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Ask the Builder: Water movement in the ground
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Ask the Builder: Water movement in the ground

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This is perforated drainage pipe that collects groundwater and redirects it to another spot in your yard.

Your home may be plagued by a wet basement or crawlspace where water enters at will whenever it rains. In addition, your yard may be the neighborhood bog, rendering it useless in the wet season. Would you be interested in knowing how to make your basement, crawlspace or yard as dry at the Atacama Desert? I thought so.

I was quite fortunate to major in geology in college. One of the classes I took was hydrogeology, which focuses on groundwater. The knowledge I obtained allowed me to build homes in dense clay soil that had bone-dry basements. Just about every other homeowner in Cincinnati suffered from wet basements, but not my customers.

Not a week goes by that I don’t do a specialized consultation for a homeowner who suffers from water issues. Just a few days ago I solved a chronic drainage problem a woman had spent over $10,000 to fix to no avail. She was stunned to discover she would have a dry yard and crawlspace for less than $500 and a few weekends of moderate labor.

The homeowner was thrilled to see the customized drawing I made for her showing her exactly how to capture the subsurface water that was causing the issue. It gave me great pleasure to ratchet back her stress. She was a mess each time rain was in the weather forecast. Thanks, Dr. Pryor, for teaching me what’s going on with groundwater so I can continue to help others!

To understand groundwater and how it moves, you need to get a handle on the soil around your home. Not all soils are the same. Many years ago, the government spent millions of dollars creating soil maps that you can access for free online. Just do a search similar to this: Cincinnati OH soil maps.

Some soils are very well-drained, such as the one around my house here in central New Hampshire. The typical soils here are thin and very sandy, a result of the erosion of the granite bedrock. However, the soil in Cincinnati, my hometown, is the exact opposite. There’s thin topsoil and under it are feet of dense clay you can use to make pottery. Water doesn’t travel down through the clay at all. This is why it’s easy to build a pond in southwest Ohio.

Topsoil almost always has lots of air in it. Insects help create these void spaces. When it rains, water soaks into the topsoil displacing the air. When the topsoil can absorb no more water, it starts to flow overland. Some weather warnings talk about flood risk when the ground or soil is saturated from snowmelt or heavy rains in previous days. Now you know why it’s happening.

Gravity takes over after the rain gets in the soil. This amazing force of nature starts to pull the water down but another law of physics comes into play. The water wants to take the path of least resistance on its journey back to the ocean. This means that the water in the soil usually starts to travel sideways through the topsoils. In most soils, the resistance to movement increases with depth.

A dense clay subsoil, as well as bedrock close to the surface, can aid in this sideways movement. Realize that some of the water moving through the soil finds cracks in the bedrock and flows down to fill them. This is what keeps drilled wells wet as the water fills up miles of tiny and large cracks that are like 3D spiderwebs in the bedrock under your feet.

To stop water from entering your basement or crawlspace, you need to capture the water moving sideways through the soil. Remember, water likes to take the path of least resistance, so create one for it to bypass your basement, crawlspace or yard!

I’ve had fantastic success for over 40 years digging a 6-inch-wide trench down about 24 inches. I put in 2 inches of washed rounded gravel in the bottom of the trench. The gravel is about the size of green grapes, maybe as large as walnuts. It can be angular, but be sure there are no smaller stones or sand in the gravel. You want lots of air spacing in this gravel because the water will much rather flow down through this gravel and into a buried drainpipe than try to push its way through the topsoil.

I then put a 4-inch perforated drain pipe on top of the 2 inches of gravel. I prefer to use the solid white ABS or PVC plastic pipe that has rows of holes drilled in it. I’ve never been a fan of the black corrugated drain pipe with the narrow slits in it. Always install the pipe so the rows of holes point down. If you don’t, the water in the trench needs to raise all the way up to where the holes are to get into the pipe. What’s more, if the holes point up, gravel that you put on top of the holes can block the holes. That’s bad.

The bottom of the trench can be parallel with the top surface of the ground around your home. At the very least the pipe in the trench needs to be level or have 1/8 per inch of fall to it. Even if the pipe is level, water will readily flow through it. Think about how lakes don’t overflow. Once the pipe is installed, fill the trench to within 1 inch of the surface with the rounded or angular gravel.

The pipe needs to extend to a low spot on your land. As the ground falls away from your house to the low spot, the pipe will naturally pop to the surface if you install the pipe level or have a minimal amount of fall to it. The pipe needs to break through to the surface so it can disgorge the water it’s collected sending it on its way to the ocean.

Be sure you don’t put a fabric sock on the pipe or line the trench with a filter fabric. Those are designed to keep gravel under parking lots and roadways from being fouled with mud. Water flowing through topsoil has already been filtered of this silt, so the fabrics are unnecessary.

(Subscribe to Tim’s’ free newsletter and listen to his new podcasts. Go to: AsktheBuilder.com.)

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