Retaining wall
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A retaining wall is a structure that holds back earth materials. Retaining walls stabilize soil and/or rock from downslope movement and/or erosion and provide support for vertical or near-vertical grade changes. Cofferdams and bulkheads, structures that hold back water, are sometimes also considered retaining walls. Retaining walls are generally made of masonry, stone, brick, concrete, vinyl, steel or timber. Once popular as an inexpensive retaining material, railroad ties have fallen out of favor due to environmental concerns. They also decompose over time.
Segmental retaining walls have gained favor over poured-in-place concrete walls or treated-timber walls. They are more economical, easier to install and more environmentally sound.
The most important consideration in proper design and installation of retaining walls is that the retained material is attempting to move forward and downslope due to gravity. This creates a soil pressure behind the wall which depends on the angle of internal friction (phi) and the cohesive strength (c) of the retained material. This pressure is smallest at the top of the wall and increases toward the bottom. Earth pressures will push the wall forward and/or overturn it if not properly addressed. Also, any groundwater behind the wall that is not dissipated by a drainage system causes an additional horizontal hydraulic pressure on the wall.
There are three common types of retaining structures: gravity, cantilevered and sheet piling walls.
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[edit] Gravity walls
Gravity walls are made from a large mass of stone, concrete or composite material. Gravity walls depend on the size and weight of the wall mass to resist pressures from behind. Gravity walls will often have a slight setback, or batter, to improve wall stability by leaning back into the retained soil. For short, landscaping walls, gravity walls made from dry-stacked (mortarless) stone or segmental concrete units (masonry units) are commonly used. Dry-laid gravity walls are somewhat flexible and do not require a rigid footing below frost.
Earlier in the 20th century, taller retaining walls were often gravity walls made from large masses of concrete or stone. Today, taller retaining walls are increasingly built as composite gravity walls such as: geosynthetic or steel-reinforced backfill soil with precast facing; gabions (stacked steel wire baskets filled with rocks), crib walls (cells built up log cabin style from precast concrete or timber and filled with soil) or soil-nailed walls (soil reinforced in place with steel and concrete rods).
For reinforced-soil gravity walls, the soil reinforcement is placed in horizontal layers throughout the height of the wall. Common soil reinforcement materials include steel straps and geogrid, a high-strength polymer mesh, that provide tensile strength to hold soil together. The wall face is often of precast, segmental concrete units that can tolerate some differential movement. The reinforced soil's mass, along with the facing, becomes the gravity wall. The reinforced mass must be built large enough to retain the pressures from the soil behind it. Gravity walls usually must be a minimum of 50 to 60 percent as deep (thick) as the height of the wall, and may have to be larger if there is a slope or surcharge on the wall.
[edit] Cantilevered walls
Prior to the introduction of modern reinforced-soil gravity walls, cantilevered walls were the most common type of taller retaining wall. Cantilevered walls are made from a relatively thin stem of steel-reinforced, cast-in-place concrete or mortared masonry (often in the shape of an inverted T). These walls cantilever loads (like a beam) to a large, structural footing; converting horizontal pressures from behind the wall to vertical pressures on the ground below. Sometimes cantilevered walls are butressed on the front, or include a counterfort on the back, to improve their stability against high loads. Buttresses are short wing walls at right angles to the main trend of the wall. These walls require rigid concrete footings below seasonal frost depth. This type of wall uses much less material than a traditional gravity wall.
[edit] Sheet piling
Sheet pile walls are often used in soft soils and tight spaces. Sheet pile walls are made out of steel, vinyl, fiberglass or plastic sheet piles or wood planks driven into the ground. Structural design methods for this type of wall exist but these methods are more complex than for a gravity wall. As a rule of thumb; 1/3 third above ground, 2/3 below ground. Taller sheet pile walls usually require a tie-back anchor "dead-man" placed in the soil some distance behind the wall face, that is tied to the wall face, usually by a cable or a rod. Anchors must be placed behind the potential failure plane in the soil.
Proper drainage behind the wall is critical to the performance of retaining walls. Drainage materials will reduce or eliminate the hydraulic pressure and increase the stability of the fill material behind the wall (assuming of course, that this is not a retaining wall for water...).
[edit] See also
[edit] Books
- 2002, Design Manual for Segmental Retaining Walls, National Concrete Masonry Association, Herndon, VA
- Joseph E. Bowles,1968, Foundation Analysis and Design,McGraw-Hill Book Company, New York
- Terzaghi, K., 1934, "Large Retaining Wall Tests," Engineering News Record Feb.1, March 8, April 19
- Terzaghi, K., 1943, Theoretical Soil Mechanics, John Wiley and Sons, New York
