Soil mechanics Soil mechanics

Soil Mechanics – History, composition, formula, compaction, Importance

Soil mechanics, often known as dirt or earth, is a mixture of organic materials, minerals, liquids, gases, and organisms that work together to sustain the lives of plants and soil organisms. Some definitions in the field of science limit the use of the term “dirt” to refer only to relocated soil. Soils are natural constituents of weathered environments with variable characteristics.

What is soil mechanics?

Soil mechanics is an intriguing scientific field that comes under the broader field of civil engineering. It delves into the mechanical behavior of soil, exploring the various factors that influence its properties and how it interacts with other materials. Through extensive research and experimentation, soil mechanics experts obtain a deeper understanding of the complicated properties of soil and its impact on construction projects.  These projects include buildings, bridges, tanks, embankments, dams, and tunnels. Understanding soil mechanics is essential for ensuring the stability and safety of these structures.

Soil mechanics is unique from classical fluid and solid mechanics. Soil is a complicated mixture of solid particles including gravel, rock, sand, silt, clay, liquid, and gas. It’s a three-phase system, unlike classical mechanics’ two-phase systems. Soil is particulate, distinguishing it from other mechanics materials.  Understanding and predicting soil behavior is difficult. It is stress-dependent and nonlinear. These considerations make soil behavior prediction challenging. Thus, studying soil behavior is crucial to understanding its qualities and traits. Thus, we may create more sustainable and responsible soil resource management techniques.

Soil mechanics is an important part of geotechnical engineering that includes the investigation of the mechanical characteristics of soil. In-situ and laboratory testing help engineers understand these features. Analytical or constitutive models simulate soil behavior after data collection. Engineers must precisely forecast soil behavior to construct safe and effective structures. Soil mechanics is essential to construction. Soil mechanics aims to stabilize soil and restrict deformation while managing groundwater flow. Soil mechanics’ function depends on the project’s nature and scale, but it’s the same. Engineers can build safe, durable, and efficient structures by knowing soil qualities and behavior.

Learn More What is Mechanical Engineering? – Everything you need to Know!


History of soil mechanics

At the beginning of the 20th century, soil mechanics started to develop. Analysis of soils behavior arose in many nations, frequently as a result of spectacular incidents like landslides and foundation failures. In 1918, a railway embankment at Weesp in the Netherlands collapsed, prompting the first systematic soil mechanics study by a government commission. At that time, many soil mechanics principles were recognized, but their unification into an engineering discipline was not.

The earliest significant contributions to soil mechanics were made by Coulomb, who wrote a significant treatise on the failure of soils in soils, and Rankine, who wrote an article on the potential states of stress in soils in 1857.

For the water supply of the city of Dijon, Darcy published his renowned work on the permeability of soils in 1856. Newton, Cauchy, Navier, and Boussinesq popularised the mechanics of continua, including statics and material strength, in the 19th century. The 20th century brought these elements together into a unified discipline. The commission investigating the Weesp catastrophe found that prolonged rains had raised water levels in the railway embankment, which was too weak to handle strong water pressures.

Also Read What is Fluid Mechanics: Understanding the Fundamentals

Classification of soil

Classification systems for soils assist engineers in determining the suitability of various soils for various purposes and applications. Soil is categorized according to specific criteria such as:

  • Soil classification must be determined by its engineering properties.
  • The classification must only comprise a limited number of groups with comparable characteristics and properties.
  • It should be straightforward and uncomplicated to learn.

The classification of soil is as follows:

  1. Massachusetts Institute of Technology System (MIT)
  2. Indian Standard System of Soil Classification
  3. Unified Soil Classification System
  4. Textural Classification of Soils
  5. AASHTO System of Soil Classification

Soil Composition

Unit weight of soil

It is the weight of soil solids Wd per unit volume of solids (Vs): γs=Wd/Vs

As a result, the dry weight is known as the dry unit weight when calculated using the total original volume V, and the unit weight of soil solids when calculated using the volume of solids


Consistency of soil

It is the strength with which soil materials are kept together, as well as the soil’s resistance to deformation and rupture.


The volume of voids (Vv) to the total volume (V) of the given soil mass is the ratio used to determine the porosity of a specific soil sample. Typically, ‘n’ stands for it and it is expressed as a percentage. Another name for it is percentage voids.

Hence, n = Vv/V.


