Module 1- Nature of soil &
Functional relationships
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Geotechnical Engineering
Geotechnical Engineering deals with the application of Civil
Engineering Technology to some aspects of earth.
Geotechnical Commission of Swedish State Railways (1914‐
1922) was the first to use the word Geotechnical in the sense
that we know it today: the combination of Civil Engineering
technology and Geology.
Geotechnical Engineering deals with;
Design of Foundation
Stability of Slopes and Cuts
Design of Earth Structures
Design of Roads and Airfield etc
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• Soil Mechanics is defined as the branch of engineering science which
enables an engineer to know theoretically or experimentally the behavior
of soil under the action of
1. Loads (static or dynamic)
2. Gravitational forces
3. Water and
4. Temperature.
• According to Karl Terzaghi, Soil Mechanics is the applications of Laws of
Hydraulics and Mechanics to engineering problem dealing with sediments
and other unconsolidated accumulations of solid particles produced by
Mechanical and Chemical Disintegration of rocks.
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Soil Mechanics is the branch of science that deals with study of
physical properties of soil and behavior of soil masses subjected to
various types of forces.
Civil Engineer must study the properties of Soil, such as its
origin, grain size distribution, ability to drain water,
compressibility, shear strength, and load bearing capacity.
Soil GeoTech.
Civil Engg.
Mechanics Engg.
Geotechnical Engineering is the sub discipline of Civil Engineering
that involves applications of the principles of Soil Mechanics and
Rock Mechanics to design of foundations, retaining structures and
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earth structures.
KARL VON TERZAGHI
Austrian m echanicalengineer,geotechnicalengineer,and geologist
know n as the "fatherofsoilm echanics and geotechnicalengineering
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Karl Terzaghi has often been called the father of Soil
Mechanics.
Academically, he earned an under graduate degree in
Mechanical Engineering.
In 1925, he accepted lectureship at MIT.
In 1939, he accepted professorship at Harvard University till
his death.
His recognition and formulation of the effective stress
principle and its influence on settlement analysis, strength,
permeability and erosion of soils was his most prominent
contribution. But Terzaghi also pioneered a great range of
methods and procedures for investigation, analysis, testing,
instrumentation, and practice that defined much of the field
we currently know as geotechnical engineering.
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• Unfortunately, soils are made by nature and not by man, and
the product of nature are always complex... As soon as we pass
from steel and concrete to earth, the omnipotence of theory
ceases to exist.
• Natural soil is never uniform. Its properties changes from point
to point while our knowledge of its properties are limited to
those few spots of which the samples have been collected.
• In soil mechanics, the accuracy of computed results never
exceeds that of crude estimate, and the principal function of
theory consists in teaching us in what and how to observe in
the field. (Karl Terzaghi)
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Pioneers of soil mechanics
C.A. Coulomb William Rankine Albert Atterberg D W Taylor
(1736‐1806) (1820‐1872) (1846‐1916) (1900‐1955)
Arthur Casagrande A W. Skempton Ralph B Peck
(1902‐1981) (1914‐2001) (1912‐2008) 20
Why we study Soil Mechanics?
Virtually every structure is supported by soil or
rock. Those that aren’t either fly, float or fall
over.1
Various reasons to study the properties of Soil:2
1. Foundation to support Structures and Embankments
2. Construction Material
3. Slopes and Landslides
4. Earth Retaining Structures
5. Special Problems
Ref. 1. Pg # 1. Geotechnical Engg. (Principles and Practices) by Donald P. Coduto. 2nd Ed.
2. Pg # 3 to 16 , Chap # 1. Soil Mechanics by T. William Lambe. 21
Why we study Soil Mechanics?
Various reasons to study the properties of Soil:
1. Foundation to support Structures and Embankments
• Effects of static loading on soil mass
• Shear failure of the foundation soil
• Settlement of structures
• Stability criteria (Solution)
There should be no shear failure of the foundation soil.
The settlement should remain within permissible limits.
• Firm Soil -> Spread Footing (Spread Foundation)
• Soft Soil -> Pile Foundation (Vertical members
transferring load of structure to ground i.e. rock)
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Why we study Soil Mechanics?
