Retaining wall

From Academic Kids

Missing image
Retaining_wall_12-2404a_(NARA).gif
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 (or bulkhead) is a structure to provide a barrier to downslope movement of soil, rock, or water. Retaining walls are generally made of masonry: stone, brick, concrete or steel sheet piles. In recent years, railroad ties have been a popular material for these walls.

Some masonry walls are dry-laid, and some are mortar-laid. If dry-laid, then the placement of the elements is especially important.
There is a definite science to building retaining walls, but they are often built without proper knowledge of the technology, and so often fail. The most important consideration is that any material behind a retaining wall is attempting to flow downslope due to gravity, and this creates a horizontal soil pressure behind the wall (depending on φ - the angle of internal friction of the material), smallest at the top and increasing toward the bottom. Also any groundwater behind the wall causes a horizontal hydraulic pressure on the wall.

In a gravity wall, if the resulting net force (resulting) vector of the vertical weight and the horizontal force of the fill material is within the middle 1/3 (the core) of the retaining wall, there will only be compressive stress inside the wall. To obtain this, the wall has a trapezoidal cross section with the widest part at the bottom, very similar to a dam construction. Drainage behind the wall 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...).
A new material for retaining walls are special angled concrete blocks with tabs on the back bottom. These are meant to be dry-laid, with the tabs positioned to secure the block on top of the one below. These walls, like most good retaining walls, are meant to be sub-vertical, gradually leaning back into the slope.
Another way of building retaining walls is the use of 'gabions'; stacked steel wire 'boxes' filled with rocks and boulders.

An arched retaining wall has stabilizing features; "dead-men", corners, buttresses, again similar to a dam construction. Dead-men are elements placed in the soil or fill some distance back from the wall, to which the wall is tied in some way. Buttresses are short wing walls at right angles to the main trend of the wall. In some cases, masonry steps are worked into a wall and become a stabilizing feature.

Retaining walls can also be made out of L shaped prefabricated reinforced concrete elements. The weight of the soil on top of the base of the 'L' will prevent tipping over. This type of wall uses much less material than a gravity wall.

In soft soils, retaining walls can be made out of steel sheet piles or wood driven into the ground. Structural design methods for this type of wall exist but these methods are a bit more complex than for a gravity wall. As a rule of thumb; 1/3 third above ground, 2/3 below ground.

This type of retaining wall can also be anchored by rods or cables extending from the back of the wall to a smaller wall in the fill material. This smaller anchor wall must be far enough away, outside the shear circle of the backfill behind the wall.

See also

Books

  • 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 Yorknl:Damwand
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