Retaining walls are important structures that support and hold back loads caused by both static and dynamic loads. They must be designed appropriately to prevent the failure of it. Designing a retaining wall requires the understanding of the retaining wall system in all of its aspects and the loads that it will support and experience. Thankfully most manufacturers have designs available for each of their retaining wall blocks that can be adhered to given it is a smaller retaining wall of three feet or less that is also not supporting a major structure. Some manufacturers have an engineering department that will also work with you to achieve an engineered drawing for your specific project or you can always hire an engineering firm that specializes in retaining wall structures. Additionally, retaining walls also need to be aesthetically appealing in their design. This is why the design of retaining walls is so important.
Design of Retaining Wall
There are several aspects that contribute to the design of a retaining wall. The height of the wall, the length and shape of the wall, the surcharge that acts on the wall, the subsoil, and the wall block dimensions and weight all play roles in considering what the design of the retaining wall will be.
Though secondary to that actual engineering design of the wall itself, the aesthetic appeal is important. You need to decide the shape and dimensions of your retaining wall in order to achieve the functional purpose. Curved retaining walls are more complex in the planning and design process. They are also more costly than a straight retaining wall.
When it comes to curved retaining walls, you need to think about the wall block that you will be working with. If the wall block only comes in linear units, in order to achieve a curved retaining wall you will need to cut each individual piece. This is a costly and time consuming process. If your retaining wall does come with tapered units that allow you to create curves, then it is easier to shape your wall to the radius that the wall blocks naturally create. Not doing this will also cause you to need to cut each block to achieve your custom curve.
When it comes to linear retaining walls, there are not as many variables that you need to consider. The one thing that presents challenges is when you come to a corner that is not a 90 degree corner. Corners are a weak spot in any wall and wall units need to alternate in corners to provide some strength much like walls blocks are staggered in a wall. If there is a seam moving up the entire corner without a stagger, the corner will fail. Wall blocks that have a rock face where they have been split to a rough face are easy to chisel or split to create a rock face at any degree allowing you to easily create that alternation in the wall blocks in a corner at any angle. Wall blocks that have a smooth face present a challenge. These cannot be cut or split to provide the same smooth face look, so corners that are not 90 degrees are more complex in their design. With these, there will likely be a seam moving up the corner. The alternation is achieved where the wall block is not present in the back with some offset cutting techniques.
The taller your retaining wall is, the more limitations you will have for wall block choices. The dimensions and the weight of the retaining wall block, as well as the locking mechanism of the wall block, are all important in how tall you will be able to build your retaining wall. Always refer to the manufacturer for specifications on each of their retaining wall blocks when beginning to plan your retaining wall.
Wall caps are the final step in the design process as not every wall block has a corresponding wall cap. A lot of manufacturers create a wide range of caps that can be used to be installed on many different lines of their retaining wall products. Another option would be to use natural stone caps.
Engineering a Retaining Wall
It is important to note that no two retaining walls are the same. There are several variables that need to be considered prior to planning a retaining wall. There will always be varying aspects of any job site that need to be considered and these designs featured below are a guide to help you in planning your retaining wall. Typically a retaining wall above 3 feet in height requires engineered drawings in regions. Consult with an engineer prior to planning your project to discuss what would be required. For retaining walls 3 feet and below that are not holding up significant dynamic or static loads, these designs will help guide you.
As discussed, the loads that are placed on the retaining wall from behind are in important variable that needs to be considered in the engineering of a retaining wall. These forces are called the retaining wall surcharge. They can be static or dynamic loads. An example of a static load that needs to be considered is a building or structure in proximity to the back of the wall. An example of a dynamic load would be vehicles moving in close proximity to the back of the retaining wall. These may not be present in your retaining wall build, but there are still forces acting on your retaining wall such as the soil that it is retaining or the water that will enter the system from a rainfall.
The retaining wall needs to be engineered to retain these forces that act upon it. This begins with a properly compacted base on a compacted subsoil with a non-woven geotextile separator. There should be a minimum of 6″ of the retaining wall embedded and 6″ of base extending past the front of the retaining wall. This prevents the toe of the wall kicking out from the surcharge that it will experience. There must be a minimum of 12″ of base past the back of the retaining wall as well. On top of this will be a drainage pipe to collect water that enters the system and properly graded drainage stone that will allow water to flow with minimal resistance to the base of that wall and collected into the pipe.
That pipe needs to exit the wall every 50 feet and have a 1/8″ per foot slope to an area that it can drain past the wall without negatively affecting it. The wall is then built up with properly graded backfill, a possible setback on the wall, and possible geogrid installed in the retaining wall as specified by the engineered drawings. Geogrid helps to stabilize the aggregate placed on top of it reducing the pressure placed on the retaining wall from the back.
The final grade on top consists of a low permeability soil on top of a non-woven geotextile with a top soil on top of that soil graded to a swale to direct water away from the back of the wall and limiting the amount of water that will enter the system. That swale should not hold water and should be sloped to an area that it can also drain.
Design of Retaining Wall Example
Here is a design of a retaining wall that has been built according to engineered specifications. This includes a proper base, embedment of the retaining wall block, drainage pipe, drainage stone, geotextile, geogrid, and final grade.
Geogrid may need to be extended past the drainage stone area in behind the wall where it will extend into the soil backfill behind the drainage stone. To achieve this, the geotextile will return to the retaining wall where the geogrid will be placed on top. It is also debated whether or not geotextile is recommended up the vertical backfill of the retaining wall.
Adding batter to the wall where each wall block is setback according to the manufacturer’s specifications allows for a taller wall to be built using the same wall blocks. For example with a wall block that is specified for a max height of 2 feet, you may be able to build using that same wall block to 2.5 feet in height if you add a setback to each row of wall blocks. This helps to retain the surcharge that is placed on the retaining wall.