Lecture 1 Introduction to Soil and 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 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.





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.





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.





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





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





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.





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





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.

Lecture 1 Introduction to Soil and Soil Mechanics

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Transcript of Lecture 1 Introduction to Soil and Soil Mechanics