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Planning and Estimating your Segmental Retaining Wall

Tuesday, February 17th, 2009
Planning and Estimating

Planning and Estimating

Our backyard wall was going to be a big project for a small backyard. We needed to have a staging plan for all the material, but before that we needed to estimate the project and figure our budget. The first step in building a retaining wall is to make a plan. A good plan will lead to a good project. To start, figure out how much grade change you are working with. In our case we had around 7 ft. We decided to cover this height with two terraced walls. But how tall should each wall be? We decided the lower wall should be about 3 ft tall for easy access for planting and gardening and the upper wall should cover the rest of the grade change and be as close to the property line as possible.

Now that we knew the rough wall height and wall length (property line length) we used the incredibly easy AB Landscape Estimating Tool that we downloaded from allanblock.com. It works in Microsoft Excel which most people have on their computers. When using this great tool, I had to remember to add a little to the wall height for the buried block. You can also use this estimate to calculate the amount of wall rock and infill soil you will need. Don’t forget to consider equipment rental such as a skid steer, plate compactor, diamond saw and maybe even a wet saw for cutting caps at the end. It all adds up and if you plan ahead you will limit the surprises to your budget during the process.

Staging & Site Preparation

Planning and Staging

How about staging the material? I think a lot of do-it-yourselfers like me, forget that with a project of this magnitude you need to store/stage a lot of material. Our project had 23 pallets of Allan Block, 3 truck loads of washed wall rock and 1 truck load of sand, drain piping and geogrid. We also had to have a place to store the spoil, or the earth that we removed

during the excavation of the hillside. This was a huge pile that was there from day one until the end of the project when it was used as topsoil backfill. Because we had a good plan for all the material and spoil, the neighbors were surprised we had space to actually build the wall!

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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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AB Commercial Retaining Walls Manual

Wednesday, January 7th, 2009

January 7th, 2009 MINNEAPOLIS, MN For complete details on how to build segmental retaining walls for commercial, residential, DOT or others projects using the Allan Block products, see the AB Commercial Retaining Wall Manual . From start to finish this document will guide you through everything you need to know for your project to be successful. Learn how to plan and design out your project before beginning, then use the detailed information, graphics and photos to install a gravity, reinforced or patterned retaining wall.
The AB Commercial Retaining Wall Manual is a very useful tool for everyone involved with SRW projects. Engineers and architects can understand how the system is engineered, use the detailed designs, reinforcement charts and specifications to specify the AB products with confidence. Contractors find the helpful installation information makes their jobs more efficient by saving them time and money. Owners of construction companies are known to be handing out this manual, in English and in Spanish, to their crews when building Allan Block Retaining walls because of the detailed information that is available.
Besides the basic installation information that is available, check out some of what is also included:

  • Building curves, corners and stairways
  • Projects with water, its effects and requirements for proper construction
  • Proper techniques for incorporating terraces
  • Finishing and ending wall options
  • AutoCAD Details on common topics
  • Detailed soil information
  • Internal and external analysis plus design considerations

Allan Block Corporation has been in the segmental retaining wall business for over 20 years, with millions of square feet of their products installed worldwide. The AB Commercial Retaining Wall Manual has the latest and most up to date information available. Visit our website at allanblock.com to view or print your copy, as well as check out all of the other tools and information that are available to help you Plan, Design and Build your next Allan Block Retaining Wall Project.

Media Contact:
Allan Block Corporation,
Phone: 952-835-5309
Fax: 952-835-0013
Email: info@allanblock.com
http://www.allanblock.com

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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

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