[Note that this article is a transcript of the video embedded above.]

Before the SpaceX South Texas launch facility on South Padre Island near Boca Chica supported crazy test launches of the Starship spaceflight program, it was just a pile of dirt. Contractors brought in truck after truck of soil, creating a massive mesa of more than 300,000 cubic yards or 230,000 cubic meters of earth. That’s a lot of olympic-sized swimming pools, not that you’d want to go swimming in it. After nearly two years, they hauled most of that soil back off the site for disposal. It might seem like a curious way to start a construction project, but foundations are critically important. That’s true for roads, bridges, pipelines, dams, skyscrapers, and even futuristic rocket launch facilities. The Texas coastline is not known for its excellent soil properties, so engineers had to specify some extra work before the buildings, tanks, and launchpads could be constructed. Building that giant dirt pile was a clever way to prevent these facilities from sinking into the ground over time. Why do some structures sink, and what can we do to keep it from happening? I’m Grady and this is Practical Engineering. Today, we’re talking about soil settlement.

The Earth’s gravity accelerates us, and everything else on our planet downward. To keep us from falling toward the center of the planet, we need an equal and opposite reaction to keep us in place. If you’re at the top of a skyscraper, your weight is supported by floor joists that transfer it to beams that transfer it to columns that transfer it downward into massive concrete piers, but eventually the force of you must be resisted by the earth. It’s ground all the way down. You might not think about the ground, and its critical role in holding stuff up, but the job of a geotechnical engineer is to make sure that when we build stuff, the earth below is capable and ready to support that stuff for its entire lifespan.

Every step you take when walking along the ground induces stress into the subsurface. And every rocket launch facility you build on the Texas coastline does the same thing. This isn’t always a big deal. When constructing on bedrock, there’s a lot less to worry about, but much of the earth’s landscape consists of soil: granular compositions of minerals. Stress does a funny thing to soils. I mean, it does some funny things to all of us, but to soils too. At first consideration, you might not think there’s really much difference between rock and soil. After all, soil particles are just tiny rocks, and many sedimentary rocks are made from accumulated soil particles anyway. But, soil isn’t just particles. In between all those tiny grains are empty spaces we call pores, and those pores are often filled with water. Just like squeezing a sponge forces water out, introducing stress to a soil layer can do the same thing.

Over time, water is forced to exit the pore space of the soil and flow up and out. As the water departs, the soil compresses to take up the void left behind. This process is called consolidation. It’s not the only mechanism for settlement, but it is the main one, especially for soils that are made up of fine particles. Large-grained soils like sand and gravel interlock together and don’t really act like a sponge so much as a solid, porous object. To the extent they do consolidate, it happens almost immediately. You can squeeze and squeeze, but nothing happens. Fine-grained soils like clay and silt are different. Like sand or gravel, the particles themselves aren’t very compressible. However, unlike in coarse-grained soils, fine particles aren’t so much touching their neighbors as they are surrounded by a thin film of water. When you squish the soil, the tiny particles rearrange themselves to interlock, pressurizing the pore water and ultimately forcing it out. The more weight you add, the more stress goes into the subsurface, the more water is forced out of the pores, and thus the further the soil settles. Geotechnical laboratories perform these tests with much scientific rigor.

This may seem obvious, but when we build stuff, we don’t want it to move. We want the number on that dial to stay the same for all of eternity, or at least until the structure is at the end of its lifespan. That idea - that when you build something, it stays put - is essentially all of geotechnical engineering in a nutshell. It encompasses the entirety of foundation design, from the simplest slabs of concrete for residential houses, to the highly sophisticated substructures of modern bridges and skyscrapers. The way movement occurs also matters. It’s actually not such a big deal if settlement happens uniformly. After all, in many cases the movement is nearly imperceptible. I’m using a special instrument just so you can see it on camera. Many buildings can take a little movement without much trouble. But often, settlement doesn’t happen uniformly.

