A retaining wall is an architectural structure that is designed to resist the lateral force of soil when the desired elevation change is greater than the angle of repose for the soil. Retaining walls are used to support soil laterally, so it can be maintained at different levels from both sides. Retaining wall structures are designed to hold soil in a slope it wouldn’t naturally maintain typically, a steep slope or a near-vertical slope. They are often used to bind soils between different elevations, especially in terrain with undesirable slopes.
Considerations for retaining wall design.
Retaining wall structures are vertical or nearly vertical and designed to hold material in place on one side. They prevent it from slipping, collapsing, or erosion. They support terrain when the angle of repose of soil is exceeded; otherwise, it would collapse and take a more natural shape. A retaining wall’s main characteristic is its ability to resist the pressure of the material that is retained, usually soil.
In the proper design and construction of retaining walls, it is important to counteract gravity’s tendency to cause the material to slide downslope. The lateral earth pressure created behind the wall depends on the angle (phi) of internal friction and the cohesiveness (c) (of the retained material), as well as the direction and magnitude movement of the retaining structure.
Inhomogeneous soil, the lateral earth pressure is zero at the top and increases proportionately to the maximum value at its lowest depth. If not addressed properly, earth pressures can push the wall or even cause it to topple. Hydrostatic pressure is also caused by any water behind the wall which has not been dissipated through a drainage system. For lengthways stretches of uniform thickness, the total pressure or thrust can be assumed to act one-third below the lowest depth.
Drainage behind the wall is essential to limit pressure to the design value of the wall. Drainage materials reduce or eliminate hydrostatic pressure, improving the stability of the material behind the wall. Drystone retaining walls are usually self-draining. For example, according to the International Building Code, retaining walls must be designed so that they are stable against overturning and sliding, excessive pressure on foundations and water uplift. They also need to have a safety factor of 1.5 for overturning and lateral sliding.
Types of retaining walls.
There are many types of retaining walls in perth. Some of the most popular are listed below.
Gravity Wall
Gravity walls rely on their mass to resist pressure. They can be made of stone, concrete, or another heavy material. A ‘batter setback’ may also improve stability by slanting toward the retained soil. They are usually made of mortar less stone or segment concrete (masonry) units. Dry-stacked gravity walls are flexible and don’t require a rigid foundation. Today, taller retaining walls are being built with composite gravity walls. These include geosynthetics like geocell cellular confinement Earth Retention or with the precast face; gabions (stacked wire baskets with rocks); cribs (log cabin-style cells built from precast concrete or wood and filled with granular materials).
Reinforced Retaining wall
The weight of reinforced concrete or reinforced masonry walls and the reinforcement bars within the wall provide stability against overturning. There are three main types of walls:
Concrete Cantilever wall
A cantilever wall consists of an attached wall to the foundation. Cantilevered walls hold back significant amounts of soil and must therefore be designed well. These are the most commonly used retaining walls. Cantilevered walls rest on slab foundations. The slab foundation is also weighed down by the backfill, and the weight of this backfill and the surcharge stabilizes the wall to prevent it from overturning or sliding.
Buttressed Retaining Walls / Counter-fort
Cantilever walls can be strengthened by counter-forts that are monolithic to the base and wall slab. Counter-forts are tension stiffeners that connect the wall slab to the base in order to reduce bending and shearing stress. Counter-forts can be used to reduce the bending moment in walls that are taller than 8-12 m. They are spaced apart at a distance equal to or slightly greater than half the height.
Cantilever Wall
These walls cantilever loads (like a beam) to a large, structural footing. These walls cantilever (like a beam), converting horizontal forces behind the wall into vertical pressures below. Cantilevered walls can be buttressed in the front or have a counter-fort at the back to increase their resistance to high loads. Buttresses consist of short wings at an angle to the main wall’s direction. These walls need rigid concrete footings that are below the seasonal frost depth. This wall type uses less material than the traditional gravity wall.
Reinforced Soil Retaining Wall
The earth walls that are mechanically stabilized are the ones made with steel or geotextiles soil reinforced in layers and a controlled granular material. Reinforced soils are also suitable for retaining walls if they form part of a design. They can be built to replace reinforced concrete or to suit the conditions of the ground.
