Retaining wall design

What is a Retaining wall

A structure that is designed to hold or keep soil behind it is called a retaining wall. There is a wide variety of material that may be used in the construction of retaining walls. Some examples of these materials are concrete blocks, poured concrete, treated timbers, pebbles, or boulders. All of them are able to hold soil, however some of them are simpler to use than others and some of them have shorter lifespans.

Why You Need a Retaining Wall

A retaining wall is a wall built to support soil or rock on one side while resisting the pressure of the soil or rock on the other side. The most common type of retaining wall is a gravity wall, which is built with heavier materials at the bottom and lighter materials at the top. Gravity walls rely on their weight to resist the pressure of the soil or rock behind them. Other types of retaining walls include cantilevered walls, sheet pile walls, and buttressed walls.

Types of Retaining Walls

Gravity wall

Gravity retaining walls are typically sloped on their faces and have a wider base, which enables them to better withstand the higher lateral earth pressures that are found at greater depths. Because of this, although they are simple to construct and appropriate for retained heights of up to about 3 metres, if they are built any higher they have a tendency to consume an excessive amount of space and can end up being too heavy for the ground below, which can cause the bearing capacity to fail. Because of this, the wall may not be able to keep soil behind it.

Semi-Gravity Retaining Wall

A gravity retaining wall that has had a little amount of reinforcement added to it in order to lessen the overall mass of the concrete is referred to as a “semi-gravity retaining wall,” and the phrase “semi-gravity retaining wall” is used to describe this kind of wall. Figure illustrates this arrangement for your reference. When it comes to a gravity retaining wall, the majority of the lateral ground pressure is met by resistance provided by the wall’s own mass.

Cantilever retaining wall

A slope is also added to the rear of the wall on the heel side of the structure. This results in an increase in the breadth of the wall proportional to the rise in the lateral earth pressure that occurs with increasing depth.

Cantilevers are comprised of the vertical wall, which is also referred to as the stem, the heel slab, and the toe slab. Cantilevers are anchored at the place where the heel slab, toe slab, and vertical wall meet, and reach all the way to the other end. The stem is being forced outward away from the backfill as a result of the lateral earth pressure it is experiencing.

Both the heel slab and the toe slab are subjected to the upward soil pressure that is caused by the bottom of the structure, and as a consequence, they bend upward. Therefore, reinforcement is supplied on the tension side, namely in the form of a vertical brace on the reverse side of the stem as well as a horizontal brace at the base of the heel slab and the toe slab respectively.

When compared to the weight of a gravity retaining wall, the weight of a cantilever retaining wall is noticeably lower, and as a result, it simply needs foundations that are less extensive.

Backfill may be retained to modest heights by using cantilever retaining walls, which are suited for heights between 4 and 7 metres. The majority of cantilevered walls have a cross section that resembles a “L” or an inverted “T.” Another alternative to consider is using earth tieback retaining walls in areas with inadequate foundation soils. These walls are not only supported by a substantial foundation, but also by a sequence of horizontal bars or strips that reach out from the vertical surface into the backfill.

Together, these elements work to counterbalance the weight of the structure. Wood, metal, or synthetic materials such as geotextiles may be used in the production of the bars or strips. When an earth tieback retaining wall has been backfilled, the structure is anchored by the weight of the fill and the friction that it experiences against the horizontal components.

Counterfort Retaining Wall

A counterfort retaining wall is a kind of retaining wall that resists all lateral pressures via the flexural action of its material rather than through the weight of the material itself. Therefore, such a wall has a large base foundation, a vertical stem that is reinforced with bar, and support provided by thin transverse slabs that are referred to as counterforts that are set at regular intervals.

The slab has been specifically engineered for high tensional stress so that it may be placed within the portion of the structure that is responsible for the retention of the soil mass. But if the slab is maintained outside, in an area where the soil mass does not need to be held, then the walls are known as buttresses and are often referred to as buttress retaining walls.

In addition, if the wall does not include any counterforts or buttresses, then it is referred to as a cantilever retaining wall. Because a cantilever wall with a longer stem needs a massive base, Counterfort walls are constructed with transverse support in order to circumvent this constraint.

Crib Retaining Wall

The figure depicts the crib’s wall. The walls of a crib are built up of a series of boxes that are filled with granular soils. These boxes may be formed out of wood, precast concrete, or steel components. It functions as a gravity wall and has the added benefit of being erected rapidly.

Due to the flexible nature of the soil, the wall can resist significant movement of the soil. The batter on its face is often set at 1 in 6, since this is the standard setting. The width of the wall may range anywhere from 0.5 to 1.0 H and is appropriate for walls of heights up to about 7.0 m in height. It is important to keep in mind that the crib wall should not be loaded with surcharges.

Gabion Retaining walls

One of the earliest types of gravity wall, gabion retaining wall systems have been around for a very long time. Gabion walls are produced by making box cages out of galvanised hexagonal wire mesh of varied sizes in a manufacturing setting. These box cages are filled on-site with stones that are readily accessible in the area, then sealed off completely and arranged in a pattern according to the design.

In regions where the ground conditions are not suitable for the adoption of any other kind of retaining structure, they are used in such regions. The provision of box cages inside the framework of a gabion wall is intended to make the rock more resistant to shear forces. They are able to handle significant ground shifts without experiencing any problems. Gabion structures are open-topped, self-draining boxes that may significantly lessen the effects of hydrostatic pressure.

The fact that these kinds of systems do not need a specific foundation construction is the primary benefit associated with them. The design specifications call for the installation of gabion barriers immediately on top of the surface in a variety of predetermined configurations.

