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Why is there a limit to how tall a gravity retaining wall can be built?

Friday, February 13th, 2009

Yesterday I received a call from a contractor wondering why a chart on the Allan Block website limits the height of a gravity wall. After talking to them for a while I had a sketch in front of me of the wall they wanted to build.

The contractor sounded like he had a good head on his shoulders, so I started explaining the concepts of external stability. Since he mentioned the website, I directed him to the page that shows external stability so that he could see pictures and the recommended factors of safety for each of the design criteria.

External stability can be broken down into 4 main components:

  • Sliding: The wall is pushed forward
  • Overturning: The wall tips or leans forward typically pivoting on the toe of the base course.
  • Bearing: The retaining wall settles due to the weight of the retaining wall itself.
  • Global Stability: The site the retaining wall is built on has soils that are not stable. This analysis should always be done by a local geotechnical professional, especially when dealing with a site where you will have a slope above or below the retaining wall.

After our lengthy discussion on external stability, and looking at the website, the contractor had a firm grasp of why gravity walls are limited in height. In the end, he decided that geogrid reinforcement would be necessary for his wall.

Tom

Posted in Commercial Retaining Walls, Landscape Retaining Walls | 1 Response »
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Can I Use a Simple Gravity Wall?

Friday, December 12th, 2008

The question often comes up whether a wall can be built by just stacking block using the installation recommendations found on the Allan Block website, or if geogrid reinforcement is necessary behind the wall. Every time this question is asked, four equally important issues need to be addressed: how tall is the retaining wall, what is going to be above the wall, what type of soil will be behind the wall, and what type of block is being used?

To help simplify this decision, take a look at the Maximum Gravity Wall Height Chart on the Allan Block website. This handy chart can give you an idea of the maximum wall height for a variety of situations. The AB Commercial Wall Manual also describes the fundamental principles of gravity walls and how to build them using Allan Block retaining wall blocks.

How tall is the retaining wall?

AB Ashlar Blend Pattern

AB Ashlar Blend Pattern

This is an important question to answer because the taller the segmental retaining wall (SRW), the more soil behind the wall is going to be trying to push the wall forward or tip it over. This is called the wall’s external stability. In order for a gravity wall to work, the weight of the block and crushed stone within the block must be greater than the forces pushing against the wall.

What is going to be above the wall?
It is intuitive that a landscape wall with a level planter bed of flowers above it would have less force pushing on the wall than if you were building a retaining wall to hold up your driveway, but what about a slope above the wall? When your wall has a slope above it, there is more weight above the wall trying to push the wall over. What another wall above, forming a terrace? True, you might only have two 3-foot (0.9 m) walls, but that top wall of the terrace is going to be pushing on the bottom wall. In summary, any time you have a surcharge (some type of additional weight) above the wall, your overall gravity wall height will be reduced.

What type of soil is behind the wall?
Clay soils are going to push against the wall more than granular sandy soils. This is because a typical clay soil has a lower internal angle of friction than a sandy soil. By clicking the link above, the AB Commercial Wall Manual may be reference to learn more about soils and their influence on the wall.

Which Allan Block retaining wall block is being used?
A wall made from AB Stones will tilt into the hill more than a wall made from AB Classic blocks. This is because the AB Stones have a setback of roughly 12-degrees, while the AB Classic has a setback of about 6-degrees. Therefore, under the same conditions, a gravity wall constructed from AB Stones will have greater stability than a wall built using the AB Classic.

Next time you are planning your landscape or retaining wall project, keep these four questions in mind. Then, reference the Allan Block website to assist in designing a gravity retaining wall that you can enjoy for a lifetime.

Tom

Posted in Landscape Retaining Walls | No Responses »
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