Free-standing retaining walls are structural elements designed to resist the lateral pressure of soil while providing support for changes in elevation, such as slopes, embankments, and excavations. These walls are constructed in open excavations, and once the wall is built, the retained soil is backfilled. This article will explore the types of free-standing retaining walls, the principles of their stability, and the various applications in civil engineering and construction.
I. Introduction to Free-Standing Retaining Walls
Free-standing retaining walls are typically used in areas where space allows for stable slopes to be compacted on the sides of an excavation. These walls are structurally independent, meaning they do not rely on other building elements for support. This makes them suitable for locations where the wall is intended to retain soil at a specific level or to manage changes in grade, such as in landscaped gardens, highway underpasses, or embankments.
In some cases, free-standing retaining walls are temporarily designed as cantilevers during construction. Once the wall is completed, it may be propped up by adjoining structures, such as floors in a building. One of the most common applications of these walls is in highway underpasses, where soil needs to be retained on both sides of a road.
II. Stability Requirements for Free-Standing Retaining Walls
For a free-standing retaining wall to perform its function effectively, it must be stable against two key forces:
- Stability Against Overturning: The wall must resist the moment (rotational force) that tries to tip it over. This is achieved by ensuring that the center of mass of the wall is positioned in a way that the forces acting on it do not cause it to rotate uncontrollably.
- Stability Against Sliding: The wall must also resist horizontal forces that could cause it to slide along the ground. This is typically achieved by ensuring the wall has adequate weight or by incorporating additional structural features, such as a base key, to prevent displacement.
The design of free-standing retaining walls takes these stability concerns into account, and different types of retaining walls use distinct mechanisms to resist overturning and sliding.
III. Types of Free-Standing Retaining Walls
Free-standing retaining walls come in various designs, each with its own unique way of providing stability. The three most common types are gravity retaining walls, cantilever retaining walls, and counterfort retaining walls.
1. Gravity Retaining Wall
Gravity retaining walls rely primarily on their own weight to resist the forces acting on them. Made from mass concrete, these walls do not require any additional structural reinforcements to provide stability. The toe of the wall, the part that extends outward at the base, plays a crucial role in resisting both overturning and sliding.
In terms of design, the primary concern is to keep the resultant force—i.e., the combined effect of all the forces acting on the wall—within the middle third of the base. This ensures that the wall remains stable and that the base stays in contact with the soil at all times.
Because of the significant volume of concrete used in these walls, it’s important to manage hydration temperatures during construction to prevent cracking. This can be done through careful mix design, construction procedures, and curing methods. Reinforcement mesh is also commonly placed in the wall faces to control thermal and shrinkage cracking.
Gravity retaining walls are typically economical for lower walls, often up to around 3 meters in height.
2. Cantilever Retaining Wall
Cantilever retaining walls rely on both the self-weight of the wall and the weight of the soil behind it (the fill) to provide stability. The structure consists of a vertical stem (the main part of the wall), a base slab, a toe, and a heel. Each of these components is designed as a cantilever slab, meaning they extend outward without additional support.
The distribution of bearing pressure under the heel and toe varies: under the heel, the downward pressure from the fill is greater than the upward bearing pressure; under the toe, the opposite is true. To maintain stability, the line of the resultant force must also stay within the middle third of the base.
If there is insufficient resistance to sliding, a key can be added beneath the base to improve stability. Cantilever walls are typically more economical for heights ranging from 3 to 6 meters.
3. Counterfort Retaining Wall
Counterfort retaining walls are generally used for larger wall heights, typically between 6 and 12 meters, where the design of a cantilever wall may not be cost-effective. These walls function similarly to cantilever walls in that they rely on the weight of the fill and the self-weight of the wall to resist overturning. However, counterfort walls incorporate additional elements called counterforts—diagonal bracing walls that are spaced at regular intervals along the stem and heel.
The role of the counterforts is to act as tension ties, effectively transforming the cantilever action of the stem and heel. These counterforts help distribute the forces more evenly, making the structure more efficient. The heel slab is supported on three sides, while the stem spans horizontally between the counterforts.
Counterforts are typically pitched at a 70-degree angle, which can sometimes result in a shorter base, reducing the wall’s resistance to sliding. To compensate for this, a key may be added beneath the base. Counterfort retaining walls are ideal for taller walls, typically in the range of 6 to 12 meters in height.
IV. Conclusion
Free-standing retaining walls are essential elements in civil engineering, serving a wide range of applications from landscaping to highway construction. Their design depends on the height of the wall, the type of soil being retained, and the required stability against overturning and sliding. The three main types of free-standing retaining walls—gravity, cantilever, and counterfort—each offer distinct advantages based on the scale and specific needs of the project.
- Gravity retaining walls are best suited for smaller walls up to 3 meters in height, where the wall’s own weight is sufficient for stability.
- Cantilever retaining walls are typically used for medium heights (3 to 6 meters), with additional design considerations for sliding resistance.
- Counterfort retaining walls are ideal for taller walls (6 to 12 meters) where the addition of counterforts improves structural efficiency.
Ultimately, the selection of a free-standing retaining wall design depends on factors such as wall height, soil conditions, and the overall cost-effectiveness of the structure. Each design ensures that the wall can resist the forces acting on it, providing safe and stable support for the surrounding soil.