There are three common types of retaining structures:
gravity, cantilevered, and sheet pile walls.
Gravity walls are made
from a large mass of stone, concrete, or composite materials.
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 (mortar less) 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 pre-cast
facing; gabions (stacked steel wire baskets filled with rocks),
crib walls (cells built up log cabin style from pre-cast
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 pre-cast, 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.
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 buttressed on the front, or
include a counter fort 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 pile walls are often used in soft soils
and tight spaces. Sheet pile walls are 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 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.