October 10, 2019

What do I need to know as I plan/design my retaining wall?

There is a lot of planning that goes into a retaining wall design when things begin to start moving forward.  To provide more information, you can visit the Allan Block Residential Retaining Wall Manual to help organize and formulate your plan. To summarize some of the main talking points, lets break it down into the following 5 items:

           Soils – What type of soils do you have at your project?  Are they sandy? Clay? Gravel?  This can increase the pressure on the wall, and require additional reinforcement depending on the height of your wall.  Since the purpose of the wall is to retain soil, this remains one of the most important aspects in the design.

           Slopes – Are there slopes above the retaining wall?  If the grade is going to slope above the wall, this is an added load and therefore may also require additional reinforcement.  Slopes above a retaining wall remain one of the largest loads possible; the steeper the slope, the more load the wall will carry.  What about slopes below the wall?  If there is a slope below the wall, then you will want to verify with your local engineer that global stability isn’t a concern, or that your wall will remain stable.  Slopes above and below a wall are an immediate red flag for a global stability concern.

          Surcharges – A surcharge can be anything above the wall that adds weight to the soil behind it.  This can be a parking lot, a driveway, a building, equipment, snow loads, or as mentioned above, slopes.  Anything that may be placed above the wall with a sizable mass to it is considered a surcharge.

      Water Management – Water is the #1 reason why walls fail.  With that being said, you will want to make sure your grades around the retaining wall don’t bring water towards it.  Instead, you’ll want to use berms and swales to guide water around the wall, keeping the soil behind it as dry as possible.

Examples of permeability in different soils and gravel.

      Retained Height – The height of the wall plays a major role.  As the height increases, more pressure is exerted on the wall.  Many local building codes govern when a retaining wall needs engineering and permitting, but even small walls can have issues if the previous notes aren’t planned for and taken into consideration.  As the wall increases in height, you will want to make sure you have additional reinforcement to make sure your wall lasts a lifetime!

These items may not be the only items to check and consider prior to construction of your retaining wall, but it is a quick list to run down as you begin to plan your wall project.

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