A Retainig Wall is a structure thet holds back soil or rock from a building, structure or area. Retainig Walls prevent donw slope movement or erosion and provide support for vertical or near-vertical grade changes.
Cofferdams and bulkeads, structures thet hold back water, are sometimes also considered retaining walls.
Retainig Walls are generally made of masonry, stone, brick, concrete, vinyl, steel or timber. Once popular as an inexpensive retaining materia, railroad ties have fallen out of avo due to enviromental 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 enviromentally sound.
The most important consideration in proper design and installation of retaining walls is that the retained materials is attempting to move forward and downslope due to gravity. This creates lateral earth pressuree behind the wall which dependes on the angle of internal friction (phi) and the cohesive stregth (c) of the retained material, as well as the direction and magnitude of movement the retaining structure undergoes.
Lateral earth pressures are typically samallest at the top of the wall and increase toward the bottom. Earth pressures whill push the wall forward or everturn it if not properly adresses. Also any, groundwater behind the wall that is not dissipated by a drainage system causes and additional horizontal hydrostatic pressurre on the wall.
As an example, the International Building Code requires retaining walls to be designed to ensure stability againt overturing, sliding, excessive foundation pressure and water uplift; and that they be designed for safety factor of 1.5 against lateral sliding and overturning.
GRAVITY
Gravity walls depend on their mass (stone, concrete or other heavy material) 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 (masonry units). Dry-stacked gravity walls are somewhat flexible and do not require a rigid footing.
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 geocell cellular confinement earth retention 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 granular material); or soil-nailed walls (soil reinforced in place with steel and concrete rods).
SHEET PILING
Sheet pile retaining walls are usually used in soft soil 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.
CANTILEVERED
Prior to the introduction of modern reinforced-soil gravity walls, cantilevered walls were once the most common type of taller retaining wall. Cantivelered 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 cantivelerd walls are butressed on the front, or include a counterfort on the back, to improve their stability against high loads. Butresses 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 opressurized concrete, eich expands to form a bulb in the soil. Technically complex, this method is very useful ehre high laods are expected, or where the wall itself has to be slender and would otherwise be too weak.
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 opressurized concrete, eich expands to form a bulb in the soil. Technically complex, this method is very useful ehre high laods 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 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.
MECHANICAL STABILIZATION
Mechanically stabilized, 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.