Retaining Wall Installation Considerations
Retaining walls add style and durability to your landscape. The purpose of a retaining wall is to keep soil in place when the desired changes in ground elevation exceed the angle of repose (the angle at which the soil on the slope is on the verge of sliding). By this definition, basement walls are a type of retaining wall. The type we’re talking about is the landscape variety.
There are some important things to keep in mind, other than proper equipment and tools, as you plan your retaining wall project.
Access to the Site
Make sure you can get to your project site with the necessary materials and equipment easily, which will save you time and stress. If access to your project site is restricted, then plan a staging area for all of your blocks, wall rock and materials.
The material you use for the base (in and behind your block) is called “wall rock.” It’s usually smooth or crushed stone that is aggregate, easily compacted, well-graded and from ¼-inch to 1 ½-inches in size. Wall rock gives you the perfect foundation for your retaining wall. The compaction and drainage that wall rock provides increases the performance and quality of your wall.
Necessary Tools & Equipment
It’s always best to have everything ready before you begin. Here is a list of tools & equipment you’ll need:
- Various hand tools
- Safety glasses
- Ear protection
- Dust masks
- Knee pads
- Tape measure
- String line
- 4’ level
- Torpedo level
- Dead blow hammer
- Hand tamper
- Round & square shovels
You’ll also need some power tools, such as a concrete saw with a diamond blade, a skid loader, a plate compactor and a transit/site level. Rather than purchase them, you can get them at most equipment rental centers.
Proper Soils for Retaining Walls
While retaining walls are built to keep soil in place as part of your landscape design, the type of soil you use in building your wall is a very important component of the wall itself. The typical reinforced retaining wall is comprised of three basic materials: the blocks, the geo-grid reinforcement and the soil used as infill around the geo-grid layers.
Knowing your soil is an important part of building a quality retaining wall. Soil type determines the length of time needed for proper compaction, as well as the amount of reinforcement you’ll need and even the cost of the project in some cases.
The best soils for retaining walls are granular; they outperform clay soils every time. Granular soils, such as gravel and sand, need less reinforcement. They also drain and compact better than clay soils.
One of the defining characteristics of soils is their friction angle, also called the soil’s internal strength measurement. The friction angle is equal to the natural angle of repose (the angle at which the soil on the slope is on the verge of sliding). A qualified geo-technical engineer can provide an accurate soil classification if you need one.
You may find that your on-site soil (under or behind the wall) isn’t the best quality for building a retaining wall. If that’s the case, it is better to remove the soil and replace it with stronger soil. You’ll need less reinforcement, have a faster rate of compaction and get better long-term performance from your wall.
This is not an area for cutting corners. Organic soils and heavy clays are not suitable for use in the reinforced areas. Silty sands and sand mixed with clay also need extra care in terms of drainage management when you put them in place and compact them. The proper placement and compaction of your infill soil is vital to the success of your project.
Compaction is basically applying stress to the soil to displace air from the spaces between the soil grains. The process creates a denser layer of soil. Compaction is important in any construction project because it adds strength and durability to whatever is being built.
Soil is placed in layers, called “lifts.” It’s key to the success of your project that the lifts are less than 8 inches deep. Lifts deeper than 8 inches don’t compact well enough to provide adequate soil strength.
You should always backfill and compact after every level of blocks is set into place. Proper compaction equipment must be used for the best results. The type of soil you’re compacting determines the sizing of the compaction equipment. Your local equipment rental center will know which equipment is right for your project.
An area called the consolidation zone stretches from the back of the block 3 feet into the infill soil. The only type of compaction equipment that should be used in the consolidation zone is a walk-behind plate compactor. It will take a minimum of two passes to properly compact the soil. The passes begin on the top of the block and run parallel to the wall, working to the end of the excavated section.