Retaining Wall Design

Imagine if the world we lived in was nothing but flat? Pretty boring, right? Luckily for us, we live in rich landscapes with a diversity of elevation, soil cover, and shape. However, sometimes this same diversity can be an issue for structural integrity- and that’s where retaining walls come into the picture.

Retaining walls allow us to stabilize and ‘hold back’ soil that can otherwise slip and erode, strengthen the landscape for buildings, roadways, and other structures, and ensure that we have beautiful properties.

This means they allow us to make better use of every lot, protect property and life from unstable natural landscape areas, and add beauty to lot designs.

CMU Block Retaining Wall

Why to Use a Retaining Wall: Dealing with Soil

Retaining wall designs are created by engineers to support soil laterally in sloping landscapes, and must resist the lateral pressure of soil to avoid collapse.  Soil can be remarkably heavy, and can exert different forces due to expansion and contraction during icy weather, when water present in the soil, and sometimes due to changes made to the landscape for buildings, foundations, or traffic. All of these factors must be considered when designing a retaining wall.

Because of the vagaries of soil movement, your lot can look great, but be at considerable risk from slipping or falling earth, depending on the inherent and weather-related soil properties. This movement can also put buildings in peril, as well as endanger users. However, a well-designed retaining wall should insure an effective wall to last for many years.

Retaining walls act to provide a ‘shelf’ that prevents soil movement and erosion, and they also help better distribute soil weight, improve graded landscapes, and keep your commercial landscape safer and more stable. Plus, they make a fantastic decorative feature when used correctly!

Retaining wall design is not one-and-done, however. A retaining wall design that’s right for your needs is heavily dependent upon the weight that is put upon it. The majority of this force is caused by the soil and aspects acting on it. Xpress Engineering’s skilled structural engineers create appropriate retaining walls after calculating the soil burden the retaining wall must be able to hold.

The soil burden is based on four different factors: (a) the strength parameters of the soil, (b) the unit weight of the soil, (c) the type and amount of wall movement, and (d) the drainage conditions of the backfill material used. A proper retaining wall design considers and accounts for all of these factors and conditions to insure that your wall can do it’s job for many years to come.

Retaining Wall Design Engineering: the First Step

So, determining which forces(s) are involved in the design is one of the first steps in the creation of any retaining wall structure. In fact, it’s critical to solid retaining wall construction.

The answers science gives us here will determine how many retaining walls will be needed, their depth/height and design, and the materials used in their construction.

While we won’t go in-depth into the mechanics, here are some key points that will be considered:

  1. If the wall is successfully restrained from moving, the lateral pressure is called at-rest, or lateral pressure
  2. Due to the soil pushing on the wall, it may deflect or tilt outward. With sufficient tilt outward, a triangular ‘failure wedge’ will be created. This type of pressure is called active earth force.
  3. In certain situations, the wall may be pushed back into the soil it is retaining. The pressure for this condition is passive earth force.

Together, these create different scenarios, and need the design of the retaining wall to shift to match (and counter) them. Another key element is bearing pressure, which simply expresses the load carried over a specific area.

Ideally, your retained soil exerts less force on the walls than the ‘push-back’ from the engineered strength and design of the retaining wall project, holding everything in place effortlessly.

As you can see, this gets quite complex- but that’s why you can trust the experts to do this work for you. We will then advise on the best type of retaining wall design, as well as the best materials, for what you need.

Designing Retaining Walls: Thinking Beyond Lateral Earth Pressure

Lateral earth pressure considerations for retaining wall design

There are a variety of types of retaining walls, each with special landscape uses and a ‘maximum load’ they can hold back.

The typical design from Xpress Engineering is a Cantilever Wall, but other types are: Gravity Retaining Walls, Semi-gravity Retaining Walls, or Counterfort Retaining Walls.

Cantilever Retaining Walls are typically the most economical design up to a height of approximately 24 feet and can be incredibly strong when concrete and steel are added to the mix.

The wall design consists of two phases. Firstly, once the lateral force type and magnitude are known, the wall is engineered for overall stability. The wall is investigated for sliding, overturning, and soil bearing capacity failures. Secondly, each element of the wall is checked for strength, and the steel reinforcement is sized accordingly.

Creating a Cantilever Retaining Wall

A typical retaining wall uses 4 key construction areas, all of which are a vital part of the overall structure:

  • The Stem: This is the vertical part of the wall, which holds backfill and steadies the wall. Stem thickness is a critical part of the overall wall design.
  • The Toe: This is the part of the wall at the base on the front.
  • The Heel: This is the retaining wall footing behind the wall on the 'filled' side
  • The Shear Key: This projects down under the wall footing and can be up to one third of the overall wall height, even though you will never see it beneath the soil of the finished wall.

These key parts of the wall structure work together to offset the forces active on the wall and behind the wall. They each help to determine the maximum wall height you can install with the materials you choose.

