Building on sloping land, in a hilly area, or by a river often necessitates the construction of a retaining wall. These structures are crucial for holding back earth, stabilizing hillsides, supporting roads, protecting river banks, and preventing erosion. While they may appear simple, retaining walls perform a complex function and must withstand significant loads. Therefore, proper design and construction are essential to ensure their effectiveness and prevent costly failures.

Understanding the Basics

Gravity constantly works to level land, and understanding the angle of slip is critical. This angle represents the maximum natural slope that earth can form without slipping, and it varies depending on the type of earth and its moisture content. Wet soil, for example, is more prone to slipping than dry soil. Additionally, adding a load to the top of a slope increases the pressure and the likelihood of slippage.

When terracing land, creating level terraces with gentle slopes between them is ideal. These slopes should be gentle enough that the earth’s natural resistance prevents slipping. However, if steeper slopes are needed, stabilization is necessary. This can be achieved by building a solid foundation and covering the slope with a stabilizing layer of materials such as natural stone, interlocking concrete blocks, railway sleepers, or pre-cast concrete planks.

Types of Retaining Walls

When a vertical change in level is needed, a retaining wall becomes necessary. There are two primary types of retaining walls:

Gravity Retaining Walls

• These walls rely on their own mass to resist overturning and hold back the earth.
• They are typically constructed from materials such as brick, concrete blocks, or stone.
• There are three forms of gravity retaining walls:
  • Walls with two parallel vertical faces.
  • Walls with one vertical face and one sloping face, which provides a buttressing effect.
  • Walls with one vertical face and one stepped face, where the weight of the backfill on the steps adds stability.
• The thickness of a gravity retaining wall at its base should be at least one-third of its height.
• The strength of gravity walls can be increased by using materials that interlock, such as concrete blocks with steel bars.

Cantilever Retaining Walls

• Cantilever retaining walls are much stronger than gravity walls, but they require reinforced concrete.
• These walls feature a vertical wall with a horizontal “foot” that extends into the slope at the bottom.
• The weight of the fill on top of this foot holds it down, thus supporting the vertical face.
• Cantilever walls can be thinner than gravity walls due to the strength provided by the concrete and must be designed by a structural engineer.

Essential Construction Elements

Foundations

A good solid foundation is essential for all types of retaining walls, and these foundations must be keyed into the ground beneath. When built on sloping ground, deeper foundations are required to prevent the wall from sliding down the slope.

Drainage

Drainage is a critical factor in retaining wall stability because waterlogged ground behind the wall can greatly increase the pressure it needs to withstand, leading to failure. Effective drainage can be achieved through two primary steps:

• Backfilling the land immediately behind the wall with rubble or crushed rock to prevent moisture retention.
• Implementing a drainage system to remove water.

Common drainage methods include:

• Installing pipes through the wall to allow water to escape.
• Laying perforated pipes along the inside of the foundation before filling with rubble.

Proper drainage will prevent the build-up of pressure behind the wall.

Conclusion

Retaining walls are crucial for managing landscapes with slopes. Understanding the principles of their design and construction is essential for ensuring stability and preventing costly failures. Whether building a gravity or cantilever wall, the use of appropriate materials, a strong foundation, and effective drainage are vital for any successful project. By focusing on these key aspects, construction professionals can create safe and long-lasting retaining wall solutions.