In this blog post, we will examine whether the common belief that “a house built on sand will collapse” is scientifically valid by looking at the settlement characteristics and differences between clay and sand soil.
There is an old saying, “a house built on sand.” The dictionary defines it as “a house built on sand, meaning something that has a weak foundation and is likely to collapse, or something that is impossible to achieve.” Think back to your childhood memories of playing in the playground or on the beach. When you use clay to build something, the clay sticks together easily because of its viscosity, but when you use sand, it scatters easily and does not hold together well. You may also remember that a house made of clay is quite sturdy, but a house made of sand collapses as soon as you take your hands away. Based on this experience, people probably thought that houses built on sand would collapse quickly, just like sticks or toad houses, and thus coined the idiom “sand castle.” But are houses built on sand really “sand castles”?
The most important thing when building a house is to understand the subsidence characteristics of the site. If you build a building without properly understanding the subsidence characteristics of the ground, not only will the building lean due to the difference in subsidence on both sides, like the Leaning Tower of Pisa, but when the subsidence exceeds the allowable limit, you may experience the unfortunate situation of your hard work collapsing in an instant. Imagine that the Leaning Tower of Pisa is your house. It would be terrible to spend every day worrying about when your house might collapse. Therefore, before building a house, it is very important to estimate how much the ground will subside and prepare accordingly.
Soil can be broadly divided into clay and sandy soil. Clay has electrical properties that cause its surface to become polarized, attracting water from its surroundings. When this happens, the water right next to the clay particles sticks to them so tightly that it cannot be distinguished from the soil. This water is called “adsorbed water,” and it is what gives clay its viscosity. In addition, clay particles are small and closely spaced, so once they absorb water, they do not easily release it and retain water for a long time. On the other hand, sandy soil has large particles and wide spaces between them, so water drains easily and quickly, leaving the soil dry and non-viscous. This is why sand castles built at the beach collapse when you leave them unattended after wetting the sand with water.
We usually think of soil as a solid, but if that is the case, how can we explain the sinking of the ground? The most common answer is that structures weighing thousands of tons weigh down on the soil, causing the soil particles to break down and the ground to subside. This is not wrong, but this type of subsidence accounts for a very small portion of the total subsidence, so it cannot be said to be the main cause. In fact, soil is not a solid, but a mixture of three states: solid, liquid, and gas. What people usually think of as “solid soil” is called “soil skeleton” in soil mechanics. The spaces between the soil skeleton are filled with water and air. Clay contains a lot of water, so when you touch it, it feels slightly sticky and moist, while sand contains little water, so it is relatively dry and not sticky. Imagine putting a soil skeleton made of clay or sand in a box of a certain volume, then adding a lot of water to the clay and a little sand to the sand. In this way, solids (soil skeleton), liquids (water), and gases (air) come together to form “soil.”
Since soil is composed of not only solids but also liquids and gases, we can guess that ground subsidence occurs when water and air escape. When a load is applied to soil, the air inside the soil is discharged to the outside at the same time as the load is applied, and the volume of the soil decreases in a relatively short period of time. The subsidence that occurs when the air is discharged is called “immediate subsidence.” After immediate settlement is complete, “consolidation settlement” occurs, in which the ground subsides as water in the soil is gradually discharged over a long period of time. After immediate settlement and consolidation settlement are complete, a long period of time passes (depending on the size of the structure, but usually more than 50 years after completion), and “secondary settlement” occurs as the soil skeleton itself is rearranged. However, the mechanism of secondary settlement has not yet been clearly identified, and the amount of settlement is small, so it is generally ignored or included in the calculation of “consolidation settlement.” Ground settlement can be divided into “immediate settlement,” in which air is discharged, “consolidation settlement,” in which water is discharged, and “secondary settlement,” in which the soil skeleton is rearranged. The total amount of settlement is the sum of these three types.
Now, let’s imagine building a house on clay and sand. Since the spaces between the soil particles in clay are mostly filled with water, immediate settlement is minimal, and consolidation settlement dominates the total settlement. On the other hand, since the spaces between the soil particles in sandy soil are mostly filled with air, consolidation settlement hardly occurs, and immediate settlement accounts for most of the total settlement. If you build a house on clay, there is little air that can escape from the soil, so immediate settlement rarely occurs, which may make construction easier. However, after completion, the water in the soil is discharged to the outside, causing consolidation settlement, which can cause the building to tilt or collapse if not done correctly. However, if a house is built on sandy soil, “immediate settlement” occurs immediately after construction begins, but there is almost no ground settlement thereafter, so it is stable. Contrary to the proverb we know, it is safer to build a house on sand.
However, this does not mean that structures cannot be built on clay soil. When constructing on clay, various methods are used to solve the problem of ground subsidence, such as applying a “preloading method” in which a load greater than the weight of the structure is applied in advance to complete subsidence before starting the main construction, or using “deep foundations” in which large piles are driven deep into the hard ground to secure the structure. Therefore, houses built on clay soil are safe, and there is no need to worry too much.
Methods for solving ground subsidence continue to evolve. Recently, various new construction methods have been developed to reinforce the foundation soil, among which methods that use vibrations to rearrange soil particles and methods that artificially drain water from the soil to promote consolidation are gaining attention. In this way, modern civil engineering is striving to solve the problem of ground subsidence more effectively by supplementing traditional techniques and introducing new technologies. Ultimately, whether the ground is sand or clay, if its characteristics are well understood and appropriate construction methods are used, it is possible to build safe and sturdy structures.
