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.
Lance 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- 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.
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 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.