The Grid That Doubles the Strength of the Ground — Practical Engineering

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[Note that this article is a transcript of the video embedded above.]<br>The Port of Long Beach in Southern California is the second-busiest container seaport in the United States, a major gateway for goods flowing to and from the west coast. Together with its next door neighbor, the Port of Los Angeles, roughly half a TRILLION dollars worth of trade moves through the port every year. To keep up with the staggering growth of shipping demand, the port is in a nearly constant state of improvement and expansion.<br>In the early 2000s, Long Beach was expanding the container storage yard at the Pier T Marine Terminal, but engineers hit a problem. The area they were expanding into was an old, disused dry dock, much lower than the rest of the yard. They were going to need a lot of fill to bring it up. There aren’t many more difficult paving challenges than a container terminal. You have constant heavy traffic of cranes, forklifts, and yard trucks, and the local fill materials that are available are junk: soft, water-logged silt scraped from the bottom of the ocean that has the structural integrity of a cake in the rain.<br>Traditionally, you would have to dig out all the muck, then haul in a literal mountain of expensive backfill, a process that would cost millions of dollars and take years. Instead, the engineers turned to a solution that looks more like a giant plastic accordion than a structural foundation. This 3D network did more than just hold the soil in place; it physically transformed those mushy dredged spoils into a high-capacity platform capable of supporting 100-ton machines and rows and rows of heavy container stacks.<br>The prefix “geo” gets a lot of work in the world of engineering. Just tack it on something semi-technical-sounding, and there’s a good chance there’s a product out there. Geotextile, geogrid, geofoam, and more. Today I want to show you how geocells work, and of course I built a little model in the garage so you can see it in action. I’m Grady and this is Practical Engineering.<br>When you think about engineering, it’s easy to conjure the typical materials we use: steel, concrete, bolts, gears, electrical components, pipes, beams, and so on. The first thing to come to mind typically isn’t dirt. Most geotechnical engineers prefer the alternative four-letter words of “rock” or “soil” to describe their medium of choice. But whatever they call it, they don’t really get to choose what it is. Where most engineers work with materials that adhere to stringent specifications, geotechnical engineers work with the materials nature gives them, which, by the way, are often terrible. So much of our built environment completely depends on our ability to predict and improve the behavior of soil and rock below our feet. And those materials are so entirely different from really anything else that engineers work with, particularly in how they fail.<br>When you walk on the ground, you don’t really even consider that it can quote-unquote “fail” under load. But just like a bridge has a maximum weight limit, every patch of dirt, sand, clay, or loam has a specific amount of pressure it can take before it physically gives up. Of course, most soils can squish a little bit, what we normally call settlement. But it gets worse than that. Soil can’t just disappear under a vertical load, it doesn’t crush like concrete or bend like a steel beam. Instead, it shears. The friction between the soil particles isn’t enough to resist the pressure in the soil, so they slide along each other in a plane. Generally, it looks like this: the soil moves down, then out, then up, causing anything on top to sink or tip.<br>This is called a bearing capacity failure. It’s one of the most important failure modes of soil, and it has to be considered when designing just about any type of structure (since everything sits on the ground). Engineers for buildings, dams, retaining walls, and of course, roadways have to contend with this limitation.<br>Spreading the load is the simplest fix. If you’re wearing high heels on a muddy lawn, you sink; if you put down a wide piece of plywood and stand on that, you don't. In engineering, this means building wide concrete footings to distribute the weight over a larger area of the ground. The problem is that concrete is expensive. For many structures, spread footings make a lot of sense. But there’s a reason not every roadway is paved with it.<br>Another option is just to replace the subgrade. If the dirt is mushy, you dig it all out and haul it away. Then you bring in so-called, “select-fill” an ambiguous term that really just means whatever material the engineer has determined will be able to support the load. For roadways, it’s usually high-quality, angular crushed stone. But you don’t want just large chunks of gravel; you need some finer particles to help lock everything...