How To Properly Add Drainage To Your Retaining Wall

Water is the major cause of retaining wall failures. Below is a stone wall that failed due to improper drainage behind the wall. The combination of the ground sloping toward the wall, no drainage stone behind the wall, and no clear weep holes or drainage outlets lead to this wall failure. Due to the height of this wall, the pressure that built up behind the wall due to water sitting behind the stones caused this blow out.

When a Drainage Pipe is Required Behind the Wall

Every retaining wall should include drainage stone behind the wall. Though it is a good idea to install a drainage pipe on all walls, there are certain situations where a perforated drain pipe is absolutely necessary. Below are several scenarios that require a drainage pipe behind the wall:

• Walls with a height greater or equal to 4-ft as measured from the foundation to the top of the wall. The segmental blocks cannot hold the weight of that much water by themselves. Also, walls greater than 4-ft can cause catastrophic damage if the wall fails.
• All poured concrete or cinder block retaining walls. These walls do not have natural joints for water to drain through like segmental blocks and wood walls do. These walls need a drainage system regardless of the wall height.
• If there are poor draining soils such as clay behind the wall, there needs to be drainage incorporated the wall system. Clay when wet is very weak, so it is essential to provide a way for water to escape from behind the wall.
• Walls that have buried water sources within 50-ft of the wall site, such as irrigation, water main, or a hose line, requires a drainage system
• Groundwater is present. This can be a little more difficult to detect. If the area is wet when you excavate for your wall, or you notice water collects near your proposed wall location even in dry conditions, this is a good indicator of groundwater.
• The ground slopes toward the wall. Water will naturally drain downhill. A drain system will be required to remove that water.
• You are building a tiered or terraced wall.
• Surface water also needs to be accounted for. Check for any gutter downspouts nearby and check where water drains within your property to ensure water is diverted away from your wall. If there is a downspout behind your wall, plan to install an additional pipe to outlet water to the front of the wall. Do not use a perforated pipe to transport water from a downspout behind the retaining wall. Use swales or berms to redirect surface water away from the wall. When possible, place the swale or berm is at least two times the wall height (2H) away from the wall face. If a swale or pipe is impracticable, plan to install a drainage system behind the wall.

What is a Drainage System?

A drainage system consists of several key components: drainage stone, filter fabric, perforated pipe, and outlets through the wall face. Before we dive into these components, remember NEVER use grout between the blocks on segmental block walls. Water should drain between the blocks, and grout blocks these gaps that would allow water to drain through the face. Proper segmental retaining wall blocks have a lip or pins to keep the blocks from shifting. Attach the top block and capstone to one another using construction adhesive* which keeps the wall system flexible.

Drainage Stone

All walls should include drainage stone, even if they don’t require a drain pipe. Install drainage stone at the back of the retaining wall and extend 12-in behind the blocks. Start the drainage stone near the base of the wall and extend up to within 6-inches of the top of the wall. To estimate the quantity of drainage stone, take the area of the wall in square feet and multiply by 1-ft to get the cubic feet of drainage stone. Divide the cubic feet by 21.6 to convert to tons or divide cubic feet by 27 to covert to cubic yards.

The amount of fines (material passing through a No. 200 sieve) should be less than 10%. Do NOT use pea gravel or river rock as drainage stone. The smoothness of this material will make it difficult to retain at the end of the wall or if you ever need to remove a block. In addition, smooth material is difficult to compact and will settle over time. I recommend a well graded compactable aggregate that is angular. The size should be 0.25” – 1.25” ideally. This includes ¾” crushed rock, #57 stone, #67 stone, or Class I or II backfill. Using one of these materials will also allow you to use the same material for the drainage stone, base material and wall rock.

Filter Fabric

Place filter fabric* or landscape fabric above the drainage stone and below the topsoil. This prevents fine material and organic matter from clogging up the drainage stone and staining the face of the wall.

The filter fabric needs to be a minimum of 3-ft wide and non-woven. Provide 6-in of fabric up the back face of the retaining wall. Use construction adhesive* to attach the filter fabric to the back of the retaining wall. Install 6-in minimum of overhang past the end of the drainage stone. Overlap the ends of the fabric 4-in minimum along the length. For estimating the quantity needed, the length of fabric is simply the length of the wall plus 5% for waste and overlap. You may purchase 6-ft wide rolls and cut them in half.

Note if the drainage stone is wider than 12-in, which is common when filling the entire excavation area with drainage stone, plan to supply a wider section of fabric to fully cover your drainage stone. You do not need filter fabric behind the drainage aggregate when a full 12-in of aggregate is installed behind the wall. If your site has clay or dark backfill, you may place filter fabric along the back of the retaining wall blocks to prevent the fine material from seeping through the blocks and staining your wall face.

Perforated Drainage Pipe

The perforated pipe* should be slotted all around the pipe. Some corrugated pipes have holes on only one side, and you want to avoid this. The pipe will run the full length of your wall (L) and should be 3-in or 4-in in diameter. In all cases, the drain pipe needs to have positive drainage of at least 2%, meaning it slopes toward an outlet location. A good rule of thumb, if your outlet locations are spaced 30-ft apart, the pipe should slope 7-in from the high side to the outlet at the low side.

Pipe Outlet

There are three different options that I will go over to outlet your drainage pipe, with the third one being my favorite. No matter how you choose to outlet the drain pipe, place an outlet a minimum of every 30-ft to 50-ft along the wall. If your wall retains clay, or if there is ground water present, place outlets at 30-ft along the wall.

1. Outlet a pipe through the wall face. This is ideal when the ground line in front of the wall is flat. Place impermeable material (soil that does not easily allow water to pass through) in front of the blocks, behind the blocks, and within the hollow core of the block (if applicable) up to the ground line at the front face. The impermeable material may be onsite fill that was excavated for the trench or fine grained sand, silt, or clay. Do not use any soft or organic matter. Lightly water then compact all the impermeable material with two passes of the plate compactor.

Cut a hole in a block using a concrete saw just big enough for the pipe to extend through the wall. The hole in the wall may be in the second course of blocks. Place the pipe on top of the compacted impermeable material. Place filter fabric at the interface between the back of the block and the pipe to prevent backfill from migrating through the opening. You can also grout around the pipe at the front face of the block.

You will need a tee connector* to connect the perforated drain pipe to the outlet pipe. I also recommend installing a grate* over the pipe outlet. This prevents rodents from getting into your drain pipe and building a nest.

2. Outlet the pipe underneath the wall. This is ideal when there is a toe slope in front of the wall. This avoids cutting the blocks and allows the pipe to be placed lower on the wall. In this case, extend drainage stone all the way to the bottom of the wall. Where the pipe daylights, or exits the slope, you can install a sloped end cap* like the one to the left to help avoid a tripping hazard.

3. Use a Universal Wall Drain*. This is an innovative product that is not as widely known. Place the Universal Wall Drain vertically between segmental blocks so you avoid the need to cut the blocks. It then connects directly to the tee connector in the perforated drain pipe.

The universal wall drain is an aesthetically pleasing option that avoids rough cut marks typical in pipe outlets. It also has a built in grate to keep rodents from crawling into your drain and building a nest. This outlet option helps reduce labor time and reduce the number of materials needed.

Closing Thoughts

Drainage is a critical part of any retaining wall system. It cannot be overstated how important managing water is for the long term performance of your wall. If you have questions about drainage design, or anything else related to your retaining wall project, leave a comment below or hit the email icon at the bottom of the page.

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