In order to prevent the mesh from rusting in places where there is a constant flow of water, it is either covered with PVC or coated with a specific coating. These walls gave a gratifying rock finish appearance in addition to a good visual appeal.

Gabion walls are built primarily with the purpose of stabilising the soil behind the wall. They are also capable of functioning as a cover wall. The gabion boxes are arranged in layers using the correct construction method. They are a very cost-effective replacement for concrete retaining walls and rock anchors in the context of projects involving the stabilisation of soil and the protection of slopes.

Sheet Pile Retaining Wall

It is built using a series of interconnecting piles that are each driven into the foundation soil independently. Sheet piles built in the modern world are often fabricated from steel, although wood and precast concrete sections are also acceptable alternatives.

The active and passive forces that work on the lowest component of the structure may occasionally keep cantilever sheet pile walls securely rooted in the ground. These forces operate on the lowest portion of the structure.

Anchored Earth Retaining Wall

These kinds of retaining walls have their foundations firmly set in place, and their upper portions are held up by one or two rows of horizontal ties or struts.

Diaphragm Retaining Wall

Reinforced concrete or sheet piles may be used as the material for the walls of a diaphragm. It consists of a vertical concrete reinforced concrete slab that is securely fastened in place. The passive and active forces that operate on the wall’s bottom half are what keep the wall in its current place.

Reinforced Earth Retaining Wall

It has been common practise for millennia to include reinforcement into retaining walls in order to fortify the soil behind them.

The principle behind reinforced earth walls is based on the idea that by embedding lengths of a material that is capable of carrying tension into a mass of soil in the desired direction, tensile strength can be imparted to the soil in a particular direction and given to the mass as a whole.. This is the primary phenomenon at work in reinforced earth walls. Numerous geotechnical applications have benefited from the use of reinforced earth.

Read more: Shear Wall- How to construct, Types, Advantages & Disadvantages

Retaining Wall Design

The purpose of retaining walls is to prevent the collapse, sliding, or erosion of the material that is being maintained on one side of the construction. These walls may be vertical or nearly vertical in their design. When the angle of repose of the soil is crossed, these walls provide the appropriate terrain support and stability to prevent it from collapsing in a natural way.

The following are the primary types of failure that may occur in a retaining wall:

• Failure of this kind occurs when the lateral ground pressure reaches a point where it becomes greater than the value which the wall can withstand. This might result in the wall toppling over or turning over completely.
• Failure Due to Exceeding the Bearing Capacity of the Soil The soil under the retaining wall collapses when the load is greater than its bearing capacity.
• When there is inadequate friction at the base of the retaining wall, it is possible for the wall to collapse due to sliding.
• Failure on the inside occurs when either the material from which the retaining wall was constructed or the structural structure of the retaining wall breaks down. This is known as internal failure.

Read more: What Is Underpinning

For an accurate assessment of the site’s structural integrity as a whole, geotechnical experts need to be consulted. The following factors need to be considered when designing a retaining wall properly:

1. Determine where you want the retaining wall to place.

• Reduce the amount of soil that has to be excavated and backfilled.
• Improve the grade, as well as the drainage patterns.
• Think about the features that are already there on site.

2. Determine the height of the retaining wall and its geometry

• Determine how high the retaining wall is when it is at its highest position.
• Determine the slopes that are located above and below the wall.
• Consider the additional costs associated with automotive or construction traffic.
• Choose the correct wall batter or setback for your structure.

3. Evaluate structural needs

• Determine the necessary amount of reinforcement based on the gravity wall table.
• If you need geogrid, check out the geogrid charts to get an estimate of how long the grid will be.

4. Determine the overall wall structure.

• When calculating the entire wall setback, use the Setback Chart as a guide.
• To calculate the overall wall envelope, just add the necessary grid lengths.
• Recheck wall envelope against the amount of space that is available at the retaining wall location.

Considerations for Retaining Wall Design

When designing and installing a retaining wall, the most important factor that has to be taken into account is determining the reason for and the natural inclination of the material that is being held to slide down the slope owing to the force of gravity. This action of the material causes lateral pressure to be exerted on the retaining wall from behind it. The amount of this pressure is determined by the angle of friction (φ) and the cohesive strength (c) of the material that is being retained, in addition to the direction and magnitude of movement that is being experienced by the retaining wall.

The lateral pressure is assumed to be zero at the top of the wall and to grow proportionately (only in homogeneous soil) as the depth increases. When it comes to a building that is intended to retain water, it is very necessary to equip it with an effective drainage system that can relieve the hydrostatic pressure. This will help to reduce the pressure that is exerted on the retaining wall while also improving the stability of the material that is being held. For parts that are longitudinal and are all the same height, it is safe to assume that the total pressure or thrust is acting one-third from the lowest depth.

Conclusion

There are many reasons why you might need a retaining wall. Perhaps you have a slope on your property that is starting to erode. Or maybe you want to create a terraced garden. Whatever the reason, a retaining wall can be a great addition to your property.

A retaining wall can help to stabilize a slope, preventing further erosion. It can also create a flat surface for planting or other activities. If you have a sloped property, a retaining wall can make it much more usable.

Retaining walls can be made from a variety of materials, including stone, brick, concrete, and even wood. The type of material you choose will depend on a number of factors, including the purpose of the wall, the look you want to achieve, and your budget.

If you are considering adding a retaining wall to your property, talk to a landscape architect or contractor to get started. They can help you determine the best location for the wall and the type of material that will work best for your needs.