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Al-Rafidain University Collage Civil Engineering Department
Soil Mechanics Lectures for Third Year Students By Dr. Ahmed Al-Adly
Ph. D Civil / Geotechnical Engineering
Lecture 1: Introduction to Soil and Soil Mechanics 1
Lecture 1
Introduction to Soil and Soil Mechanics
Most of the civil structures rest on the soil, so the life of these structures depends on the
soil and its behaviour under the loads. The behaviour of soil under the loads depends on
the various properties of soil. The different soil properties can be determined by
studying soil mechanics.
1.1 Soil
The word “Soil” is derived from the Latin word “Solum” which has different meanings to
different professions as:
① In agronomy: the term means the upper layer of the earth that may be plowed;
specifically, the loose surface material of the earth in which plants grow.
② In geology: the term means the materials, which are produce from weathering of rocks
and cover the upper layer of earth’s crust.
③ In civil engineering: the term means, uncemented inorganic material, composed of
solid particles, produced by the disintegration of rocks. The void space between the
particles may contain air, water or both.
Since ancient ages, engineers have been handling soils as an engineering material for
various construction projects. Construction of the Egyptian pyramids, Iraqi ziggurats,
Roman aqueducts, and China’s Great Wall.
In civil engineering, soil is used as (Figure 1-1):
① A constructional material (fill materials) such as earth dams, embankments, slopes,
railways, highways, and airports.
② To supports loads from the foundations of buildings.
Al-Rafidain University Collage Civil Engineering Department
Soil Mechanics Lectures for Third Year Students By Dr. Ahmed Al-Adly
Ph. D Civil / Geotechnical Engineering
Lecture 1: Introduction to Soil and Soil Mechanics 2
Figure (1-1): Use of Soil in Civil Engineering
1.2 Soil Mechanics
Soil Mechanics is one of the youngest disciplines of civil engineering involving the study
of the physical and mechanical properties of soil. The first definition of soil mechanics has
been coined by so-called "father of soil" "Dr. Karl Terzaghi", Karl Terzaghi coins the
term of "Soil Mechanics" in (1925), and define the soil mechanics as following:
Soil Mechanics is the application of the laws of mechanics and hydraulics to
engineering problems dealing with sediments and other unconsolidated accumulations
of solid particles produced by the mechanical and chemical disintegration of rocks
regardless of whether or not they contain an admixture of organic constituents.
Definitions
① Foundation engineering: is a branch of soil engineering dealing with the application
of principles of soil mechanics to the design and construction of foundations for various
structures.
② Geotechnical engineering: is a broader term and refers in total to all engineering
problems involving soil and/or rock as foundation material and construction material.
Geotechnical engineering involves the application of soil mechanics, rock mechanics, and
engineering geology to engineering problems including soils and rocks.
Al-Rafidain University Collage Civil Engineering Department
Soil Mechanics Lectures for Third Year Students By Dr. Ahmed Al-Adly
Ph. D Civil / Geotechnical Engineering
Lecture 1: Introduction to Soil and Soil Mechanics 3
1.3 Applications of Soil Mechanics in Civil Engineering
Soil mechanics has vast applications in many fields of civil engineering works. Some of
the important applications are as under:
① Foundations: The loads from any structure have to be transmitted to a soil through the
foundation for the structure. Thus, the type, and details of which can be decided upon only
with the knowledge and application of the principles of soil mechanics.
② Underground and Earth-retaining Structures: Underground structures such as pipe
lines, and tunnels and earth-retaining structures such as retaining walls can be designed
and constructed by evaluating the forces exerted by the soil on these structures. This
requires application of the principles of soil mechanics.
③ Slopes and Excavations: Slopes is not horizontal surface, there is a component of
weight of the soil, which tends to move it downward and thus cause instability of slopes.
This requires study the stability analysis in soil mechanics. Likewise, excavations require
the knowledge of slope stability analysis; deep excavations may need supports (bracing).
These supports should be designed to resist the lateral forces exerted by the soil. This
requires knowledge of theories of lateral earth pressure only in soil mechanics.
④ Embankments and Dams: The construction of embankments and earth dams where
soil itself is used as the construction material requires a thorough knowledge of the
engineering behaviour of soil especially in the presence of water. Knowledge of seepage
analysis as well as compaction characteristics for achieving maximum unit weight of the
soil is essential for efficient design and construction of embankments and earth dams.
⑤ Pavement Design: A Pavement is a hard crust placed on soil for providing a smooth a
strong surface on which vehicles can move. Pavements depend more on the subgrade soil
for transmitting the traffic loads. The design of pavements depends on swelling and
shrinkage of sub-soil and frost action. Consideration of these factors is necessary and one
cannot do without the knowledge of soil mechanics.
