building retaining walls



retaining walls
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retaining walls

Structure in the foreground is called a 'mud box' - a kind of retaining wall built to hold the flood waters in check. Note the buildings on the opposite side are half-submerged. 05/1973, Mississippi river, USA.

A retaining wall is a structure that holds back earth. Retaining walls stabilize soil and rock from downslope movement or erosion and provide support for vertical or near-vertical grade changes. Cofferdams and bulkheads, structures to hold back water, are sometimes also considered retaining walls. Retaining walls are generally made of masonry, stone, brick, concrete, 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 (depending on the angle of internal friction (phi) and the cohesive strength (c) of the material). This pressure is smallest at the top and increases toward the bottom and will push the wall forward 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.

Contents

  • 1 Gravity walls
  • 2 Cantilevered walls
  • 3 Sheet piling
  • 4 See also
  • 5 Books

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.

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.

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...).

See also

  • Civil engineering
  • Geotechnical engineering
  • Structural engineering

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
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Bans on Using CCA Treated Lumber For Seawalls Helps Create New Industry 

[Press Release] PR Web - Oct 05 12:17 AM
When a few counties and townships in New Jersey introduced bans on CCA treated lumber for retaining walls in marine environments, a new industry developed to provide an environmentally friendly solution. Since the onset of the bans, the industry has experienced significant growth and costs for these alternatives have decreased considerably. (PRWeb Oct 5, 2006) Trackback URI:

Team to build canal for mudflow emergency 
The Jakarta Post - Oct 03 9:30 PM
In a bid to reduce the volume of mud held in retaining ponds before the arrival of the rainy season, the government-appointed team coordinating the response to the Sidoarjo mudflow disaster is transforming a 15-kilometer irrigation ditch into a mud canal.

Community Development Corporation amends FY06 budget 
Allen American Star - 45 minutes ago
Members of the Community Development Corporation met briefly Tuesday evening to amend the FY06 budget and to look at future projects.

Habitat house shapes up 
Daily Gate City - 28 minutes ago
The intensity of focus and controlled excitement volunteers are showing at the site of the newest Habitat for Humanity house in Keokuk is nearly tangible.

Designer home 
Manila Bulletin - Oct 05 9:26 AM
This is a home which captures the essence of understated opulence: Glass, tile and metal elements create clean and light-weight lines; sleek and sophisticated designer furniture define modern, tasteful preferences; wide spaces and ambient lighting enhance moods and tame the senses.

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