Retaining walls are more than just a stack of blocks or stones piled up to keep soil from spilling onto a driveway or basement seating area. Retaining walls should be well-designed and carefully engineered for longevity. Part of that design is drainage.
Why do retaining walls need drainage? Drainage is a critical feature of retaining walls because it prevents water and heavy soil from putting pressure on the wall from the backside. Drainage redirects water away to an area where it can be beneficial while reducing settlement and erosion in the wall or its foundation.
The article below explains the importance of good drainage and how to evaluate your drainage options.
The Importance of Drainage
Retaining walls made of high-quality material are supposed to last at least 40 to 50 years. Most retaining walls fail because of water issues caused by a lack of drainage.
A retaining wall is like a dam holding the earth back. Without drainage, the water sits in the soil behind the all and the weight builds. Eventually, the weight and pressure are going to be too much causing the wall to blow out.
Water will flow on the easiest path and that is usually along the bottom of the all. This causes soil to erode behind the wall and move to the front. That creates more space for water and pressure in the back which will eventually shift or move the wall.
Water runoff on a retaining wall will erode the soil around it and kill plants and lawns.
The Benefits of a Drainage System
Not only does a drainage system protect your wall from erosion and failure but it diverts the water to areas better served. That means fewer puddles, less mud, and less standing water for bugs like mosquitos and flies.
A solid drainage system also makes a retaining wall easier to repair because it lowers the chance that you will need wall drilling to fix a crack. Home experts state every retaining wall should have a drainage system. However, some need it more than others. These include:
- Walls taller than 4 feet. These larger walls are holding back a lot of weight.
- Cinder block or poured concrete walls. There aren’t natural joints for water flow in these materials.
- Those with a lot of clay in the soil. Clay doesn’t drain or absorb water.
- Areas where there are water sources within 50 feet of the retaining wall. This includes a water main, irrigation, hose lines, and groundwater tables. This is because if there is a leak or if there is significant groundwater, your wall needs to be able to withstand the pressure.
- Retaining walls that face a slope. Water flows downhill.
- Terraced or tiered walls.
- Walls near downspouts.
What a Drainage System Includes?
A good drainage system includes several things including:
- Gravel in the trench below the wall and clean gravel as the backfill. It includes a drainage pipe running the entire length of the wall along the base.
- The wall should have outlets and a pipe that drains water away.
- Handymen have tips for those who want to ensure there is good drainage along the retaining wall.
- First, make sure there are 12 inches of space for backfill. Use clean crushed stone or gravel.
- The drainage stone should have filter fabric covering it. It lays between the stone and the topsoil.
- The drainage pipe needs slots on all sides rather than one side.
Don’t include river stone or pea gravel in your drainage stone. Both are too smooth to compact and settle. It also makes it harder to remove a block. Most suggest using an angular compactable aggregate. Suggestions include #57 or #67 stone or 3/4-inch crushed rock.
Several options exist for water outlets. Water outlets are holes in the wall so water can flow from the backside to the front. Excess water is moved to the front to flow that way and that relieves pressure on the backside.
Weep holes strategically placed along the base of your retaining wall allow water to flow. Their size and placement depend on the wall’s height. Weep holes can be simple holes drilled into the stone but they usually include an outlet pipe running through the wall.
The problem with an open pipe outlet is rats and other rodents can crawl up into the pipe and build a nest within the wall.
Another version is an outdated method of filling a small burlap bag with gravel and putting it in the hole to allow for drainage.
Pipe Underneath the Wall
Some newer methods include a pipe outlet underneath the wall. This is beneficial because you don’t need to cut the blocks, it can be placed lower on the wall and is perfect when a toe slop exists. It’s placed where the pipe exits the slope.
Universal Wall Drain
A universal wall drain is a new product that isn’t widely known yet. It sits vertically between the blocks. This means you don’t have to cut the blocks. The wall drain directly connects to the perforated drain pipe system.
It comes with a grate to prevent vermin from building a nest within the wall.
Where does the perforated pipe go?
The perforated pipe runs the length of the backside of the wall, where water pressure builds. It connects to a solid pipe that flows away from the structure.
What is a French drain?
A French drain is a common drainage system that includes a perforated pipe laying over gravel in a trench and coved by soil and soil. It is considered an effective a good looking drainage system that is fairly easy to install.
Does ever retaining wall need weep holes?
Most walls taller than a few feet should have weep holes. They are holes that form a grid pattern and are equally spaced in a vertical direction.
Does a two-foot retaining wall need drainage?
All retaining walls need drainage regardless of height. Those fixed into poor-draining soils like clay need it badly.
Do I need a structural engineer to design a retaining wall?
Most home advisors say you need a structural engineer if walls are 6 feet or taller. It might also be helpful to have one if you plan on building terraced or tiered retaining walls because there are guidelines for height, slope, and distance from each other.
Can I install drainage for a retaining wall myself?
Yes, it is a project you can do DIY as long as you do the research. Installing drainage is easier when you are constructing the wall rather than after the fact. You can install drainage on an existing wall but may need to tear the wall down and rebuild it to install it properly.
How much does it cost to install a retaining wall?
The cost depends on the height and length of the wall. The average cost is between $2,000 and $10,750. You can calculate between $10 and $30 per square foot for materials and $2 per square foot for site prep.
Drainage costs between $.50 and $70 per linear foot.
Much of the cost depends on the materials and type of drainage system you select, as well as the length and height. Some materials cost far more than their cheaper counterparts.