drainage should be designed to minimize erosion of the topsoil in front of the wall toe and to direct surface water away from the structure.

Wall embedment is the depth of the wall face that is below grade. The primary benefit of wall embedment is to ensure the SRW is not undermined by erosion of the soil in front of the wall. Increasing the depth of embedment provides greater stability when site conditions include weak bearing capacity of the underlying soils, steep slopes near the toe of the wall, potential scour at the toe — particularly in waterfront or submerged applications — and seasonal soil volume changes or seismic loads.

Vertical surcharge loadings are often imposed behind the top of the wall in addition to the load due to the retained soil. These loads increase lateral pressure on the structure and can be caused by a sloped backfill; a uniform surcharge due to buildings and/or parking lots; or line or point loads from heavy footings or continuous footings close to the wall. Segmented Retaining Walls

According to Carter, "It's highly recommended that the backfill be placed in thin lifts of 6 to 8 inches. If you look from the back of the block toward the fill, the first three feet should be compacted with a lightweight, walk behind compactor. One of the biggest construction pitfalls that an inexperienced contractor will have is not taking the time and effort to properly place the block and backfill. Often to speed construction, the contractor backfills behind the wall in 12-, 14- and 18-inch lifts. Either you don't get full compaction, or to achieve full compaction you move the alignment of the wall so it starts getting a bulge or lacks the proposed batter. In the worst case scenario, improper placement of wall units and backfill can result in unacceptable performance.

Success is in the details
According to NCMA, the success of any segmental retaining wall installation depends on complete and accurate field information, careful planning and scheduling, the use of specified materials, proper construction procedures, and inspection. As the mason contractor knows, it is good practice to have the retaining wall location verified by the owner's representative. Existing and proposed finish grades shown on the drawings should be verified to ensure the planned design heights are in agreement with the topographic information from the grading plan.

The contractor should coordinate the delivery and storage of materials at the site to ensure unobstructed access to the work area and availability of materials. Materials delivered to the site should be accompanied by the manufacturer's certification that the materials meet or exceed the specified minimum requirements. Construction occurs in the following sequence:

  1. Excavation and construction of the leveling pad.
  2. Setting, leveling and backfilling base course.
  3. Placement and backfilling of units in succeeding courses.
  4. Placement, tensioning, and backfilling of soil reinforcement (when required).
  5. Compaction of backfill to the specified density.
  6. Capping and finish grading.

As with any structure used to retain soil, careful attention should be paid to the compaction equipment and procedures used during construction. When compacting soil within 3 ft (0.9 m) of the front face of a wall, compaction tools should be limited to hand operated equipment, preferably a vibrating plate compactor. Reinforced soil can be compacted with walk-behind or self-propelled riding compaction equipment.

Tips and tricks
NCMA's Carter considers the backfill process to be extremely important for a good, solid SRW. He tells mason contractors, "One trick, when placing the soil in lifts of 6- or 8-inches, the height of the unit, right behind the wall, they can fill in behind the wall at a thickness of say half the height and then walk that half height lift in with their foot. Generally the soil behind the wall is placed with a front-end loader, but as they spread it out the mason can walk down and push that soil against that block with his foot. A simple method but it saves time and effort in the compacting of the backfill. Then it will take less effort to compact the material directly behind the wall face with light-weight compaction equipment."

According to Steve Hooker, national sales manager for Rockwood Retaining Walls, Rochester, Minn., all SRW systems require some alteration at the job site. "A hand splitter that can split concrete units easily, versus a hammer and a chisel, can save a lot of time and time is money," he says. "Ensure that the base pad or leveling pad is 100 percent level. This will prevent adjustments or shims between successive courses. Be sure to obtain proper compaction and compact every course to ensure that the system will work. It also will save time in the long run." Segmented Retaining Walls

Hooker makes a commonsense statement saying, "Building one wall twice will erode profits from other jobs. That means do the wall right the first time. Follow the design manual or instructions for the block being installed and the engineer's design. This will release the installer from liability in case of problems if the wall proves to have been installed correctly."

He adds, "Always have a soil test done. It is a small price to pay to ensure that the soil layers below and around the wall are taken into consideration during the design phase."

Carter knows that some contractors are not familiar with the geosynthetic material, and when doing soil-reinforced SRW that can be a problem. "Geosynthetic material, especially those used for retaining walls, generally has a high-strength direction and a weak-strength direction. There's one direction that has a higher tensile capacity and that's the strength that needs to be placed perpendicular to the wall, because that provides the reinforcement to help stabilize the structure. In relatively small roll widths, it's fairly obvious when you roll them out that the strength direction is in the direction of the roll width. With wide rolls, some of the manufacturers do put markings on the outer edges of the roll with arrows saying strength direction. Yet, it is quite common for no clear marking to be present on the geosynthetic roll and the contractor should take care to label the materials in the field prior to cutting. Colored paint is commonly used to identify both strength direction and material types."
Segmented Retaining Walls
Image courtesy of National Concrete Masonry Association

So there you have it, a possible new area for you to explore in expanding your business. It's not hard to make the transition as Carter says, "Once they've mastered placing the block and getting the fill in 6- to 8-inch lifts, it's just a repetitive sequence as they continue up the wall."



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