This document provides an introduction to soil mechanics. It defines key terms like soil, soil mechanics, and phase diagram. It discusses the important applications of soil mechanics in foundations, pavement design, underground structures, earth retaining structures, embankments, and earth dams. Properties of soil like bulk unit weight, dry unit weight, saturated unit weight, submerged unit weight and specific gravity are explained. Volumetric relationships between void ratio, porosity, degree of saturation and air content are defined. The document also introduces the concept of relative density for coarse-grained soils. Textbooks and the father of soil mechanics Karl Terzaghi are cited.

1. Soil Mechanics
(AGRI.ENGG)
UNIT-1
By
Y. Naga Lakshmi
Assistant Professor
SWCE Department
Aditya Engineering College,
Surampalem,
Kakinada, A.P.
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2. 2
Lecture no. 1:
Introduction of soil mechanics, field of soil mechanics, phase diagram,
physical and index properties of soil.
Y.NAGA LAKSHMI (ASSISTANT PROFESSOR)

3. 3
Text Books:
1. Soil Mechanics and Foundations Punmia B C, Ashok Kumar Jain & Arun Kumar Jain – 16th
edition 2005 - M/S Laxmi Publications(P)Ltd. 113, Golden
House, Daryagani, New Delhi – 110002.
2. Basic and Applied Soil
Mechanics
Ranjan Gopal and Rao A S R. 1993, Welley Easters Ltd.,
New Delhi.
3. Soil Engineering (Vol. I.) Singh Alam. 1994, CBS Publishers and Distributions,
Delhi.
Y.NAGA LAKSHMI (ASSISTANT PROFESSOR)

4. FATHER OF SOIL MECHANICS:
“KARL VON TERZAGHI”
(1883-1963)
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Y.NAGA LAKSHMI (ASSISTANT PROFESSOR)

5. 5
SOIL:
“The term Soil is defined as an unconsolidated material, composed of solid particles,
produced by the disintegration of rocks”.
(or)
“Soils are formed by weathering of rocks due to mechanical disintegration or
chemical decomposition. When rock surface gets exposed to atmosphere for an appreciable
time, it disintegrates or decomposes small particles and thus soils are formed”.
Y.NAGA LAKSHMI (ASSISTANT PROFESSOR)

7. 7
SOIL MECHANICS:
“Soil Mechanics is the application of laws of mechanics and hydraulics to engineering
problems dealing with the 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 constituent”.
Y.NAGA LAKSHMI (ASSISTANT PROFESSOR)

8. Field of Soil Mechanics:
• Soil Mechanics has vast applications in the construction of various civil engineering
works. Some of the important applications are:
 Foundations design and construction-bearing capacity of soil, pattern of stress
distribution in the soil beneath the loaded area, effect of groundwater and the effect of
vibrations.

10.  Pavement design-
Busy pavements, intensity of traffic is high, the effect of repetition of loading and
consequent fatigue failure.
Note: Fatigue failure is when the surface of a material begins to crack or fracture, causing
the part to weaken. Typically, the first stage of fatigue failure is crack initiation. Crack
initiation occurs once applied stress exceeds tensile strength. The next stage that occurs is
crack growth.
Plate:Crack or fracture Plate: Frost Damage

11. Design of underground structures and earth retaining structures:
Underground structures Ex: Tunnels, underground buildings, drainage structures and pipelines
Earth retaining structures Ex: Gravity retaining wall, anchored bulk heads and coffer dams.
Plate: Gravity retaining wall
Plate: Anchored bulk heads

12. Plate: Coffer dams
Note:
Coffer dam is an enclosure built within a body of water to allow the enclosed area to be
pumped out. The pumping creates a dry working environment so that the work can be
carried out safely.

13. Design of embankment and excavation:
• Thorough knowledge of shear strength of soils and related properties of soil is
required to design the slope and height of the embankment.

14. Design of earth dams
It involves the determination of the following properties of soil:
Index properties: density, plasticity characteristics, specific gravity, particle size
distribution, permeability, consolidation and compaction characteristics and shear
strength.

15. 3-PHASE DIAGRAM:
In general the soil mass consists of solid particles, water and air. The three
constituents are blended together to form a complex material (Fig.2.1a). However, for our
convenience, all the solid particles are segregated and place at the bottom of the three phase
diagram (Fig.2.1b). Likewise, water & air particles are placed separately. The three phase
diagram is also known as Block Diagram.

