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How Groundwater Moves

This sand-and-water model confirms mathematical predictions about how groundwater moves through soil
July 05, 2021
Read time: 3 mins

[lightly edited transcript]

There's an old joke about the different kinds of engineers that say mechanical engineers design the weapons and civil engineers design the targets. Well, it's even worse than that for geotechnical engineers, who really just care about what's underneath the targets. Yet despite the frequent association of “dirt” with “grit,” “grime,” and “gossip,” its importance in civil engineering can't be overstated.

There's hardly a single structure out there that doesn't sit on the ground—or at least sit on something that sits on the ground—and there's really more to earth than first meets the eye. For the most part geotechnical engineers are content to perform their analyses quietly knowing full well that the general public does not share their devotion to dirt and reverence for rock. But occasionally they find themselves with the desire to educate and inform, in which case models often speak louder than words.

Flow Net Diagram

In the field of civil engineering, it's often important to be able to characterize the flow of groundwater. Water in the subsurface can have a major impact on civil structures by causing uplift pressure and seepage, and by changing the strength characteristics of soil, among other things. We have fancy computer models that do a good job simulating groundwater flow, but even today one of the most important tools still used by geotechnical engineers is the flow net. Without getting into the nitty-gritty details, a flow net consists of two sets of perpendicular lines that create a curvilinear grid. One set is equipotentials or lines that connect points with the same pressure. Once the equipotential lines are drawn, the flow lines are just drawn perpendicular to them, forming squares.
Sand-and-Water Model

These kinds of simplified drawings make pretty pictures, but do they really reflect how groundwater flows in real life? It's always important to ground-truth your calculations, sometimes even with real ground. This model is designed to do just that. It simulates the flow of groundwater around an obstruction to illustrate the morphology and velocity of the flow. It's made of 1/4-inch acrylic sheets cut to size on a table saw. The acrylic is solvent welded to create a narrow box. All of the plumbing is composed of aquarium bulkhead fittings and clear nylon tubing. Everything was leaked tested before the sand was added.

Here's how it works. Potassium permanganate is added to three spots in the top of the sand. A pump in a bucket below keeps a constant head pressure on one side of the model, and the differential between the water level in both sides is what drives the water to flow from one side to the other. Groundwater flowing through the sand creates traces of the flow lines over the course of several hours, illustrating exactly what we estimated earlier with the flow net.

The flow of groundwater hasn't always been so well understood. In fact, for some states, regulation of the pumping of groundwater is established on explicit ignorance of its behavior. For example, in a landmark case which submitted the rule of capture into Texas water law in 1904, the court said that groundwater movements are so secret, occult, and concealed that regulating the use of groundwater would be practically impossible.

Fortunately, geotechnical engineers have since developed a foundation of knowledge around the flow of water in the subsurface. It can be a dirty job, but a large part of geotechnical engineering is relegated to abstract calculations and computer models.

I was glad to get a chance to take a concept away from the desk and illustrate it with some real dirt and water.

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