Soil mechanics in civil engineering is a crucial subject of understanding as all the structures are ultimately erected on soil. The load on a structure is ultimately transferred to the soil.
Let us understand what is soil mechanics, which deals with different aspects of soil and the word ‘soil’.
What is Soil Mechanics in Civil Engineering?
The term ‘soil mechanics’ was given by Dr. Karl Terzhagi in 1925 in his book Erdbaumechanic.
Soil mechanics is a branch of mechanics, which deals with the action of forces on soil and with the flow of water in soil.
The scope of soil mechanics is limited to the study of soil, it’s behaviour, and application as an engineering material.
‘Soil’ in Soil Mechanics
The word soil comes from the Latin word solium, which means the upper layer of the earth that may be dug or ploughed.
Soil is an unconsolidated, complex material formed by the disintegration of rocks as they undergo weathering. The meaning of soil varies with the profession.
According to agriculturists, soil is the substance on earth’s surface on which the plants grow. According to geologists, soil is the material lying in the relatively thin zone where roots of plants are present. Rest is considered as rock irrespective of its hardness.
According to engineers, soil is the unaggregated or uncemented deposits of minerals &/or organic particles. Soil particles may be as small as 10-4 centimetre to large boulders. Boulders, sand, clay, gravel, silt, etc are included in soil.
Definition of soil mechanics by Terzhagi:
Soil mechanics is the application of laws of mechanics and hydraulics to engineering problems dealing with sediments and other unconsolidated accumulations of solid particles produced by mechanical and chemical disintegration of rocks regardless of whether or not they contain admixtures of organic constituents.
- Terzhagi
Soil Engineering vs Soil Mechanics
In contrast to soil mechanics with limited scope, soil engineering has much wider scope. It focuses on practical science rather than just fundamentals or mathematical aspects of soil.
Soil engineering is an applied science dealing with applications of the principles of soil mechanics to practical problems. Soil engineering covers Soil investigations, Design and Construction of foundations, earth-retaining structures, and earth structures.
However, many-a-times, soil mechanics is also referred to as soil engineering.
Geotechnical Engineering
Geotechnical engineering comprises soil engineering/mechanics and foundation engineering.
Contributors of Soil Mechanics
Coulomb: In 1776, he gave the wedge theory of earth pressure. This marked the beginning of present scientific study of soil behaviour.
He was the first one to introduce the concept of shearing resistance of soil. He presented that shearing resistance has two components- cohesion and friction.
Poncelet: He gave the graphical method of determining the magnitude of earth pressure on vertical or inclined wall surface for an arbitrarily broken polygonal surface.
Thus, he extended Coulomb’s theory.
Darcy and Stoke: In 1856, two important laws used extensively in soil mechanics were put forward-
Darcy’s law of water flow
Stoke’s law for settlement of soil particles in liquid
Rankine: In 1857, Rankine put forward the theory to calculate the earth pressure as well as safe bearing capacity of soil.
He ignored the cohesion of clay even though its existence was known.
Mohr: In 1871, he represented the stress at a point graphically. It became popular as Mohr’s stress circle. These stress circles are used to determine the shear strength of soils in soil mechanics.
Boussinesq: In 1885, he presented the analysis in semi-infinite elastic medium under surface point loads.
Muller–Breslau: In 1906, they performed experiments on large-scale model retaining wall to test the theories of earth pressure.
Atterberg: In 1911, he proposed that a clay soil may exist in different stages of consistency, which depends on its water content.
Leygue: In 1885, he developed a shear box to measure shear strength of sand in France.
The shear box was then modified by various scientists including Krey, Terzaghi, and Casagrande.
Praridtl: In 1920, he presented the theory of plastic equilibrium on which bearing capacity theories were developed later on.
Terzaghi: In 1923, he presented the theory of consolidation.
Who is the Father of Soil Mechanics?
Karl Terzaghi is known as the father of soil mechanics for his immense contribution in the field of soil mechanics in the nineteenth century.
Objective of Soil Mechanics/Aim of Soil Mechanics/Importance of Soil Mechanics
All structures are built on soils. To design safe and sound structures, certain properties of soil, theories and procedures are formulated.
During the design of structures, this information is utilized by the designers combining with their own practical experience to give a suitable design for the structure relevant to the field conditions.
The designer has to deal with natural soil deposits, which provide support to the foundations and its superstructure. But this soil deposit exists in an erratic manner which makes it possible to have various combinations. Hence, multiple options of possible combinations are possible. This affects the choice and design of structure.
A foundation engineer is thus required to have the ability to interpret the principles of soil mechanics to suit the field conditions. Thus, soil mechanics is the basis for foundation engineering.
Scope of Geotechnical Engineering/Application of Soil Mechanics
Soil engineering covers a vast scope as discussed below:
1. Foundation engineering
Foundation is a requirement for any structure to transmit the loads coming on it to the soil safely and efficiently. Whether the structure is a building, bridge, highway, canal, dam, or tunnel, it is founded on soil.
Following factors are to be considered when designing the foundation for any structure
- Bearing capacity of soil
- Stress distribution pattern in soil below the loaded area
- Probable settlement
- Effect of groundwater
- Swelling and shrinkage characteristics of of soil
The selection of a particular foundation type depends on the soil strata, load coming on structure and groundwater conditions. Various types of foundation are discussed in detail – Foundation
Types for a Structure.
