2. 2
Definition of Soil
The term soil according to engineering point of view
is defined as the material, by means of which and upon
which engineers build their structures. The term soil
includes entire thickness of the earth’scrust (from
ground surface to bed rock), which is accessibleand
feasible for practical utilization as foundation support
or construction material. It is composed of loosely
bound mineral particles of various sizes and shapes
formed due to weathering of rocks.
3. 3
Definition of Soil Mechanics
Soil Mechanics is a discipline of Civil Engineering
involving thestudy properties of soil, behavior of soil
masses subjected to various types of forces,and its
application as an engineering material.
4. 4
Definition of Soil Mechanics
According to Terzaghi (1948):
Soil Mechanics is the application of laws of
mechanics and hydraulics to engineering problems
dealing with sediments and other unconsolidated
accumulations of solid particles, which are produced
by the mechanical and chemical disintegration of
rocks, regardless ofwhether or not they contain an
admixture of organic constituents.
5. Why do you need to learn about soils?
Almost all structures areeither constructed of
soil, supported on soil, or both.
5
6. 6
Soil is generally formed by disintegration and
decomposition (weathering) of rocks through
the action of physical (or mechanical) and
chemical agents which break them into smaller
and smaller particles.
All soils originate, directly or indirectly, from
different rock types.
Soils areformed from thephysical and
chemical weathering of rocks.
7. 7
Physical weathering
Involves reduction of size without any change in the
original composition of the parent rock. The main
agents responsiblefor this processareabrasion,
erosion, freezing, and thawing.
Physical or mechanical processes taking placeon the
earth's surface include the actions of water, frost,
temperaturechanges, wind and ice. They cause
disintegration and the products are mainly coarse soils.
9. 9
Chemical weathering causes both reduction in
size and chemical alteration of the original
parent rock. The main agents responsible for
chemical weathering arehydration, carbonation,
and oxidation.
Rain water that comes in contact with therock
surfacereacts to form hydrated oxides, carbonates
and sulphates.
Theresults of chemical weathering aregenerally fine
soils with altered mineral grains.
11. Soils as they arefound in different regions can be
classified into two broad categories:
(1) Residual soils
(2) Transported soils
11
12. Residual Soils
Residual soils are found at the same location where they have
been formed. Generally, thedepth of residual soils varies
from 5 to 20 m.
Chemical weathering rate is greater in warm, humid regions
than in cold,dry regions causinga faster breakdown of
rocks. Accumulation of residual soils takes placeas therate
of rock decomposition exceeds therateof erosion or
transportation of the weathered material. In humid regions,
the presence of surface vegetation reduces the possibility of
soil transportation.
12
13. Transported Soils
Weathered rock materials can bemoved from
their original siteto new locations by oneor
moreof thetransportation agencies to form
transported soils. Transported soils areclassified
based on the mode of transportation and
the final deposition environment.
13
16. Transported Soils
Transported soils are classified based on the mode of
transportation and the final deposition environment.
(a)Soils that arecarried and deposited by rivers are
called alluvial deposits.
(b)Soils that aredepositedbyflowing water orsurface
runoff while entering a lake are called lacustrine deposits.
Alternate layersareformed in different seasonsdepending
on flow rate.
16
17. Transported Soils
(c)If the deposits are made by rivers in sea water, they are
called marine deposits. Marine deposits contain both
particulate material brought from the shore as well as
organic remnants of marine life forms.
(d)Melting of a glacier causes thedeposition of all the
materials scoured by it leading to formation of glacial
deposits.
(e)Soil particles carried by wind and subsequently deposited
areknown as Aeolian deposits.
17
18. Gravity Soils
Gravity can transport materials only for a
short distance.
Gravity soils aretermed as talus thesesoils
aregenerally looseand porous.
18
19. Soil is not a coherent solid material like steel and
concrete, but is a particulate material. Soils, as they
exist in nature, consist of solid particles (mineral
grains, rock fragments) with water and air in the
voids between theparticles.
The water and air contents are readily changed by
changes in ambient conditions and location.
Phases System of Soils
19
20. As therelativeproportions of thethreephases vary
in any soil deposit, it is useful to consider a soil
model which will represent thesephases distinctly
and properly quantify theamount of each phase. A
schematicdiagram of thethree-phasesystem is
shown in terms of weight and volume symbols
respectively for soil solids, water, and air
.
Theweight of air can beneglected.
20
21. IUST 21
Ground surface Air
Water
Voids
Solids
The compositions of natural soils may include diverse components
which may beclassified into threelargegroups:
1.Solid phase ( minerals,
cementations and organic
materials)
2.Liquid phase(water
with dissolved salts)
3.Gaseous phase(air
or other somegas)
22. IUST 22
Ground surface Air
Water
Voids
Solids
The spaces between the solids ( solid particles) are called voids.
