Segmental retaining walls (SRWs) are gravity retaining walls that rely primarily on their mass (weight) for stability. The system consists of concrete masonry units which are placed without the use of mortar (dry stacked), and which rely on a combination of mechanical interlock and mass to prevent overturning and sliding. The units may also be used in combination with horizontal layers of soil reinforcement which extend into the backfill to increase the effective width and weight of the gravity mass.

Design Flexibility - The SRW system is composed of units whose size and weight makes it possible to construct walls in the most difficult of locations. Curves and other unique layouts can be easily accommodated. Segmental retaining walls have the ability to function equally well in large-scale applications (highway walls, bridge abutments, erosion control, parking area supports, etc.) as well as smaller residential landscape projects.
Aesthetics - Since SRW units are available in a variety of sizes, shapes, textures and colors, segmental retaining walls provide designers and owners with both an attractive and a structurally sound wall system. 

Ease of Installation - Most SRW units can be placed by a single construction worker. The dry stack method of laying units without mortar allows erection of the wall to proceed rapidly.

Economics - SRWs provide an attractive, cost effective alternative to conventional cast-in-place concrete retaining walls. Savings are gained because on-site soil can usually be used eliminating costs associated with importing fill and/or removing excavated materials, and because there is no need for extensive formwork or heavy construction equipment.

Durability - Segmental units are manufactured of high compressive strength, low absorption concrete which helps make them resistant to spalling, scour, abrasion, the effects of freeze-thaw cycles, rot, and insect damage.

Performance - Unlike rigid retaining wall structures, which may display cracks when subjected to movement, the flexible nature of segmental retaining walls allows the units to move and adjust relative to one another without visible signs of distress.

Site Design Consideration - Typical designs and specifications for segmental retaining walls should be prepared by a designer who has technical knowledge of soil and structural mechanics. Each SRW unit manufacturer can provide design information tailored to that product, which will indicate the wall heights and design conditions when an SRW should be designed by a qualified engineer. In addition, SRW systems should be designed by a qualified engineer when:

  • structures will be surcharged
  • walls will be subjected to live loads
  • walls will be founded on poor foundations
  • the nature of the design conditions requires special consideration.

Geosynthetic Length and Spacing - For soil-reinforced segmental retaining walls, geosynthetic reinforcement increases the mass of the composite SRW structure, and therefore increases the resistance to destabilizing forces. Length of the geosynthetic is typically controlled by external stability calculations. Increasing the length of the geosynthetic layers increases the SRW's resistance to overturning, base sliding, and bearing failures. In some instances, the length of the uppermost layer(s) is locally extended in order to provide adequate anchorage (pullout capacity) for the geosynthetic layers. The strength of the geosynthetic and the frictional interaction with the surrounding soil may also affect geosynthetic length.
A sufficient number and strength of geosynthetic layers must be used to satisfy horizontal equilibrium with soil forces behind the wall and to maintain internal stability. In addition, the tension forces in the geosynthetic layers must be less than the design strength of the geosynthetic and within the allowable connection strength between the geosynthetic and the SRW unit.

Drainage System - Drainage is an essential part of a properly designed SRW. Drainage materials are generally well-graded aggregates. A properly designed drainage system relieves hydrostatic pressure in the soil, prevents retained soils 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 construction. 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.

Batter - Segmental retaining walls are generally installed with a small horizontal setback between units, creating a wall batter into the retained soil. The wall batter compensates for any slight lateral movement of the SRW face due to earth pressure, ensuring that the finished wall does not appear to rotate.

Unit Size and Shear - In conventional (gravity) SRWs, where the stability of the system depends primarily on the mass and shear capacity of the SRW units, increasing the SRW unit width or weight provides greater stability, larger frictional resistance, and larger resisting moments. In soil-reinforced SRWs, heavier and wider units may permit a greater vertical spacing between layers of geosynthetic.
All SRW units provide a means of transferring lateral forces from one course to the next. Shear capacity provides lateral stability for this mortarless wall system. This is accomplished by shear keys, leading lips, trailing lips, clips, pins, or compacted columns of aggregate in open cores.

Embedment - 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 also provides greater stability when site conditions include weak bearing capacity of underlying soils, steep slopes near the toe of the wall, potential scour at the toe (particularly in waterfront or submerged applications), seasonal soil volume changes, or seismic loads.

Elements - The basic elements of each segmental retaining wall system are the foundation soil, leveling pad, segmental retaining wall units, retained soil, drainage fill, and, for soil-reinforced SRWs, the soil reinforcement.

Foundation soil: The foundation soil is the soil which supports the leveling pad and the reinforced soil zone of a soil-reinforced SRW system.

Leveling pad: The leveling pad is a level surface, consisting of crushed stone or unreinforced concrete, which distributes the weight of the SRW units over a wider area and provides a working surface during construction. The leveling pad typically extends at least 6 in. (152 mm) from the toe and heel of the lowermost SRW unit and is at least 6 in. (152 mm) thick.

Segmental retaining wall units: Segmental retaining wall units are concrete masonry units that are used to create the mass necessary for structural stability, and to provide stability, durability, and visual enhancement at the face of the wall.

Retained soil: Retained soil is the undisturbed soil for cut walls or the common backfill soil compacted behind infill soils.

Drainage fill: Drainage fill is free-draining granular material placed behind the wall to facilitate the removal of groundwater and minimize buildup of hydrostatic pressure on the wall. It is sometimes also used to fill the cores of the units to increase the weight and shear capacity. The dry stacked method of construction used for segmental retaining walls permits water to drain through the face of the wall, aiding in the removal of groundwater. In some cases, a geotextile filter is installed between the drainage fill and the infill to protect the drainage fill from clogging.

Reinforced soil: Reinforced soil is compacted structural fill used behind soil-reinforced SRW units which contains horizontal soil reinforcement. A variety of geosynthetic and steel soil reinforcement systems is available.

Construction
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.
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 project 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 leveling pad construction
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.

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