By design, retaining walls are meant to hold or retain soil, rock, and other earth materials from eroding, destroying, or collapsing into nearby structures such as the yard, garage, or garden.
However, due to the hydrostatic pressure created by water accumulation behind it and the ground settling, the wall may collapse at any time.
This is where the role of French drain pipe comes in; to prevent water pooling, by picking it and driving it away, thereby reducing pressure against the wall.
However, the same drain pipe that is meant to protect the retaining wall might suffer damage if enough care is not taken during installation. To prevent the wall from crushing the drain pipe, this article gives guidelines you should follow during installation.
French Drain pipe installed correctly
Use of a Stronger Pipe
No matter how effective your drainage pipe protection measures may be, the pipe is still at risk of damage by the wall if you use the wrong pipe. Too thin drainage pipe is weak and will be crushed by the weight of the materials within no time. Instead of a flexible corrugated drain pipe, consider using a rigid PVC for increased tensile strength. Rigid PVC is less likely to crush under the pressure of the materials created by the wall. To make it even stronger and less at risk, you go for schedule 40 PVC pipe which is thicker and more durable. These kinds of drain pipes may be a little expensive but the reduced maintenance cost will make up for that increased initial cost, going into the future.
Proper Bedding
Another major contributing factor to the retaining wall crushing a drain pipe is the wrong bedding. Simply put, bedding is the process of laying backfill materials behind the wall, on the pipe. An improper bedding can exert unsurmountable pressure on the pipe and end up crushing it. The proper bedding process includes laying a coarse gravel base directly over the pipe. A ¾-inch base of coarse gravel provides a stable and supportive bed, thereby reducing the risk of the pipe being crushed. After the coarse gravel base, add a layer of fine sand or fine gravel before backfilling with large stones or other materials. The fine materials help in distributing the load more evenly.
Drainage Fabric
Drainage fabric such as geotextile, helps in preventing overfill materials such as silt from entering the pipe perforations while allowing the free flow of water. By preventing silt from getting into the pipe, you are reducing pressure from crushing the drain.
Adequate Covering
How effectively you place the backfill materials behind the wall determines how well-protected the drain pipe is from the pressure of the wall. Ensure the backfill materials are equivalent to the height of the wall to reduce pressure on the wall and the drain pipe. Ensure to lay the materials bit by bit, alternating with proper compaction. Careful compaction of the material prevents the materials from crushing the pipe when the ground settles or shifts for any reason.
Avoiding Heavy Machinery over the Pipe
Operating heavy machinery in the area immediately after drain pipe installation can wreak havoc. Heavy machinery such as rollers, tractors, or excavators exert a lot of pressure and vibrations that may cause ground shift and settling of materials, thereby crushing the drain pipe. If you really have to operate heavy machinery over the backfill area for any reason, be sure to schedule the works later, say 3 months after the installation of the pipe to prevent it from being crushed.
Consider Drainage Design
Even though the drain pipe is meant to reduce the pressure of backfill materials on the retaining wall by carrying all the runoff water, it may never collect all the water. That being the case, consider your drainage design. Consider grading your yard in a way that runoff water is directed away from the retaining wall to prevent hydrostatic pressure from destroying the pipe. If the wall is close to the walls of the house, consider repairing your gutters and cleaning them regularly. This works by directing roof water away from the retaining wall and preventing the pressure from crushing the pipe.
By taking these precautions seriously, you can significantly reduce the risk of the French drain pipe from being crushed by the backfill materials behind the retaining wall. Most importantly, consider the input of a professional plumber if you do not understand the entire installation process or if you are not very good with tools. Getting it wrong from the start will only set you up with trouble, going into the future.