Retaining Wall Design Types

There are a variety of different types of retaining walls. The design from Xpress Engineering is a Cantilever Retaining Wall, but other types include: Gravity Retaining Walls, Semigravity Retaining Walls, or Counterfort Retaining Walls. Cantilever Retaining Walls have been found to be the most economical designs up to a height of approximately 24 feet.

Once the recommended retaining wall design parameters are known and the overall structural design is created, it’s time to choose the right material for the wall.

Strength and capability have a role to play here, and so does simple aesthetics. Here are some common retaining wall designs that work with our designs:

Poured Concrete Retaining Wall Designs

Poured concrete retaining wall designs, sometimes just called ‘poured’ retaining walls or ‘concrete’ retaining walls, combine strength and dexterity, and allow for precision construction in any space.

In fact, concrete is one of the best retaining wall materials, although it can come at a higher price. They are quite technical to create, too, but this also means they can be used for almost any space and design.

With modern concrete technology, we can also add texture and color, embed decorative or strengthening objects, and much more, ensuring the end result is as pleasing as it is stable.

Poured concrete retaining wall in a landscape

This are one of the most versatile and customizable types of retaining wall in use today, well suited to the overall shape of cantilever walls, and have a great safety factor, but they need a solid grasp of design and engineering to create successfully.

Steel reinforcement is often used in larger wall designs, allowing for stronger footing in the soil and for the wall to carry a higher minimum-maximum force without wall failures.

Cinder block retaining wall

Block Retaining Wall Designs

Block retaining walls are, as the name suggests, created with blocks (aka “cinder blocks” or “concrete blocks”) instead of poured concrete. It’s the oldest type of design for retaining walls, and technically covers anything from brick walls upward. Most modern designs use larger poured concrete blocks for better aesthetics, and to allow them to be built to a larger size.

However, past a certain height wall failures can occur, as there are more ‘moving parts’ in the design which can be acted on by soil force and weight, and hydrostatic pressure.

This means they have specific uses and designs, and this needs to be correctly determined in conjunction with the retaining soil, backfill soils and drainage, earth pressures, and necessary wall height.

They are great for already bound soils, which means they work fantastically in garden bed designs on a slope.

They’re often spaced at regular intervals over a landscape, allowing the construction of the slope to ‘step down’ in a stable and aesthetically pleasing way while providing plenty of support, and can be steel-reinforced if necessary for overall structural stability, too.

Pressure Treated Wood Retaining Wall Designs

Again, the design of retaining walls is only half the journey. The materials used must be able to support the minimum required force of the soil acting on them without sliding or pushing the footing of the wall out of place, which would lead to structural failure.

Treated wood is stronger than ‘normal’ wood, but still has its limits. This is a design suited to lower wall heights, and any structures designed with pressure treated wood must take these materials into account.

Remember that any wood will rot over time, too, so it has a much shorter lifespan than concrete and steel walls. They’re often used to bring stability to a landscape, rather than hold space for building construction or tame higher grades.

Pressure Treated Wood Retaining Wall Designs

This are one of the most versatile and customizable types of retaining wall in use today, well suited to the overall shape of cantilever walls, and have a great safety factor, but they need a solid grasp of design and engineering to create successfully.

Steel reinforcement is often used in larger wall designs, allowing for stronger footing in the soil and for the wall to carry a higher minimum-maximum force without wall failures.

Retaining Wall Installation

Once you’ve designed your retaining wall, and you know the wall can stand up to the job, it’s time to install the project!

This can need an amount of site preparation, and your retaining wall expert may even regrade parts of the lot and shift soil around to create a better, more stable platform for building and human activity on the lot.

However, your Xpress Engineering team will do their best to make the installation as hassle-free as possible for you, and we’re sure the end result will thrill you.

We will discuss all aspects of the project with you before the installation goes ahead, so you know how to plan around any limited access or other disruptions before we start. Rest assured, the final project will be well worth it!

We Provide Professionally Engineered Retaining Wall Designs in 24-Hours

Xpress Engineering will have the design, calculations, and specifications for the project in your inbox within 24-hours of your query in nearly every cases, so you can get your retaining wall project started very quickly.

Is Xpress Engineering able to design what you need? Following are factors that could complicate or delay your design:

  • You have been provided a Geo-technical (Soils) Report for your specific project.
  • You can see significant cracking or previous movement in slabs and/or foundations close to where the wall is to be installed.
  • You need a wall over 8 feet tall.
  • You will be supporting a building structure on top of the retaining wall.
  • You will be building the wall very close to a building.
  • You will be building this wall parallel and adjacent to a public roadway or highway.
  • This is the first retaining wall you or your contractor has ever built

Don’t worry if any of these factors do apply, however! While we may not be able to get you a standard design within 24-hours, we can very likely still help you create the perfect retaining wall for your needs.

Contact our helpful team at info@xpressenginering.com, and let us know what the complicating factors are. You can feel confident that we can help.

If none of the above conditions are applicable, let’s get started with your retaining wall design!