Soils are divided into clay and sandy soil (sand), and the stability of a house depends on the characteristics of each. Ground subsidence is caused by the release of air and water, and there are various methods to solve this problem, making it possible to build a stable house on sandy ground.
There is an old saying, “Sasangnugak (砂上樓閣)”. The dictionary explains that it means “a house built on sand, which may collapse due to a weak foundation, or something that is impossible to realize.” Think back to your childhood memories of playing on the playground or at the beach. When you use clay to build sticks, the clay’s viscosity makes it easy to hold the soil together, but when you use sand, the soil scatters easily and doesn’t hold well. You may also remember that toadstools made of clay are quite sturdy, but those made of sand collapse as soon as you remove them. Based on this experience, you probably thought that if you build a house on sand, it will collapse as quickly as a stick or a toadstool, so you coined the lion’s share of the word ‘sasangak’. But is a house built on sand really a house built on sand?
When building a house, the most important thing is to understand the subsidence characteristics of the place you are building. If you build a building without properly understanding the subsidence characteristics of the ground, not only will the building tilt due to the different amounts of subsidence on both sides, like the Leaning Tower of Pisa, but if the subsidence exceeds the allowable limit, you may experience the misfortune of having your hard-earned house collapse overnight. Imagine if the Leaning Tower of Pisa was your house, it would be terrifying to spend every day wondering if it would collapse at any moment. Therefore, it is very important to estimate and prepare for how much the ground will settle before building a house.
Soils can be divided into two main types: clay and sandy soil (sand). Clay has an electrically polarized surface that attracts water from the surrounding area, and water that is right next to a clay particle will stick to it until it becomes indistinguishable from soil. This water is called “adsorbed water,” and it is this adsorbed water that gives the clay its viscosity. Also, clay has small particle size and narrow spacing between the particles, so once it holds water, it doesn’t drain well and retains water for a long time. Sandy soil, on the other hand, has large particle sizes and wide spacing between particles, so water drains out easily, so even if you spray it with water, it will quickly drain away and become dry and non-viscous again. This is why sandcastles are built by wetting the sand with water at the beach and then collapsing when you leave and return.
We usually think of soil as a solid, so how do we explain the ground settling? The most common answer is that a structure weighing thousands of tons is weighing down on the soil, so the soil particles will crumble and cause subsidence. While this isn’t wrong, this kind of settlement is a very small part of the total amount of settlement, so it’s hardly the main cause of settlement. In fact, soil is not a solid, but a mixture of solids, liquids, and gases. The “solid soil” that people usually think of is called the “soil skeleton” in geomechanics. The spaces between the soil skeleton are filled with water and air. Clay has a lot of water, so you can feel a little viscous and wet when you touch it, while sand has less water and is relatively dry and not viscous. Think of an earthen skeleton of clay or sand in a box of a certain volume, with more water in the clay and less in the sand. This combination of solids (the soil skeleton), liquids (water), and gases (air) is what makes up soil.
The fact that soil is made up of solids as well as liquids and gases tells us that subsidence is caused by the escape of water and air. When a load is applied to the soil, the volume of the soil decreases in a relatively short period of time as the air inside the soil is expelled from the soil at the same time as the load is applied. This expulsion of air is called “instantaneous settlement”. After the immediate settlement is complete, the water in the soil gradually drains out over a long period of time, causing the ground to settle, called compressive settlement. After a long period of time after the immediate settlement and compressive settlement are complete (usually 50 years or more after construction, depending on the size of the structure), secondary settlement occurs as the skeleton of the soil rearranges itself. However, the mechanisms of secondary settlement are not yet clearly understood and the amount of settlement is small, so it is usually ignored or included in the calculation of ‘compression settlement’. The settlement of the ground can be divided into ‘immediate settlement’, which is the release of air, ‘compaction settlement’, which is the release of water, and ‘secondary settlement’, which is the rearrangement of the soil’s skeleton, and the sum of these is the total amount of settlement.
Now, in your mind’s eye, build a house on clay and sand. In clay, the spaces between the soil skeletons are mostly filled with water, so immediate settlement is minimal and compressive settlement dominates the total amount of settlement. On the other hand, sandy soil is mostly filled with air, so compressive settlement is almost non-existent, and instantaneous settlement dominates the total amount of settlement. If you build a house on clay, it may be easier to build because there is less air to escape from the soil, so there is very little ‘instantaneous settlement’. However, once it’s built, the water in the soil will drain out, causing compressive settlement, which can cause the building to tip or collapse if done incorrectly. However, if you build a house on sandy soil, it will only experience “immediate settlement” right after you start construction, and there will be very little ground settlement after that, so it will be stable. Contrary to popular belief, it’s actually safer to build a house on sand.
This doesn’t mean that you can’t build a structure on clay. There are several ways to solve the problem of ground settlement in clay, such as the preloading method, in which a load greater than the weight of the structure is applied in advance to allow it to settle before the main construction begins, or deep foundations, in which large piles are driven deep into the solid ground to anchor the structure. So, if your home is built on clay soil, you don’t have to worry too much because it’s safe.
The methods for solving land subsidence are constantly evolving. In recent years, new methods have been developed to strengthen foundations, such as using vibrations to rearrange soil particles and artificially draining water from the soil to promote compaction. As such, modern civil engineering seeks to complement traditional techniques and introduce new technologies to better address the problem of ground settlement. In the end, whether it’s sand or clay, a good understanding of the ground’s properties and the use of appropriate construction methods can lead to safe and sturdy buildings.