2. Retaining Structures
Retaining structures are built to retain the soil.
The soil needs to spread to a definite slope so that it can remain stable. However if there is not enough space for the soil to spread on ground, a retaining structure needs to be constructed. Or else, the soil will stumble down.
Retaining structures also help to keep the soil at different levels on its either side.
3. Stability of Slopes
When the surface of the soil is not horizontal, the vertical component of weight tends to move the soil in downwards direction. This causes instability of slopes.
Slopes are designed for embankment as well as cutting.
Knowledge of shear strength of soil and its related properties is essential for designing the slope and height of embankment or depth of cutting.
Possibility of seepage of groundwater also needs to be considered while activating the soil. If groundwater seeps into the excavation, the bearing capacity of soil is considerably reduced. Sometimes, subsoil water needs to be drained out to reduce the seepage forces. There are various methods for draining the seepage water.
Lateral braces or sheet walls need to be constructed to support the soil during deep excavations.
4. Underground Structures
Tunnels, shafts, conduits, etc are underground structures. The forces exerted by soil on these structures need to be evaluated.
5. Pavement Design
Pavement is the hard crust constructed over soil to provide smooth and strong surface for moving of vehicles.
In the design of pavement, behaviour of subgrade under various loading and environment changes needs to be studied.
Before going ahead with the design of pavement, thickness of pavement and its components needs to be decided. This depends on the characteristics of the subsoil.
On a busy payment, the effect of repetition of loading and consequent fatigue failure needs to be evaluated.
Other problems arising for the payment are
- Frost and penetration depth of Frost
- Heaving and thawing
- Pumping of clay subsoils
A technical knowledge on improving the soil properties like strength and stability is required for the stabilization of subgrade soil.
6. Earthen Dams
For earthen dams, soil is the only construction material used. Earthen dams need to be designed meticulously as its failure may cause a catastrophe.
Earthen dams may have homogenous or composite sections.
Following physical properties of soil are required for designing an earthen dam-
- Index properties of soil like density plasticity specific gravity particle size distribution and radiation of soil
- Permeability of soil
- Consolidation and compaction characteristics of soil
- Shear strength parameters under various drainage conditions
Soil survey of nearby areas also needs to be done for the borrow pit area.
Optimum moisture content at maximum density on compaction is the most important aspect of soil characteristics for design.
7. Miscellaneous Soil Problems
Miscellaneous soil problems include
- Soil heave
- Soil subsidence
- Frost heave
- Shrinkage of soil
- Swelling of soil
Key Take Away
Definition: Soil mechanics is the branch of mechanics, in which soil, its behaviour and its applications are studied.
Father of Soil Mechanics: Karl Terzaghi
Objective/Aim/Importance of Soil Mechanics: Foundation of any structure transmits loads to the soil. For safe design of foundation, fundamentals of soil mechanics like stress relations, bearing capacity, etc. are required.
Difference between Soil Engineering and Soil Mechanics:
| SR. NO. | Soil Engineering | Soil Mechanics |
|---|---|---|
| 1 | Soil engineering has wider scope. | Soil mechanics has limited scope. |
| 2 | Soil engineering focuses on practical science. | Soil mechanics focuses on fundamentals or mathematical aspects of soil. |
| 3 | Soil engineering is an applied science dealing with applications of the principles of soil mechanics to practical problems covering soil investigations, design and construction of foundations, earth-retaining structures, and earth structures. | Soil mechanics is the study of deformation of soil and the flow of fluid in the structures constructed in or on the soil. |
Geotechnical Engineering = Soil Mechanics + Foundation Engineering
Soil Mechanics includes:
- Index properties of soil
- Shear strength of soil
- Stress strain relationship of soil
- Permeability of soil
- Pore water pressure in soil
- Soil exploration
- Stability of slopes
- Lateral earth pressure
Foundation engineering covers:
- Design of different types of foundation on various soils
- Soil improvement
Scope of Geotechnical Engineering /Application of Soil Mechanics:
- In foundation engineering
- Construction of retaining structures
- Stability of slopes
- Design and construction of underground structures
- Design of pavement
- Design and construction of earthen dams
FAQ
What is soil mechanics?
Soil mechanics is a branch of mechanics, which deals with the action of forces on soil
and with the flow of water in the soil. It includes the study of soil, its behaviour, and its
application as an engineering material.
As per Terzaghi, Soil mechanics is the application of laws of mechanics and hydraulics to
engineering problems dealing with sediments and other unconsolidated accumulations of
solid particles produced by mechanical and chemical disintegration of rocks regardless of
whether or not they contain admixtures of organic constituents.
What are the basics of soil mechanics?
The basics of soil mechanics include:
- Index properties of soil
- Shear strength of soil
- Stress strain relationship of soil
- Permeability of soil
- Pore water pressure in soil
- Soil exploration
- Stability of slopes
- Lateral earth pressure
What is the importance of soil mechanics in civil engineering?
All structures are built on soils. To design safe and sound structures, certain properties of
soil, theories and procedures are formulated.
During the design of structures, this information is utilized by the designers combined
with their own practical experience to give a suitable design for the structure relevant to
the field conditions.
The designer has to deal with natural soil deposits, which provide support to the
foundations and their superstructure. All these are covered in soil mechanics.
Who is the father of soil mechanics?
Karl Terzaghi is known as the father of soil mechanics. This is because he made notable
contributions in the field of soil mechanics in the twentieth century.