No matter how big or small a construction project is, it must stand on the ground, through the so-called Foundation (probably the dictionary took the image of a chicken’s leg). If the foundation is not stable, the project can collapse (photo). Therefore, construction engineers must study a minimum number of hours about soil and foundation. This series of articles hopes to popularize this difficult knowledge, for as many people as possible who need it 🆗
1. What is soil?
The question sounds naive, but
Soil in the eyes of a builder is different from the concept of soil in farming or mining, which is any loose or easily excavated material that we will build on it (foundation) or inside it (basement) or use it for construction (dyke). You can feel it by touching it, soil consists of countless loose material particles. These particles originate from rocks.
Over millions of years, under the influence of the environment and internal activities, rocks are broken down into soil particles, called the Weathering process. Soil particles move by themselves or are dragged and then accumulate to form different types of soil, called the Sedimentation process.
In addition to soil particles (solid), soil also consists of 2 forms (phases): liquid, gas as illustrated below
Soil in construction design
The most influential on the construction properties of soil is the solid phase, consisting of different sized soil particles. When the particle size is small to a certain extent, they will appear to be able to bind with water molecules to create Stickiness, we have a familiar soil type called Clay, which is the source of raw materials for countless ceramic works from ancient times to the present. Stickiness is the ability to maintain a new shape when pressed or molded.
When the humidity is low enough (little water), the water between the soil particles creates a stronger attraction force, creating Stickiness. The lowest humidity at which the soil changes from a solid state to a sticky state is called the plastic limit WP (%). 💦When the humidity increases, the moisture absorption efficiency is worse and the stickiness decreases. The humidity increases to the point where it can no longer absorb moisture and the soil mixture behaves like a liquid, called the liquid limit WL (%).
Foundations are often located deep in the ground, even very deep up to tens of meters like pile foundations. The builder must then consider how the soil changes throughout that depth and further within the scope of influence of the foundation. Because the soil is formed from rock, it is easy to see that the deeper the soil, the better it is for bearing the load: tighter, the soil particles are harder, larger, and finally the bedrock💎. Very heavy load structures such as super high-rise buildings (over 200m high) often have to make pile foundations against the rock. The figure below is a typical example of a geological pillar consisting of many layers of soil according to depth:
Typical geological pillars in construction design
What is the basis for classifying soil layers? For builders, the most common way to classify soil is based on the size of soil particles. Many European and American systems classify soil according to: soil particles smaller than 0.002mm can be classified as Clay, which begins to have stickiness when mixed with water. Larger soil particles are not sticky, and are classified as Sand. Note the word “can”, because many types of soil have such small grain sizes but are not sticky, such as those originating from rocks (minerals) such as Quartz, Feldspar, Mica. Soil particles originating from minerals such as Kaolinite, Montmorillonite… are called clay minerals. These mineral particles have a large surface area, increasing contact with water molecules, creating a greater gravitational force between clay particles, creating stickiness.
Thus, there are 2 important types of soil: sand (loose soil) and clay (cohesive soil). Because there are many different views on land classification, we must follow the Codes, live and work according to the law. Vietnamese Standard TCVN 9362:2012 “Design of house and construction foundations” gives a very clear classification:
Soil classification of foundation design standard TCVN 9362_2012
Summary as follows:
– Sandy soil is classified into types according to grain size: gravel sand, coarse sand, medium coarse sand, fine sand, dusty sand. State according to void ratio e: tight, medium tight, loose. State according to water saturation G: Low moisture, moist, water saturated.
– Clay is classified into types according to plasticity index IP: Sub-sand, Sub-clay, clay. State according to consistency index IS: hard, semi-hard, hard plastic, soft plastic, mushy plastic, mushy.
2. Parameters of Soils
If the Earth could speak
Then the Geologist would speak with his teeth.. 😂
How does the Earth speak? Through numbers. How can we get the numbers? The only way is to conduct a geological survey and take samples for testing. Then, we will synthesize a number representing each property, also known as a physical index. The geological survey engineer needs to provide it to the design engineer, or better yet, the designer proactively asks the surveyor to synthesize the physical indexes as follows:
Physical indicators of soil survey in construction design
There are also many numbers. Physical indicators of soil include: physical indicators, mechanical indicators.
Physical indicators of soil design
Mechanical indicators include
Cohesion c, internal friction angle ϕ, deformation modulus E, which are the most important for the builder.
To understand the meaning of these numbers, it is necessary to grasp some concepts of Soil Mechanics, which is the basic science of soil mechanical properties for the builder.
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🎁Read part 2