Various reasons to study the properties of Soil:
1. Foundation to support Structures and Embankments
• Effects of dynamic loading on soil mass
• For Design and construction of roads following must be
considered: (a)) Compaction Characteristics (b) Moisture
Variation
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Why we study Soil Mechanics?
Various reasons to study the properties of Soil:
2. Construction Material
• Subgrade of highway pavement
• Earthen dam
• Land reclamation (Dubai Palm Island)
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Why we study Soil Mechanics?
Various reasons to study the properties of Soil:
3. Slopes and Landslides
• Major cause is the moisture variation resulting in:
Reduction of shear strength
Increase of moisture
Increase in unit weight
• Excavation of trenches for buildings require braced excavation.
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Why we study Soil Mechanics?
Landslide of a parking
area at the edge of a
steep slope, mainly
due to increase in
moisture content.
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Why we study Soil Mechanics?
Various reasons to study the properties of Soil:
4. Earth Retaining Structures
• Earth retaining structure (e.g., Retaining walls)are constructed to
retains (holds back) any material (usually earth) and prevents it
from sliding or eroding away.
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Why we study Soil Mechanics?
Various reasons to study the properties of Soil:
5. Special Problems
i. Effects of river water on soil mass
a) Scouring
Causes:
• Increased flow velocity due to obstruction
• Fineness of riverbed material
Stability criteria:
• The foundation of pier must be below the scour depth
ii. Land Erosion
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Why we study Soil Mechanics?
Various reasons to study the properties of Soil:
5. Special Problems
iii. Effects of frost action on soil mass
• Reduction Of Shear Strength
• Settlement Of Structure In Summer
• Lifting Up Of Structure In Winter
Causes:
• Heaving (due to formation of ice lenses)
• Increase of moisture due to thawing (MELTING)
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Soil Descriptions
• Soil is an unconsolidated agglomerate of minerals
with or without organic matter found at or near the
surface of the earth crust, with which or upon which
civil engineers build their structures
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WEATHERING OF ROCKS
Weathering of Rocks
Physical Weathering
Chemical Weathering
Rock Cycle
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Weathering is the process of breaking down rocks by
physical and chemical process into smaller particles.
There are two main types of weathering processes:
Physical (or mechanical) Weathering
Chemical Weathering
Biological weathering is caused by activities of living
organisms ‐ for example, the growth of roots or the
burrowing of animals. Tree roots are probably the most
occurring, but can often be by animals!
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Physical (or mechanical) Weathering is the
disintegration of rocks into smaller particles through
physical processes, including:
The erosive action of water, ice and wind.
Opening of cracks as a result of unloading due to
erosion of overlying soil and rock.
Loosening through the percolation and
subsequent freezing (and expansion) of water.
Thermal Expansion and contraction from day to
day and season to season.
Landslides and rockfalls.
Abrasion from the downhill movement of nearby
rock and soil.
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Mechanical/Physical weathering
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Chemical Weathering is the disintegration of rock
through chemical reactions between the minerals in
the rocks, water, and oxygen in the atmosphere.
An example of the chemical weathering : orthoclase
to form clay minerals, silica and soluble potassium
carbonate follows:1
H2O + CO2 H2CO3 H+ + (HCO3)‐
2K(AlSi3O8) + 2H+ + H2O 2K+ + 4SiO2 + Al2Si5(OH)4
Orthoclase Silica Kaolinite
(Clay mineral)
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Chemical Weathering
Chemical weathering rate depends on
1. Temperature
2. Amount of surface area
3. Availability of water or natural acid
Thus, rocks in tropical environment
experience most severe chemical weathering.
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All rock at or near Earth's surface is being modified by the
processes of metamorphism, melting, crystallization, lithification
and weathering. These processes move rock material through the
states of metamorphic rock, igneous rock, sedimentary rock, melts
and sediment. The natural and continuous cycling of rock
materials through these states is known as the Rock Cycle.1
Ref. 1. General Dictionary of Geology (Environmental Geographic Student Association (EGSA)) 43
