Digging for a retaining wall requires some planning. There are minimums that are required for the depth as well as width of the trench for your retaining wall project. However, there are always exceptions to these minimums especially with a retaining wall. Because retaining walls are important in retaining the soil in behind them, there are a few factors that need to be considered in order to calculate the necessary measurements for both the depth and width of your trench.
For example, if your retaining wall is supporting a major structure or a parking lot, that is going to add significant surcharge (load against your retaining wall) which will require some engineering. If your wall has a significant slope immediately after it, this will also require some planning. And if you are wall is over 3 feet, then it will definitely require some engineering. Whenever any of these aspects exist in your project you will require engineered drawings and you will want to follow those specifications. For most retaining walls, this is a helpful guide to get you started with the measurements for the depth and width of your trench.
How Deep to Dig for Retaining Walls
Knowing the total depth to dig for your retaining wall project relies on two different aspects. The base and the embedded course of the wall. Adding these two up will provide you with the depth you are going to need to dig for your retaining wall. Walls under 3 feet in height from the final grade to the top of the retaining wall have a fairly standard measurement for both of these aspects. If your wall is over 3 feet or you have a significant surcharge (force against the back of your wall) you are likely going to need to seek an engineer to create drawings for your wall.
- Base Depth for a Retaining Wall
- Embedment Depth of a Retaining Wall
When it comes to a base for our segmental retaining wall we install a gravel base of around 6 to 8 inches in depth. The one thing that we may want to consider for this depth range are our subsoils. If we have a well draining subsoil, such as sand, we can probably stick to a minimum on this range that I’m going to give you. Whereas a clay subsoil that is very dense and it does not drain well, you are probably going to want to dig a little bit more to give yourself a little bit more of a base. Additionally if you are going to add a screed layer before laying your initial course, you will want to add at least 1 inch to this measurement.
We also need to factor in the depth of embedment for a retaining wall. This will depend on the height, slope away from the wall, and surcharge (force against the back of the wall). For most walls that are under 3 feet in height you are likely opting for a minimum 6″ embedded course of your wall as long is there is no significant surcharge or slope after the wall.
When you add together the depth of the base, screed layer (optional), and the depth of the embedment for the retaining wall, this adds up to a total of 12 to 15 inches for the depth of your excavation for your retaining wall.
Another thing that you need to consider are any step ups or step downs in your project. You always need to consider that range of 12 to 15 inches, so when you are building a wall on a slope there will be times that you fall out of that range and your embedded course may begin to show above the final grade. At this point you are actually going to need to dig down an additional amount depending on the height of your retaining wall product to be able to step down or step up.
Width to Dig for Retaining Wall
Besides the excavation depth, we also need to consider the excavation width. To calculate the width of the trench for a retaining wall we need to add up the measurement in front of the retaining wall, the depth of the wall block, and the drainage area behind the wall.
A retaining wall requires a 6 inch toe of base material in front of the wall. The depth of the wall block depends on the wall block that you are installing which is typically at least 8 inches. The drainage area behind the wall is a minimum of 12 inches. Adding these up provides a minimum of 26″ for the width of your trench for your retaining wall.
You will be preparing your base material throughout this entire trench. In front of the wall block after your base will be your final grade, whether that is pavers or topsoil. In behind the wall block, that is your drainage area. That is going to dictate where your drainage pipe is going to go as well as filling in with a three quarter inch angular crushed clear stone and compacting that as you build up your retaining wall.