Concrete Retaining Wall Design

Xpress Engineering specializes in matching the retaining wall to the location, ensuring that you receive a product that keeps intrusion on the landscape to a minimum, while considering all the forces acting on the wall to ensure you receive a quality product you’ll love.

What goes into retaining wall design, and why is concrete so often the best material of choice for retaining walls ? Let’s take a look.

Poured concrete retaining wall

The Pressures Working on a Retaining Wall

We won’t overwhelm you with all the engineering terms that go into creating a cantilever retaining wall. However, understanding the basics of designing retaining walls always helps.

Lateral Earth Pressure

Retaining walls do exactly what the name implies, they retain soil behind them without the wall sliding, cracking, or overturning. Active earth pressure refers to the pushing of the soil behind the wall and engaging the wall with very slight movement. Passive pressure occurs on the opposite side of the wall where any soil is in contact with the footing or any portion of the wall. Passive pressure resists active pressure. Not only must the bulk of the retaining wall be able to tolerate the active pressure, but all construction and expansion joints.

Sliding Resistance

Friction and passive pressure are the two main elements that prevent a retaining wall from sliding. Static friction is a resisting force created by two materials pressing together that hinders movement of the object being pushed. In the case of a retaining wall, this is the weight of concrete wall and footing pressing against the soil which creates the friction. If the forces pushing on the retaining wall are not able to overcome the static friction then sliding is prevented.

Passive pressure is the “push back” of the soil in front of the wall. If a retaining wall does not have adequate friction to resist the forces pushing on the wall, passive soil is critical in the design. Passive soil is the undisturbed or newly compacted soil in front of the retaining wall shear key, footing, and on occasion, soil in front of the lower portion of the wall itself. The allowable soil pressure of the passive soil is typically high. This is based on failure wedges and more complex soil mechanics that is a discussion for another day.

In the event that the friction and passive pressure against the footing and wall are not sufficient, a shear key is incorporated below the footing to create additional surface area to push against the passive soil. Our qualified engineers are experts on identifying when and were a shear key is required, so you do not have to worry about it. Remember, every design element is there to prevent sliding, cracking, and overturning, working with the geometry of the wall and surrounding area to create a strong, stable wall you can rely on for years.

Backfill Material

This is everything that gets returned behind the wall, to its back face. The top layer will be productive soil you can use for planting or just enjoy the aesthetic value. However, drainage and soil stability must be considered, so the depth of backfill will be added to reduce the load on the wall, increase stability, and avoid sliding, to create optimal success conditions. Again, the less overturning and sliding pressure acting on the wall, the better the overall design, so backfill weight is kept to a minimum.

Your Xpress Engineer designer will determine the exact loading forces in play, and everything you need to create a hassle-free build, but we hope you enjoyed this peek into the world of a retaining wall contractor.

How Thick Should a Concrete Retaining Wall Be?

Thickness of a retaining wall depends entirely on the planned purpose and location of your retaining wall. The idea is to create a wall with sufficient wall thickness to tolerate the allowable soil bearing pressure behind it, without the retaining wall design being too thick, which wastes money and time.

The proper portioning of a concrete retaining wall is as important as the overall structural design. Soil type, backslope, hydrostatic pressure, surcharges, and a lot more all affect the answer. Typically, the top of the stem will be no less than 6″ thick, but a lot depends on the earth pressure, vertical load, and other key design elements. As a general rule of thumb, the stem thickness in a cantilever retaining wall is typically about 10% of the wall height. This is a general rule of thumb, but our expert engineers at Xpress Engineering will determine the exact required thickness for your wall.

How Do You Size Concrete Retaining Wall Footings?

A general rule of thumb is the footing width should be 50-70% of the wall height, and the footing thickness ranges from 12” up to 18” for walls over 10 feet in height. Similar to the stem thickness, many factors play into the footing size and site specific parameters will always need to be examined.

If the wall will need extra sliding resistance, a footing key can be used in the concrete wall design. An extension of the stem, it extends below the bottom of the base. Reinforced concrete, with steel built-in for greater integrity, can also be used (mentioned previously in this post).

However, smart retaining wall construction starts with the proper placement of all features. A poorly-designed wall that doesn’t match the soil parameters will fail no matter how thick it is.

How Do You Design a Retaining Wall?

Getting the right design for your retaining wall is critical. Luckily, it’s not a decision you have to make – your Xpress Engineering structural engineer will create an ideal design for your retaining wall with minimum intrusion into your landscape, while still having the depth and loading forces to get the job done.

Firstly, however, we would recommend hiring a licensed contractor to help your survey the entire area of where the wall will go. Planning is a critical first step for all successful projects. However, here’s a few of the steps described briefly which can be discussed with your contractor:

Smart Location

When you design retaining walls, a lot hinges on choosing the right location in the first place. The idea is to create an optimal drainage pattern and grading on the site, considering the existing features of the landscape and foundation soil. The location should also make for minimized excavation and backfill material needs.

Clever Design Parameters

Now gathering the retaining wall site parameters begins. You and your contractor will need to determine the ideal wall height (at the tallest point), with reference to the top of supporting slopes above and behind the wall. Our engineers at Xpress Engineering will need to know if there are other surrounding features that would affect the design, such as, roadways, water ponding, building structures, rock outcroppings, etc. These parameters are critical in the design of the wall to prevent sliding and overturning.

Keep in mind the appropriate building setback for the wall need to be considered and will play an important role in the design. Consult with your local building department to determine your specific property setbacks.

Matching Structural Requirements

Once the parameters are provided, one of our expert engineers will determine the retaining wall thickness, required reinforcement of the wall and footing, necessity of a shear key and more, including any special site specific notes to successfully construct the wall. These wall elements are all based on the parameters provided. We will also provide drainage recommendations, but the final drainage design is completed by your contractor who is more familiar with the seasonal rain that occurs at the project drainage. This is discussed more below. Simply, you won’t have to worry about the retaining structure with our retaining wall designs.

Adding Drainage

Retaining walls help to stabilize the landscape, but a landscape acts very differently- and has different soil parameters- when it’s wet vs dry. All good wall designs incorporate drainage, typically through the backfill placement, depth, and structural design. Sometimes, greater drainage options, or full systems, will be built in to the space behind the back face of the concrete. Due to the varying locations and geographical climates, the final drainage pipe sizing, depth, amount, and type will be done by your contractor. Xpress Engineering recommends hiring a local Civil Engineer who can determine water shed and/or ground water demands and size the drainage accordingly.

The total wall setback, thickness, length, footing size, and design of retaining walls is an art, but done correctly by a structural engineer, your retaining wall will last for decades to come.

Are you ready to get started? Contact Xpress Engineering today, and we’ll have our team get right back to you with the perfect concrete retaining walls for your needs.