Recommended Drainage Gravel and Filter Fabric for Retaining Walls. When it comes to drainage rock, use an angular aggregate that's free of fines. For CornerStone 100 retaining walls, #57 or ¾” (20mm) clean crush drainage gravel is recommended.
Actually, you want coarse sand and gravel behind a retaining wall, along with bleeders and/or drain tiles along the foundation to remove any water that might build up. There are plastic panels you can use as well that incorporate a space to intercept and drain water from behind the wall.
Backfill the wall with gravel or sand. If you use sand, first cover the backside of the retaining wall with landscape fabric. This will keep the sand from seeping between the blocks. If you're backfilling with gravel, you can pour the gravel after every course you lay.
We highly recommend using the SDR 35 pipe over the black corrugated pipe you often see at home improvement stores--especially for retaining walls. Trust us, it's worth it to avoid costly repairs down the road!
Drainage Aggregate
A layer of free-draining material behind the wall serves as a conduit for water movement. Best practices include: Material choice: Clean, angular gravel or crushed stone (typically 20-40 mm or 3/4 to 1 1/2 inch)
Absolutely! Even a 2-foot retaining wall needs an effective drainage system. Without it, water can build up behind the wall, leading to pressure that might cause the wall to lean or even collapse. All walls need systems to manage water effectively, preventing damage over time.
Pea gravel is one of the best materials for landscape drainage because its small size allows for outstanding drainage. These rocks are only about ¾ inches in size. Pea gravel is available in a variety of shades, including brown, grey, blue, and red, making it a good choice for a number of different landscapes.
Non-woven geotextile fabric is the best choice for drainage applications where water flow through (permeability) is the main concern. Non-woven landscape fabric is commonly used in French-drains, drain-fields, and erosion control.
PVC pipes keep wastewater moving without any clogs, unlike corrugated pipes. Corrugated pipes can get damaged when buried deep under, but PVC pipes withstand the pressure and last longer. PVC pipes can last up to 100 years when properly installed and have a low failure rate compared to other materials.
For proper drainage you need at least 300 mm of gravel or similar material directly behind the wall. If you're planning on landscaping behind the wall, allow for at least 150 mm of topsoil above the gravel.
Soil, sand, gravel, and crushed stone are all common backfill materials. The type of material will vary depending on the application. As an example, crushed stone can improve drainage around foundations.
One major thing to avoid when building your retaining wall is lining the back of your wall with plastic sheeting as this will surely result in the pooling of water behind the wall.
Hydrostatic Pressure and Wall Failure
When water isn't properly drained, it can accumulate behind the wall, exerting pressure on the structure. This pressure can prompt the wall to shift, crack, or even collapse. To avert such failures, it's crucial to have adequate drainage behind the retaining wall.
By incorporating a well-designed drainage system, using geotextile fabric, choosing suitable backfill materials, incorporating vegetation and planting, and implementing terracing or staircases when necessary, you can effectively prevent soil erosion and ensure the long-term stability of your retaining wall.
Sometimes retaining walls are designed as cantilevers. These typically include reinforcement and rely on the combined strength of the concrete and reinforcement to resist bending at the base of the wall. In that case, plan on waiting seven days before placing the full depth of backfill.
Cons of Using Corrugated Pipe:
Much lower flow rates. Since the pipe is flexible, water can sit in low spots after rain events. Have to be more precise to dig a flat and properly pitched trench before installing.
You can expect a service life of 100 years in typical drainage applications. Highly resistant to abrasion and chemicals, corrugated plastic pipe outperforms in environments that damage other materials.
The minimum cover required when the drainage system is in a traffic area with drainage pipe diameters from 100 mm through 1200 mm (4”-48”) must have at least 0.3 m (12”) of cover over the pipe crown, while 1500 mm (60”) pipes must have at least 0.6 m (24”) of cover.
Don't Skimp on Gravel Amount
Add an angular aggregate — clean, crushed stone or gravel between 1/2- and 3/4-in. — as a base under the retaining wall and as back fill behind it.
The material shall meet or exceed the specification for wall rock outlined in Section 1, 2.2 Wall Rock. The drainage collection pipe, drain pipe, shall be a 4 in. (100 mm) perforated or slotted PVC, or corrugated HDPE pipe as approved by engineer of record.
In order to provide proper drainage, at least 12 inches of granular backfill (gravel or a similar aggregate) should be installed directly behind the wall. Compacted native soil can be used to backfill the rest of the space behind the wall.
Landscape Fabric vs Drainage Fabric. Landscape fabric / drainage cloth is used in landscaping for water to pass through. Drain Field fabric is used in septic drain fields as a filter. They both act as a barrier, are from the same class of synthetic textiles and are similar in their construction.
These include terracotta PVC sewer pipe, twinwall drainage pipe, MDPE water pipe and land drainage pipe. Land drainage coil sizes range from 60-160mm and twinwall comes in sizes from 100-900mm.