Retaining Wall Drainage: What Is It and Why Does It Matter?
Have landscaping plans that include a retaining wall? Don't underestimate the importance of retaining wall drainage. Learn all about it here.
Drainage is a vital and often overlooked aspect of constructing a successful retaining wall. Walls play many roles in landscape design: They create privacy, prevention erosion by keeping soil in place and often provide a place to grow plants. A good retaining wall should do all these things well.
So what is the purpose of retaining wall drainage? There are various benefits and purposes, including:
- Controlling the amount and rate of water entering and exiting a property.
- Protecting buildings and vehicles from excess moisture damage.
- Maintaining lawns, gardens and surrounding streets by directing water into the soil, instead of pooling near foundations or on streets.
What Is Drainage?
When it rains, water runs down the wall. But when all is said and done, you should see little standing water in your landscape. That’s drainage.
Retaining wall drainage is critical. It ensures water does not collect behind the wall, causing it to fail. A quality drainage system collects and redirects rainwater away from the wall. It decreases pressure on the soil around the foundation and within the wall itself, reducing erosion and settlement.
Why Do Retaining Walls Need Drainage?
Here’s why why retaining walls need drainage:
- Water flowing over a retaining wall follows the path of least resistance, often toward the bottom of a wall, where construction activity loosened soil particles. In this process, more earth from behind the wall erodes and is redeposited at its front, which creates a pressure gradient. If this gradient is not controlled, the wall will move or fail.
- Rainwater that runs off the top of a retaining wall can damage the soil and plants on either side, eroding them away. Drainage systems divert this water from vulnerable areas, reducing the impact to the structure and its surroundings. The drainage system also reduces puddles and standing water that invite insects, such as mosquitoes and flies, which can be a major nuisance.
Proper Retaining Wall Drainage Setup
It’s been said half of all the problems with retaining walls involve drainage and water issues, and keeping water out of these walls is a big one. If it’s dry, there’s little to no chance any vegetation can take hold. A properly designed and installed system will keep your wall dry, saving you money on maintenance and replacement costs.
Here are some tips:
- Make sure to include backfill, the 12 inches of space behind a retaining wall. Fill it with crushed stone or gravel.
- All retaining walls should include drainage stone even if a draining pipe is not required.
- Place filter fabric above the drainage stone and below the topsoil. That prevents fine material and organic matter from clogging up the drainage stone.
- If your retaining wall needs a drainage pipe, make sure the pipe has slots on all sides, not just one. A drainage pipe might be needed if:
- The retaining wall is at least four feet high or taller.
- Clay or other poor draining soils are behind the wall.
- There are buried water sources within 50 feet of the retaining wall location
5. There are outlet options for your drainage pipe. Whichever one you choose, place an outlet at least every 30 to 50 feet along the wall.
How Does Drainage Affect the Longevity of a Retaining Wall?
Poorly drainage will shorten the life of your retaining wall. If water can’t flow away from the back of the wall, pressure will build up, causing the footing (and sometimes even the wall) to fail.
Drainage is also important because it impacts how often the wall will require repairs. A well-placed drainage system will make it much less likely you’ll need to drill into the wall to fix a crack or other issue.
Potential Problems Without Proper Retaining Wall Drainage
Effective drainage systems ensure your wall won’t suffer water damage. Pooling water behind a retaining wall can cause erosion that will eventually lead to decay in the structure. Other potential problems include damage to a home’s foundation over time, due to improper or not enough drainage.
This damage can be costly to repair, and a properly installed drain would have prevented it.