Void ratio

The volume of voids (Vv) to the volume of soil solids (Vs) in a given soil sample is known as the void ratio. It is typically written as a fraction and designated by the letter “e.”

So, e = Vv/Vs.

Moisture content

The ratio of the weights of the solids (Wd) and water (Ww) in a particular mass of soil is known as the moisture content (w). m or w stands for it. Normally, a percentage is used to denote it.

Thus, w = (Ww/Wd)* 100%


Degree of Saturation

Its definition is the ratio of the total volume of voids (Vv) to the volume of water (Vw) that is present in a specific soil mass. The symbol for it is (Sr). It also stands by the name “percent saturation,” and is typically represented as a percentage.

Therefore, Sr = Vw / Vv.

Atterberg limits

These are fundamental measures of fine-grained soil’s critical water content. These three Atterberg limits;

  • Liquid limit
  • Plastic limit
  • Shrinkage limit

Cohesive soils include

These are fine-grained soils with particles that agglomerate or clump together. In layman’s terms, it’s the material that holds things together! These soils are often soft and have a high moisture content.

Soils that are non-cohesive

These soils are not clumpy in any form. In other words, their grains remain distinct from one another.


Soil Compaction

By reducing the void space between soil particles, soil compaction is the process of adding mechanical compaction force to densify soil. When particles are pushed together to make less room between them, this is called compaction. Because there are so few spaces in highly compacted soils, the dirt weighs more per unit. At an optimum moisture content, or OMC, which is short for optimum moisture content.

Compaction makes it less likely that a building, road, airport, or parking lot will settle after it has been built. A settlement could result in premature pavement failure, expensive upkeep or repairs.

Why does compaction need for soil?

Soil compaction is required to increase the bearing capacity and stiffness of naturally occurring or chemically changed soils. Compaction increases soil shear strength by increasing friction caused by particle interlocking. Future soil settling is reduced by increasing stiffness and reducing cavities, resulting in densified soil.


What are the four factors that affect compacting?

The following are some of the factors which affect compaction: The nature of soil type, such as sand or clay, as well as grading and plasticity. The amount of water present (moisture content) at the time of compaction. Weather, site type, and layer thickness are examples of site conditions. Compaction effort: type of plant (vibration, weight, number of passes)

Also Read How to Become a Mechanical Engineer: A Roadmap to Success

what is the Importance of soil mechanics?

In soil mechanics, we learn about the different characteristics of the soil that can be used for different types of engineering construction projects.



The soil is where all civil engineering structures finally rest. We must build foundations to support these structures since they place their entire weight on the ground. We can offer shallow foundations in cases when the earth is firm or strong enough.

The type of foundation to be used can be chosen if we are aware of the soil’s strength. We must build deep foundations, such as pile foundations and well foundations if the earth is weak. Understanding how to compute how to determine how strong the soil is is crucial.

Read: What is Foundation Engineering: Building Strong Structures

Dams made of earthen materials

To keep the water in place, numerous earthen dams have been built. The soil that will be used to build these earthen dams must be suitable for usage for that purpose. To determine whether the soil has been compacted to the necessary density or not, various soil parameters, including its permeability, strength, and density, are regularly evaluated.

Studying the soil’s qualities is crucial since earthen dams are expensive structures that also have a significant danger of failing. As a result, they must be built carefully.



Since there is a danger of flooding and other disasters, embankments are built to elevate the levels of the highways on the plains. It is also necessary to keep the pavement’s foundation above the water table. The soil, whose varied qualities have been assessed, is typically used to build the embankments. A cost-effective embankment must be designed, which can only be done by researching the various soil characteristics.

Canals, other retaining structures, and underground constructions

The soil that must be built to be impermeable and strong enough also shapes the canals. The purpose of the retaining wall and structure is to hold the ground in place. It’s crucial to understand the characteristics of the planet. We get the notion to construct the retaining structure from characteristics like ground pressure and shear strength, among others.

In conclusion, soil mechanics applies mechanics and hydraulics to soil engineering challenges. Fields of study define soil differently. Geotechnical engineers define soil as agricultural material, broken-up rock, volcanic ash, alluvium, Aeolian sand, glacial material, and any other residual or transportable product of rock weathering.

 Soil Mechanics studies soil deformation and strength.  It studies soil mechanical properties and applies them to engineering challenges.  It focuses on structure-foundation interaction.  This includes conventional constructions and soil-based earth dams, embankments, and highways.

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