Al-Rafidain University Collage Civil Engineering Department
Soil Mechanics Lectures for Third Year Students By Dr. Ahmed Al-Adly
Ph. D Civil / Geotechnical Engineering
Lecture 1: Introduction to Soil and Soil Mechanics 4
1.4 Uniqueness of Soil
Soil unlike others civil engineering materials such as concrete, steel, brick, ect. The soil
problems as a material can be considered unique because:
① Stress-Strain relationship of soils is not linear. This makes the stress-strain properties
analysis of soil mass is difficult.
② Soil is not elastic material and the behaviour of soil depends on the elastic and plastic
properties. Soil is Elasto-Plasto material.
③ Soil behaviour strongly depends on pressure, time, and environments conditions. Soil
properties are controlled by changing in pressure and presence of water.
④ Soil is not homogenous material; this means the soil properties may be different from
location to other.
⑤ The nature of soil is complex (Figure 1-2). Soil composed of particles with wide range
in sizes, shapes, and grading. Soil mass is three phase system, the pours between its solid
particles filled with water and air. Because of these particulate natures of soil, the
development of mathematical models to describe and predict the soil behaviour of soil is a
difficult.
Strain
Stre
ss
Strain
Stre
ss
Strain Strain
Stre
ss
Stre
ss
Linear
Non-Linear
Elastic
Elastic-Plastic
Al-Rafidain University Collage Civil Engineering Department
Soil Mechanics Lectures for Third Year Students By Dr. Ahmed Al-Adly
Ph. D Civil / Geotechnical Engineering
Lecture 1: Introduction to Soil and Soil Mechanics 5
Soil System (solids, water, air) Soil particles Shapes of soil particles
Figure (1-2): The Nature of Soil
⑥ Most soils are very sensitive to disturbance and the properties measured by a
laboratory test may be unlike that of the in situ soil.
⑦ Soil mass cannot be seen entirely and its properties evaluated based on small sample.
Sand particles Clay particles
Al-Rafidain University Collage Civil Engineering Department
Soil Mechanics Lectures for Third Year Students By Dr. Ahmed Al-Adly
Ph. D Civil / Geotechnical Engineering
Lecture 1: Introduction to Soil and Soil Mechanics 6
1.5 Example of Soil Mechanics Problems
Engineers have to deal with many challenging soil mechanics problems even at present, as
well as in the past. A famous building illustrates the problems of soil mechanics is
leaning Tower of Pisa.
This famous building illustrates
historical soil mechanics problems.
The (56 m) high tower at Pisa, Italy,
leans about (3.9 m) at top toward
the south. The construction of the
tower started in (1173) and was
completed in (1372). It was reported
that the tower started to sink
unevenly after the construction
progressed to the third floor in
(1178) and more floors were built
up to accommodate for the tilt. The
lean is due to differential settlement
of the foundation soil. In March
(1990), the tower was closed to the
public due to the possibility of
collapse in the near future.
Engineering remediation procedures
were discussed to stop further
leaning.
Al-Rafidain University Collage Civil Engineering Department
Soil Mechanics Lectures for Third Year Students By Dr. Ahmed Al-Adly
Ph. D Civil / Geotechnical Engineering
Lecture 1: Introduction to Soil and Soil Mechanics 7
has been declared stable for at least another 300 years.
An early attempt was made to put
heavy load (800 metric tons of lead
counterweight) on the north side of
the tower foundation to compensate
for the larger settlement on the
south. A more drastic measure was
taken later to extract soil mass
(38 m3) under the north side of the
foundation soil by angled auger
holes so that the north side
experienced extra settlement. In December 2001, the tower was reopened to the public and
Al-Rafidain University Collage Civil Engineering Department
Soil Mechanics Lectures for Third Year Students By Dr. Ahmed Al-Adly
Ph. D Civil / Geotechnical Engineering
Lecture 1: Introduction to Soil and Soil Mechanics 8
Dr. Karl von Terzaghi
(October 2, 1883 – October 25, 1963)
An Austrian civil engineer and geologist, founder
and guiding spirit of soil mechanics, outstanding
engineering geologist, and preeminent foundation
engineer. Is generally recognized as a Father of
Soil Mechanics. His early professional life was
spent in search for a rational approach to
earthwork engineering problems. His efforts were
rewarded with the publication in 1925 of his
famous book on soil mechanics, this publication is
now credited as being the birth of soil mechanics.
Terzaghi was the first to make a comprehensive
investigation of the engineering properties of soils;
he created most of the theoretical concepts needed
for understanding and predicting the behavior of
soil; and he devised the principal techniques for
applying scientific methods to the design and
construction of foundations and earth structures.
To commemorate Terzaghi’s great work, the
American Society of Civil Engineers (ASCE)
created the Terzaghi’s lecture and the Terzaghi
Award.