For one, structures don’t usually impose uniform loads. If everything we built was uniform in size and density, we might be okay, but that’s never the case. No matter what you’re constructing, you almost always have some heavy parts and other light parts that stress the soil differently. On top of that, the underlying geology isn’t uniform either. Take a look at any road cut to see this. The designers of the bell tower at the Pisa Cathedral in Italy famously learned this lesson the hard way. Small differences in the soils on either side of the tower caused uneven settlement. Geotechnical engineering didn’t exist as a profession in the 1100s, and the architects would have had no way of knowing that the sand layer below the tower was a little bit thinner on the south side than the north. It didn’t take long after construction started for the tower to begin its iconic lean. I should point out that there’s another soil effect that can cause the opposite problem. Certain types of soils expand when exposed to increased moisture, introducing further complications to a geotechnical engineer. I have a separate post on that topic, so check it out after this if you want to learn more.

Settlement made the tower of Pisa famous, but in most cases it just causes problems and costs a lot of money to fix. One of the most famous modern examples is the Millennium Tower in San Francisco, California. The 58-story building was constructed atop the soft, compressible fill and mud underlying much of the Bay Area. Engineers used a foundation of piles driven deep below the building to a layer of firmer sand, but it wasn’t enough. Only 10 years after construction, the northwest corner of the building had sunk more than 18 inches or 46 centimeters into the earth, causing the building to tilt. Over time, some of the building's elements were damaged or broken, including the basement and pavement surrounding the structure. As you would expect, there were enough lawsuits to fill an olympic sized swimming pool. The repairs to the building are in progress at an estimated cost of 100 million dollars, not to mention the who-knows-how-much in legal fees.

One of the most reliable ways to deal with settlement is just to make sure it happens during construction instead of afterwards. As you build, you can account for minor deviations as they occur. Unfortunately, consolidation isn’t always a speedy process. The voids in clay soils are extremely small, so the path that water has to take in order to exit the soil matrix is long and windy. We call this windiness sinuosity. Depending on the soils and loads applied, the consolidation process can take years to complete.

It’s not a good idea to build a structure that will settle unevenly over the next several years. Hopefully it’s obvious that that’s bad design. So, we have a few options. One is to use a concrete slab that is stiff enough to distribute all the forces of the structure evenly and provide support no matter how nonuniformly the settlement occurs. These slabs are sometimes called raft foundations because they ride the soil like a raft in the ocean. Another option is to sink deep piles down to a firmer geologic layer or bedrock so that loads get transferred to material more capable of handling them. But both of those options can be quite expensive. A third option is simply to accelerate the consolidation process so that it’s complete by the end of construction.

One way to speed up consolidation in clay soils is to introduce a drainage system. Settlement is mainly a function of how quickly water can exit the soil. In a clay layer, particularly a very thick layer or one underlain by rock, the only way for water to leave is at the surface. That means water well below the ground has to travel a long distance to get out. We can shorten the distance required to exit the soil by introducing drains. This is often done using prefabricated vertical drains, called PVDs or wick drains. These plastic strips have grooves in which water can travel, and they can be installed by forcing them directly into the subsurface using heavy machinery. An anchor plate is attached, the drain is pressed into the soil to the required depth, the mandrel is pulled out, and the material is cut. It all happens in quick succession, allowing close spacing of drains across a large area. The tighter the spacing, the less distance water has to exit. One of the other benefits here is that water often travels through soils horizontally faster than it does vertically, since geologic layers are usually horizontal. That speeds up consolidation even more. Plotting the displacement over time, the benefit of vertical drains is unmistakable.

The second way we speed up consolidation is surcharge loading. This is applying stress to the foundation soils before construction to force the water out quickly. Like I described in the intro at SpaceX South Texas, it’s usually as simple as hauling in a huge volume of earth to be temporarily placed on site. The way this works is as straightforward as squeezing a sponge harder. It’s the equivalent of adding more weight to my acrylic oedometer, but it’s simpler just to show a graph. Let’s say you’re going to build a structure that will impose a stress on the subsurface. That stress corresponds to a consolidation at this red line. If you load the foundation soils with something heavier than your structure, that weight will be associated with a greater consolidation. It’s going to take about the same time to reach a certain percentage of consolidation in both cases, but you’re going to hit the target consolidation (the red line) much faster. In many cases, engineers will specify both wick drains and surcharging to consolidate the soil as quickly as possible so that construction can begin. Once you get rid of all the extra soil you brought in, you can start building on your foundation knowing that it’s not going to settle further over time.