16. • In three phase diagram, it is conventional to write volumes on one side the masses on the
other side as shown in figure below:

21. VOLUME-WEIGHT RELATIONSHIPS
Unit weight of soil mass: The unit weight of a soil mass is defined as its weight per unit
volume.
(i) Bulk Unit Weight (γ): It is the total weight of a soil mass (W) per unit of its total volume
(V).
(γ)= W/V
(ii) Dry Unit Weight (γd): It is the total weight of solids per unit of its total volume V (prior to
drying) of the soil mass.
(γd) = Wd/V
(iii) Unit Weight of solids (γs): The unit weight of soil solids is the weight of soil solids (Wd)
per unit volume of solids (Vs)
(γs) =Wd/Vs
(iv) Saturated unit weight (γsat): It is the ratio of the total weight of a saturated soil sample to
its total volume.
(γsat) = Wsat/V

22. VOLUME-WEIGHT RELATIONSHIPS
(v)Submerged Unit Weight (γ'):
It is the submerged weight of soil solids (Wd)sub per unit volume V of the soil mass.
(γsub or γ') = (Wd)sub /V
When the soil mass is submerged, the weight of soil solids is reduced due to buoyancy. The
submerged weight (Wd)sub is therefore equal to the weight of soil solids in air minus the weight of
water displaced by solids.
γ' =γsat – γw
γw = unit weight of water, 9.81 kN/m3
Note:
1g/cm3 or 1g/cc = 9.81 kN/m3
γ =9.81 x ρ

23. SPECIFIC GRAVITY OF SOLIDS:
• The specific gravity of soil particles (G) is defined as the ratio of the weight of the given volume of
soil solids at a given temperature to the weight of an equal volume of distilled water at that
temperature.
G = γs/γw
• The Indian Standard specifies 27oC as the standard temperature for reporting the specific gravity.
• The apparent or mass or bulk specific gravity (Gm) denotes the specific gravity of soil mass given by
Gm = γ/γw

24. VOLUMETRIC RELATIONSHIPS:
1.Void Ratio
• It is defined as the ratio of volume of voids to the volume of solids. It is denoted by ‘e’
e = Vv/Vs……….(a)
• It is expressed as a decimal.
2. Porosity
• It is defined as the ratio of volume of voids to the total volume. It is denoted by ‘n’
n = Vv/V……….(b)
• It is generally expressed as a percentage.

25. Relation between e and n:
From equation (b), 1/n =V/Vv
1/n = (Vv+Vs)/Vv = (Vv/Vv)+(Vs/Vv)
1/n = 1+ (1/e)
1/n = (1+e)/e……….(1)
n = e/ (1+e)
From equation (1),
1/e = (1/n)-1= (1-n)/n
1/e = (1-n)/n
e = n/ (1-n)

26. 3. Degree of saturation
• The degree of saturation is the ratio of the volume of water to the volume of voids. It is
denoted by ‘S’.
S = Vw/Vv
• The degree of saturation generally expressed as a percentage. It is equal to zero when the
soil is absolutely dry and it is equal to one when the soil is fully saturated.
4. Air content
• Air content is defined as the ratio of the volume of air to the volume of voids
ac= Va/Vv
Va
= Vv- Vw
ac = 1- Vw/Vv= 1-S

27. 5. Percentage air voids
• It is the ratio of volume of air to the total volume.
na= Va/V……….(c)
From equation (c), na= Va/V = (Va/Vv) * (Vv/V)
na= n ac

29. Basic Relations:

30. RELATIVE DENSITY OF SOIL:
• Relative density or density index is the ratio of the difference between the void ratios of a
cohesionless soil in its loosest state and existing natural state to the difference between its
void ratio in the loosest and densest states.
Relative Density (Dr) = (emax - e)/ (emax - emin)
Where, emax = void ratio of coarse grained soil (cohesionless) in its loosest state
emin = void ratio of coarse grained soil (cohesionless) in its densest state
e = void ratio of coarse grained soil (cohesionless) in its natural existing state in the
field
• Determination of relative density is helpful in evaluating compaction state of coarse
grained soils and also assessing the safe bearing capacity in case of sandy soils.

31. Functional Relationships (for competitive exams)

35. Problems