Retaining Wall FAQs

What are the different types of precast concrete retaining walls and how does each work?
There are several different types of precast concrete retaining walls, including:

Mechanically Stabilized Earth Systems

  • These walls are built from individual panel sections.
  • The panels interlock and are supported by steel straps that are mechanically attached to the back of the panel and extend back from the panel into the retained soil, resulting in a reinforced soil zone behind the wall.

 

Large Precast Modular Block – Unreinforced / Gravity Wall

  • These walls are built from individual large concrete blocks and generally stacked in a running bond fashion.
  • The wall systems usually have an interlocking feature or shear key mechanism that locks one course of block to the next.
  • They retain the soil by virtue of there size and weight. No additional reinforcement of the soil is required.

 

Large Precast Modular Block – Reinforced Wall

  • These walls are also built from individual large concrete blocks and generally stacked in a running bond fashion.
  • Given the height of the wall, soil reinforcement is now incorporated with these large blocks.
  • The soil reinforcement used is generally a form of geo-grid attached to the large blocks either frictionally or mechanically.

Cantilever Wall

  • These are typically large “L” type units with an extended footing on the front side of the wall. The footing helps resist the overturning moment.
  • These wall systems retain the soil by virtue of there size and weight. No additional reinforcement of the soil is required.

 

What methodologies are used to design retaining walls?
The American Association of State Highway Transportation Officials (AASHTO) uses the FHWA Publication Number FHWA-SA 96-071 Mechanically Stabilized Earth Walls and Reinforced Soil Slopes Design and Construction Guidelines (known as Demo 82) dated October 1996.

AASHTO is generally used for public and government projects. It calls for reinforcement depths equal to the greater of 70 percent of the wall height or 8 feet, requires uniform reinforcement depths, requires imported sand or gravel in the reinforced zone, and calls for a minimum embedment depth of 2 feet.

Other design methodologies may be available depending on the system used.

What safety factors should the retaining wall engineer calculate?
The retaining wall engineer should be calculating:

  • External Factors of Safety, including Overturning, Sliding and Bearing.
  • Internal Factors of Safety, including Pull Out.
  • Local Stability, including Shear.
  • Global Stability