From the course: Practical Engineering

Groundwater movement and the quick condition

From the course: Practical Engineering

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Groundwater movement and the quick condition

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- When you think about quicksand you probably don't feel much fear or uncertainty. What was once a popular plot device in island and jungle movies, has began to fade from our collective curiosity. But, in civil engineering quicksand is more than just a puddle of mud. It has a specific and potentially more catastrophic definition that can affect all kinds of infrastructure. I'm Grady, and today on Practical Engineering we're continuing our dive into the hazards of soil mechanics, and talking about seepage. (bright electronic music) If you've ever built a dam on a creek, or even in a storm gutter on your street you know hard it is. Water is not only powerful, it's elusive too. It seems to always find a way through. Like all fluids, water flows from areas of high pressure to low pressure. And a dam is essentially a structure which separates those two conditions. This is a prime circumstance to induce flow, whether it's through the dam itself or underneath through its foundation. Water flowing through soil is called seepage, and it follows some interesting and unexpected rules. So to give you a better understanding I'm building this demonstration out of clear acrylic sheets and some plumbing fixtures. This will let us see a cross-section through a dam so we can observe how seepage behaves. All dams have some seepage, so engineers need to be able to characterize it. Flow through soil follows Darcy's law, which is simple to understand, but difficult to calculate in two dimensions. Before computer models, engineers used an analytical tool called a flow net to estimate the behavior of seepage. Now software can do the work more quickly and accurately. Darcy's law essentially says that the rate of seepage depends on the length of the flow path, and the difference in pressure across it. The combination of these two factors is called the hydraulic gradient. If the difference in pressure is small, like in this example, the seepage will be slow. The bigger the difference in pressure the greater the seepage flow rate will be. But if we increase the difference by too much, eventually some strange things start to happen. So what's happening here? I've talked about sheer strength of soil in a previous video, so check that out if you want more details. But here's the gist, soil is a granular material that has one main way of holding itself together. Friction. Gravity pushes the soil particles together, creating friction which gives the soil strength. Seepage is the enemy of friction. Water gets in between the particles pushing them away from each other, reducing the friction and thus reducing the strength. This is important because sheer strength is really the only thing that separates a solid from a fluid. In fact, when the water pressure within the soil gets so high that it eliminates the sheer strength altogether, we call it liquefaction. Also known as the quick condition. The soil is literally behaving like a liquid. It's quicksand. You can see why seepage is bad for dams and levees. Engineers generally try to avoid building civil structures out of liquids. And soil liquefaction can be the starting point of a specific type of erosion called piping. As soil is carried away from the dam's foundation the seepage path gets shorter. Remember that seepage depends on the hydraulic gradient, which is a function of the length of the seepage path, and the difference in pressure. If the pressures stay the same but the path gets shorter, the gradient goes up creating more seepage. This is a dangerous positive feedback loop. The erosion shortens the seepage path, while the increasing seepage creates more erosion. Eventually the erosion pipe reaches the reservoir leading to catastrophic failure of the dam's foundation. This isn't just speculation, piping is the number one cause of failure for earthen levees and dams. So how do engineers deal with seepage? One common way is a cutoff wall. A subsurface wall constructed into the foundation of a dam. This is some footage from a previous demonstration I built that shows a cutoff wall in action. Notice the length of the seepage flow pass, they're longer which means the hydraulic gradient is lower. That means less seepage, and less chance for erosion. Thank you for watching, and let me know what you think.

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