Retaining walls are structures designed to restrain soil to unnatural slopes. They are
used to bound soils between two different elevations often in areas of terrain possessing
undesirable slopes or in areas where the landscape needs to be shaped severely and engineered
for more specific purposes like hillside farming or roadway overpasses.
Definition A retaining wall is a structure designed and
constructed to resist the lateral pressure of soil when there is a desired change in
ground elevation that exceeds the angle of repose of the soil.
A basement wall is thus one kind of retaining wall. But the term usually refers to a cantilever
retaining wall, which is a freestanding structure without lateral support at its top. These
are cantilevered from a footing and rise above the grade on one side to retain a higher level
grade on the opposite side. The walls must resist the lateral pressures generated by
loose soils or, in some cases, water pressures.
Every retaining wall supports a “wedge” of soil. The wedge is defined as the soil
which extends beyond the failure plane of the soil type present at the wall site, and
can be calculated once the soil friction angle is known. As the setback of the wall increases,
the size of the sliding wedge is reduced. This reduction lowers the pressure on the
retaining wall. The most important consideration in proper
design and installation of retaining walls is to recognize and counteract the tendency
of the retained material to move downslope due to gravity. This creates lateral earth
pressure behind the wall which depends on the angle of internal friction 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 zero at the top
of the wall and - in homogenous ground - increase proportionally to a maximum value at the lowest
depth. 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
hydrostatic pressure on the wall. The total pressure or thrust may be assumed to act at
one-third from the lowest depth for lengthwise stretches of uniform height.
Unless the wall is designed to retain water, It is important to have proper drainage behind
the wall in order to limit the pressure to the wall's design value. Drainage materials
will reduce or eliminate the hydrostatic pressure and improve the stability of the material
behind the wall. Drystone retaining walls are normally self-draining.
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 wall
Gravity
Gravity walls depend on their mass to resist pressure from behind and may have a 'batter'
setback to improve stability by leaning back toward the retained soil. For short landscaping
walls, they are often made from mortarless stone or segmental concrete 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: geosynthetics such as Geoweb cellular confinement earth
retention or with precast facing; gabions; crib walls; or soil-nailed walls.
Cantilevered
Cantilevered retaining walls are made from an internal stem of steel-reinforced, cast-in-place
concrete or mortared masonry. These walls cantilever loads 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 counterfort on
the back, to improve their strength resisting 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 retaining 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 the material is usually driven 1/3 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 then placed behind the
potential failure plane in the soil. Bored pile
Bored pile retaining walls are built by assembling a sequence of bored piles, proceeded by excavating
away the excess soil. Depending on the project, the bored pile retaining wall may include
a series of earth anchors, reinforcing beams, soil improvement operations and shotcrete
reinforcement layer. This construction technique tends to be employed in scenarios where sheet
piling is a valid construction solution, but where the vibration or noise levels generated
by a pile driver are not acceptable. Anchored
An anchored retaining wall can be constructed in any of the aforementioned styles but also
includes additional strength using 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. Alternative retaining techniques
Cellular Confinement Cellular confinement systems, first marketed
by Presto Geosystems as Geoweb®, have become increasingly popular for earth retention applications.
They can be constructed as a gravity wall or a "geogrid" wall which consists of vertical
layers of geocells with geogrid reinforcement installed behind the face of the wall every
few layers of the geocell depending on design. 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 or isolated soil nail heads may be
used at the surface. Soil-strengthened
A number of systems exist that do not consist of just the wall, but reduce the earth pressure
acting directly on the wall. 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", which are filled with roughly cut stone or other material. The mesh cages
reduce some internal movement and forces, and also reduce erosive forces. Gabion meshes,
especially those filled onsite, have resulted in "bearing" failures(ie: Olivers Hill Gabion
Wall collapse:Melbourne’s worst million dollar mathematics mistake). When any wall
is built above the slip failure plane, clearly it's going to slide, but this type of wall,
by it's very nature of shedding water, is more likely to be used where there is a slip
failure problem. Mechanical stabilization
Mechanically stabilized earth, also called MSE, is soil constructed with artificial reinforcing
via layered horizontal mats 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 to affix
the layers to and vice versa. [1] 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.
Cellular confinement systems are also used for steep earth stabilization in gravity and
reinforced retaining walls with geogrids. Geocell retaining walls are structurally stable
under self- weight and externally imposed loads, while the flexibility of the structure
offers very high seismic resistance. The outer fascia cells of the wall can be planted with
vegetation to create a green wall. See also
References ^ Ching, F. D., Faia., R., S., & Winkel, P..
Building Codes Illustrated: A Guide to Understanding the 2006 International Building Code. New
York, NY: Wiley. ^ Ambrose,J.. Simplified Design of Masonry
Structures. New York: John Wiley and Sons, Inc.
^ Crosbie, M. & Watson, D... Time-Saver Standards for Architectural Design. New York, NY: McGraw-Hill.
^ Commercial Installation Manual for Allan Block Retaining Walls
^ Terzaghi, K., Large Retaining Wall Tests, Engineering News Record Feb. 1, March 8, April
19 ^ 2006 International Building Code Section
1806.1. ^ a b "Segmental Retaining Walls". National
Concrete Masonry Association. Archived from the original on 2008-03-04. Retrieved 2008-03-24.
^ Terzaghi, K., Theoretical Soil Mechanics, New York: John Wiley and Sons
^ Leshchinsky, D. “Research and Innovation: Seismic Performance of Various Geocell Earth-retention
Systems,” Geosysnthetics, No. 27, No. 4, 46-52
Retaining Walls Industry The Retaining Walls Industry web portal Italiantrivelle is the
number one source of informations regarding the building of retaining walls.
Ambrose,J.,(1991). Simplified Design of Masonry Structures. New York: John Wiley and Sons,
Inc. Bowles, J.,(1968). Foundation Analysis and
Design, McGraw-Hill Book Company, New York Building Code. New York, NY: Wiley.
Ching, F. D., Faia., R., S., & Winkel, P.. Building Codes Illustrated: A Guide to Understanding
the 2006 International Crosbie, M. & Watson, D... Time-Saver Standards
for Architectural Design. New York, NY: McGraw-Hill. External links
Segmental Retaining Walls - Case Studies Pictures of Gravity Walls
Information on the design of retaining walls Information on the analysis and design of
retaining walls NCMA is the national trade association representing
the concrete masonry industry Images of all types of residential retaining
walls Information on the construction of retaining
walls PINCRIB
Retaining wall design