A Wall With Soil Nailed
The Passive Bars are used to reinforce the soil in the excavation area. They are typically parallel and slightly angled downward. These bars are also capable of working in partial bending or shear. The nails are tensed by the friction of the soil with the nails.
Anchored Wall
Anchored retaining walls can be built in any style, but they also include additional strength by using cables or other supports anchored into the rock or soil. Anchors are usually driven into the material using boring. They can then be expanded by either mechanical means or by injecting pressured concrete which expands and forms a bulb within the soil. This method, although technically complex, is useful when high loads are anticipated, or the wall must be thin and otherwise too weak.
Sheet Piled Wall
The technique of earth retention is to use sheet sections that have interlocking edges. The pile acts as a temporary wall or certificate that is driven into an excavation or slope to support soft soils from lower to higher ground. It is lightweight and provides high resistance against driving stresses. Sheet piles can be reused for many years above or below the water and have a long service life. It is easy to adjust the pile length either by attachment or bolting, and joints are less likely to deform during driving.
Pile Wall (secant piles and tangent piles).
The intersection of concrete piles is what shapes these walls. These piles are designed using a lubricator or augering. Secant piles overlap by about three inches. The tangent walls are another option where the piles don’t overlap. These piles are flushed with each other. Secant and tangent walls’ main advantage is their increased alignment flexibility. The walls may also be stiffer, which makes the construction process less frantic. The joints are difficult to waterproof, and the cost is high. Also, the vertical tolerances for deeper piles can be hard to achieve.
Berlin Wall (soldier heap)
The Roman military engineers used this type of wall for deep excavations. This is a relatively cheap system that’s easy to build. It is only suitable for temporary construction and cannot be used in conditions of high formation. It is not as rigid as other styles of retentive walls.
Soil Nailing Wall
It is possible that soil nailing will not strengthen and reinforce existing ground. The technique involves putting closely spaced bars in a slope as a construction income. The soil nailing method is an economical and efficient way to build a wall that can be used for supporting excavations, hill cuts, bridge abutments, and highways. This method works well in conditions of cohesive soil, rock that is broken, sedimentary rock, or fixed faces. This type of wall is used for the stabilization of railroad and road cut slopes, excavation of retentive structures in urban areas, and high-rise buildings and underground facilities. This wall is also useful for tunnel portals on steep, unstable slopes. Bridge abutments can be retrofitted with wall support under piled-up foundations.
Bored pile wall
The assembly of bored piles is followed by the removal of excess soil. The bored pile retaining walls may be constructed with earth anchors and reinforcing steel beams. They can also include soil improvement operations, shotcrete reinforcement layers, or a combination of these. This technique is used in situations where sheet piling would be a good construction solution, but the noise or vibration levels produced by a piledriver are unacceptable.
Mechanical Stabilisation Wall
MSE walls can tolerate some movement. The face of the wall is filled with granular earth while retaining the backfill. MSE walls are easy to construct since they don’t require formwork or curing. Soil nailing is used in MSE walls to install slender steel bars into the soil. They are placed on an incline, parallel to each other, and then grouted. Anchored earth wall
Crib Wall
When timber, steel, or concrete boxes or cages are interlocked, they can be called crib walls.
Green Retaining Walls
To retain gentler slopes, green retaining walls are an option. Geocellular structures such as honeycomb cells or a series can be embedded in the slope surface to stabilize it. The individual cells can then be planted.
Barrette Retaining Walls
The barrette retaining walls are constructed of reinforced concrete columns with a rectangular form and a long axis pointing in the direction of retention.
Conclusion
Retaining walls are used to prevent soil erosion, create beds from steep terrains, and add decorative or functional features to landscaping. These walls can be standalone structures or part of larger construction projects, like a building. If the wall will be higher than 1 metre and close to a path or road or 2 meters high in other places, planning permission is needed. Freestanding, independent retaining walls do not always require approval from the building regulations. However, they must be structurally sound.
