News

Retaining Walls 101

May 30, 2018

By Andrea Mitchell, Structrual EIT

 

What do canals, basements, and overpasses have in common?

All of them utilize, or are a form of, retaining walls.  This article will teach you about the different types of retaining walls, when they are used and when you should consider hiring an engineer for your site grading needs.

Retaining wall are structures that hold back soil. These structures are used for significant changes in ground elevation. Retaining walls prevent soil erosion and are designed to resist the lateral earth pressure exerted by the soil. There are many types of retaining walls, each specific to their application. They can be constructed of various materials.

Types of Retaining Walls

The most common retaining wall include:

  • Gravity retaining walls
  • Cantilever retaining walls
  • Anchored retaining walls

Gravity retaining walls resist soil pressures by using its own weight. Gravity retaining walls are often made from stone blocks, gabion baskets, or concrete units. They may have a “battered” setback (inclined toward the higher elevation) to improve stability. Gravity walls work well when there is limited room for excavation, or if utilities need to be accessed behind the wall after construction. However, with large elevation differences, they can become quite expensive.

 

Gravity Retaining Wall

Gravity Retaining Wall

Cantilever retaining walls are used with large changes in grade elevation. They consist of a concrete wall and base concrete slab. Cantilever retaining walls use the weight of the backfill material to resist soil pressure. This type of retaining wall is the most cost effective; however, they are not always possible because they require more space.

Cantilever Retaining Wall

Anchored (or tieback) retaining walls use tension supports, usually cables driven and bonded to the soil or rock, to resist the soil pressure. The retaining wall is usually constructed of cast-in-place concrete. Anchored retaining walls are often used when space is limited, such as an encroaching property line, or on a site with high soil pressures. They can be used for temporary shoring or permanent applications. Anchoring can also be combined with either a gravity or cantilever retaining wall.

Anchored or Tieback Retaining Wall

Design Considerations for Retaining Walls

When designing a retaining wall, the site conditions include:

  • Change in ground elevation
  • Properties of the surrounding soil backfill material
  • Surrounding land use

The change in ground elevation determines the required height of the wall, soil pressures, and approximate material costs. The properties of the surrounding soil and backfill material will determine the stability of the foundation, as well as soil pressures. Sloping earth can exert great pressure, and this pressure increases when the soil is wet. A geotechnical survey defines these soil properties. The use of the surrounding land (i.e., pedestrian traffic, or roadways) will also contribute to the forces exerted on the retaining wall.

Other design considerations include:

  • Drain backfill material to prevent soil saturation. Drains should be installed both within and at the base of the retaining wall. Saturated soils result in increased pressures on the retaining wall. Without free draining backfill, these soil pressures could crack the retaining wall.
  • Design a firm foundation with compacted soil to prevent settling.
  • Provide control and expansion joints for long retaining walls to prevent cracking.
  • Place footings below the frost line to avoid the effects of frost heave. Alternatively, insulation can be placed at the top of the footing if sufficient frost cover cannot be achieved.

When do you need an engineer?

Local municipal councils typically regulate when an engineer is required to design a retaining wall.

Consult your municipality or get in touch with one of Crozier’s Professional Engineers to determine the regulatory requirements if you have unusual site conditions or if there will be any vehicular traffic or a heavy object placed on top of the retaining wall.  Our experts in civil engineering, water resource management, and structural engineering have extensive experience in lot grading and retaining wall design.

Not all retaining walls will require engineering drawings. However, when in doubt, check with your municipality and ask the experts before you start.

 

 

This article is for informational purposes only, and should not be used as a substitute for recommendations or services provided by a licensed engineer. C.F. Crozier & Associates is not liable for guarantees regarding the accuracy, reliability or completeness of the information presented. Please consult with a Professional Engineer before undertaking any of the above activities.

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