The Art and the Science of the Trench Drain

Q: DEAR TIM: Do you think a trench drain will solve my soggy yard and the chronic leak in my basement? Many of my neighbors suffered from a recent heavy rainfall, and we all are tired of dealing with water in and around our homes. Will a trench drain really work? What is a proven design, and how does one go about installing such a drain? -- Robin R., Dawson Springs, Ky.

A: DEAR ROBIN: A trench drain, if you install it correctly, will solve your swampy yard and stop the water from entering your basement. Thousands of people have used a trench drain design I perfected years ago to redirect water that was coming from my neighbor's yard. It worked so well that I started using the same system on all of my jobs where customers had poor drainage.

My college degree is in geology, and I had a special interest in hydrogeology, the study of groundwater. To understand how to drain land and keep your house dry, one needs to understand the movement of water after it enters the soil. Many people believe that water travels straight down through the soil on its way to the water table. It can do so in certain areas, but often, as it moves to a layer of porous rock, it travels sideways through the soil. This is especially true in areas where the top layers of the soil horizon have lots of clay, which blocks water. (This is why clay serves as a great liner for lakes and ponds.)

Topsoil has a very open structure and is filled with lots of air voids when the soil is dry. As rain enters topsoil, it enters this network of interconnected air spaces. If the water travels downward under the pull of gravity and hits a dense clay subsoil, it starts to move sideways in a downward slope toward creeks, streams and rivers.

A trench drain installed on the high side of your land can intercept water as it moves toward your home. The trench drain acts like a gutter on a roof, collecting the water and redirecting it to another location. The water enters the trench drain and flows through it because the drain offers the path of least resistance.

My trench-drain design is simple. You dig a six-inch-wide trench about two feet deep. The bottom of the trench stays parallel to the top surface of the ground on the high side of your land as it passes on either side of your home. Once the trench passes your house, the bottom of the trench should be made nearly level with a minimal slope. As the ground slopes from your house, the bottom of the trench gets closer and closer to the surface until it pokes through to the surface, much like a horizontal mine shaft.

The trench contains rounded, washed gravel and a perforated four-inch-diameter drain pipe. The pipe is simply a high-speed conduit that acts like an underground river in times of heavy rainfall. You install two inches of gravel on the bottom of the trench and then the pipe. Cover the pipe with additional washed gravel to within one inch of the surface. You can then add soil or sod if you want to hide the drain.

There is no need to install a geotextile sock around the pipe or use it to line the sides of the trench. These materials are designed to stop silt from clogging gravel or drains, and they are needed when you install fluffed dirt over drainage systems. In my design, the small amount of soil added to the top of the gravel will not be a factor.

Water flowing through soil does not contain silt except in the situation described above. In virgin soil or compacted soil, silt is a surface phenomenon. Creek water in a storm is muddy because of dirt that is eroded from the surface and carried overland to the stream, brook or river. Proof of this is the crystal-clear water that flows into wells and out of surface springs. You will also see clear water flowing from the outlets of your trench drain once you install it.

Tim Carter can be contacted via his Web site, http://www.askthebuilder.com/printer_Submit_Question.shtml.

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