Most of the time walls are necessary for grade changes. Sometimes,
walls are a great addition to terrace your garden aesthetics and divisions.
There are multiple options. We have built many types and sizes over the
years and have extensive knowledge in this area. Finding out what would
suit your situation and budget is the first step.
A retaining wall is a structure that holds back soil or rock from a building,
structure or area. Retaining walls prevent downslope movement 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.
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 lateral earth pressure behind the wall
which depends on the angle of internal friction (phi) and the cohesive strength
of the retained material, as well as the direction and magnitude of movement the
retaining structure undergoes.
Lateral earth pressures are typically smallest at the top of the wall and
increase toward the bottom. Earth pressures 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
hydrostatic pressure on the wall.
As an example, the International Building Code requires retaining walls to be
designed to ensure stability against overturning, sliding, excessive foundation
pressure and water uplift; and that they be designed for a safety factor of 1:5
against lateral sliding and overturning.
Types of Retaining Walls
Gravity
Gravity walls depend on the weight of their mass (stone, concrete or other heavy
material) to resist pressures from behind and will often have a slight 'batter'
setback, to improve stability by leaning back into the retained soil. For short
landscaping walls, they are often made from mortarless stone or segmental
concrete units (masonry units). Dry-stacked gravity walls are somewhat flexible
and do not require a rigid footing in frost areas.
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 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).
Sheet Piling
Sheet pile walls are usually used in soft soils and tight spaces.
Sheet pile walls are made out of steel, vinyl or wood planks which are driven
into the ground. For a quick estimate we usually drive the material 1/3 third
above ground, 2/3 below ground, but this may be altered depending on the
environment. Taller sheet pile walls will need a tie-back anchor, or "dead-man"
placed in the soil a distance behind the face of the wall, that is tied to the
wall, usually by a cable or a rod. Anchors are placed behind the potential
failure plane in the soil.
It is very important to have proper drainage behind the wall as it is critical
to the performance of retaining walls. Drainage materials will reduce or
eliminate the hydrostatic pressure and will therefore greatly improve the
stability of the material behind the wall, assuming that this is not a retaining
wall for water.
Cantilevered
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.
Anchored
This version of wall uses cables or other stays anchored in the rock or soil
behind it. Usually driven into the material with boring, anchors are then
expanded at the end of the cable, either by mechanical means or often by
injecting pressurized concrete, which expands to form a bulb in the soil.
Technically complex, this method is very useful where high loads are expected,
or where the wall itself has to be slender and would otherwise be too weak.
Soil Nailing
Soil nailing is a technique in which soil slopes, excavations or retaining walls
are reinforced by the insertion of relatively slender elements - normally steel
reinforcing bars. The bars are usually installed into a pre-drilled hole and
then grouted into place or drilled and grouted simultaneously. They are usually
installed untensioned at a slight downward inclination. A rigid or flexible
facing (often sprayed concrete) or isolated soil nail heads may be used at the
surface.
Soil-strengthened
A number of systems exist that do not simply consist of the wall itself, but
reduce the earth pressure acting on the wall itself. These are usually used in
combination with one of the other wall types, though some may only use it as
facing (i.e. for visual purposes).
Gabion Meshes
This type of soil strengthening, often also used without an outside
wall, consists of wire mesh 'boxes' into which roughly cut stone or other
material is filled. The mesh cages reduce some internal movement/forces, and
also reduce erosive forces.
Mechanical Stabilization
Mechanically stabilized earth, also called MSE, is soil constructed
with artificial reinforcing via layered horizontal mats (geosynthetics) fixed at
their ends. These mats provide added internal shear resistance beyond that of
simple gravity wall structures. Other options include steel straps, also
layered. This type of soil strengthening usually needs outer facing walls (S.R.W.'s
- Segmental Retaining Walls) to affix the layers to and vice versa.
The wall face is often of precast concrete units that can tolerate some
differential movement. The reinforced soil's mass, along with the facing, then
acts as an improved 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 or thick as the height of the wall, and may
have to be larger if there is a slope or surcharge on the wall.
