What makes a Retaining Wall Work?

There are two types of walls to consider when asked the question “what makes a retaining wall work?”  Gravity walls and reinforced walls.  First, let’s look at gravity walls.

Gravity walls require two things to perform well – the Allan Block unit (its weight) and the setback the unit has.  What is “setback?”  Setback is the degree to which a retaining wall “leans” back from vertical as shown in the gravity wall section below.  This setback, coupled with the weight of the block itself, will allow the wall to perform as it should in a “gravity wall” application.  Check out our setback charts for additional information.

Reinforced walls require three things – the Allan Block unit (its weight), setback, and geogrid.  Since we have identified what setback is, the only potential unknown is geogrid.  Geogrid is a woven textile that has been used to strengthen soil for decades.  Reinforced segmental retaining walls utilize geogrid to strengthen soil and create a mass heavy enough to resist the active forces of the soil behind the wall, ultimately allowing us to build taller walls.  A typical reinforced Allan Block section is here:

To learn more about gravity walls or reinforced walls please visit allanblock.com and/or call us today to discuss in more detail. 

Should I be Worried About Water Runoff While Building My Retaining Wall?

Water is enemy #1 for all retaining walls, and must be managed with post construction management details as you will see in our water management section on allanblock.com.  When water is present in soil behind a retaining wall, it will add significant weight and pressure to the wall which may cause concern.  For that reason, routing water away from the wall after—construction and even during construction—is a Best Practice for Segmental Retaining Wall Design and should be done whenever possible. 

As read in Chapter 6 of the Allan Block’s Best Practices for SRW Design manual, backfill should be graded to avoid water accumulation behind the wall, or in the reinforced zone at the conclusion of each day’s construction. This may require a temporary berm at the back of the reinforced zone as outlined in 6-11 on the detail below, indicated by the red arrow.  This is done to help reduce the amount of water that may accumulate behind the wall, which adds weight and pressure that the wall was not designed for.  For more tips on construction, including water management details, visit our Retaining Walls by Allan Block guide or our Design for Water section that outlines post construction recommendations and the potential need to temporarily control water during construction.

How do I Figure Out How High My Retaining Wall Should be?

Retaining walls help us offset grade changes to create usable land on our property.  There are two ways a wall will be constructed, and they are referenced as a “cut” or a “fill” site.  A cut site is one in which you will cut into the hillside to add flat land, and a fill site is one which you will add soil to and raise the land to flatten out the area.

In order to properly size the wall, we will need to have a detailed understanding of site elevations and grade changes to determine wall heights.  Starting at the lowest point on your site, mark your grade changes in 1 ft. (0.3 m) increments on the plan. Mark your elevations on a drawing like the one pictured. 

You may find it easiest to use string lines and line levels to create the elevations on your site. Determine the elevation (height) you want to “fill to” or the elevation point you want to place your wall and “cut” into.  This point, or elevation, will be your standard/benchmark.  From the point you just created, run a string line away from it towards the low spot on your property and/or to the point that will be the low part of your wall.  Keep the string level by using a “line level” and pull the string all the way to the low spot on your site.  Use a stake to hold the string in place and ensure it is pulled tight.  Now, mark the elevations in 1 ft. (0.3 m) increments up to the standard/benchmark identified at the beginning.  Using this technique will help you understand how tall of a retaining wall you will need to build. 

How Much of the Base Course Should I Bury?

One of the great things about segmental retaining walls (SRWs), is the fact that the installer does not need to excavate to frost depth and/or pour a concrete footing below the structures. This helps reduce cost and speed up the installation of the systems.  SRWs are flexible in nature, and only require minimal burial below grade.  So, how much block do I need to bury on my first course?  The industry recommends that your first course be buried a minimum of 1ft (2.5cm) for every foot (.3m) in wall height, or 6ft (15cm) (whichever is greater) if the ground in front of your wall is flat.  

For unique applications where a slope is running away from the face of the wall (pictured below), the industry has a different recommendation of 5ft (1.5m). This means that though you have a slope downward, the soil against the block is still 5ft (1.5cm) deep. the burial outlined in both situations helps reduce the likelihood of erosion at the front face of the wall.