Water is often the predominant liquid and air is the predominant gas.
We will use the terms water and air instead of liquid and gases.
23. Soils can be partially saturated (with both air and
water present), or befully saturated (no air content)
or be perfectly dry (no water content).
In a saturated soil or a dry soil, the three-phase
system thus reduces to two phases only, as
shown.
IUST 23
24. IUST 24
Partially saturated soil
Solid Particles
V
oids (air
or water)
Idealization:
ThreePhases Diagram
Water
Air
Solid Particles
28. For the purpose of engineering analysis and
design, it is necessary to express relations between
theweights and thevolumes of thethreephases.
Thevarious relations can begrouped into:
Weight relations
V
olumerelations
Inter-relations
Three - Phases System
IUST 28
29. W
T
W
S
W
W
W
a
≈
0
Water
Air
Solid Particles
IUST 29
(1-1)
w𝑡 = w𝑠 + ww
where,
W𝑡 = 𝑡o𝑡𝑎𝑙 wei𝑔ℎ𝑡 of 𝑠oi𝑙 𝑠𝑠𝑎𝑚𝑝𝑙e
W𝑠 = wei𝑔ℎ𝑡 of 𝑠oi𝑙 𝑠o𝑙i𝑑𝑠
Ww= wei𝑔ℎ𝑡 of w𝑎𝑡e𝑟
W𝑎 = wei𝑔ℎ𝑡 of 𝑎i𝑟 ≈ 0
Thefollowing arethebasicweight relations:
water content or moisture content
specificgravity (Gs)
30. IUST 30
Water content
The ratio of the mass of water present to the mass
of solid particles is called the water content (w𝑐), or
sometimes the moisture content.
𝑐
w𝑠
w % =
ww
× 100% (1-2)
Thewater content of a soil is found by weighing a sample
of the soil and then placing it in an oven at 110 ∓ 50𝐶
until theweight of thesampleremains constant , that is, all
theabsorbed water is driven out.
31. SpecificGravity,
Themass of solid particles is usually expressed in
terms oftheir particle unit weight or specific
gravity (Gs) ofthe soil grain solids
The specific gravity of a solid substance is the ratio
of the weight of a given volume of material to the
weight of an equal volume of water (at 20°C).
IUST 31
𝑠
𝐺 =
w𝑠
=
𝛾𝑠 𝑉𝑠
=
𝛾𝑠
ww 𝛾w 𝑉
w 𝛾w
(1-3)
w
𝛾 = 𝑢𝑛i𝑡 wei𝑔ℎ𝑡 of w𝑎𝑡e𝑟 = 9.81
𝑘𝑁
𝑚3
32. Weight Relations
SpecificGravity,
The specific gravity of soil solids is often needed
for various calculations in soil mechanics.
For most inorganicsoils, thevalueof Gs lies
between 2.60 and 2.80.
Thepresenceof organicmaterial reduces the
value of Gs.
IUST 32
33. IUST 33
3. Degree of saturation (S)
4. Air content (a)
Thefollowing arethebasicvolumerelations:
1. V
oid ratio(e) V
olumeSymbols
2. Porosity (n)
V
a
V
S
V
T
V
W
V
V
Water
Air
Solid Particles
𝑉𝑡 = 𝑉𝑠 + 𝑉w+ 𝑉𝑎 (1-4)
𝑉𝑉 = 𝑉w + 𝑉𝑎
34. IUST 34
Void ratio (e)
V
oid ratio (e) is theratio of thevolumeof voids
(Vv) to the volume of soil solids (Vs), and is
expressed as a decimal.
The void ratio of real coarse grained soils vary
between 0.3 and 1. Clay soils can have void ratio
greater than one.
e =
𝑉𝑣
𝑉𝑠
(1-5)
35. IUST 35
Porosity (n)
Porosity (n) is the ratio of the volume of voids to the
total volumeof soil (Vt ), and is expressed as a
percentage.
The range of porosity is 0 %< n < 100%
𝑉𝑡
𝑛 100% =
𝑉𝑣
× 100% (1-6)
36. IUST 36
Void ratio (e) & Porosity (n)
Void ratio and porosity are inter-related to each
other as follows:
𝑛 =
𝑉𝑣
𝑉𝑆+ 𝑉𝑣
=
𝑉𝑣
𝑉𝑆 1 + 𝑉𝑣
𝑉𝑆
=
e
1 + e
e =
𝑉𝑣
=
𝑉𝑠 𝑉𝑡 − 𝑉𝑣 𝑉𝑡 1 − 𝑉𝑣
𝑉𝑡
= =
𝑉𝑣 𝑉𝑣 𝑛
1 − 𝑛
(1-7)
(1-8)
37. IUST 37
Degree of saturation (S)
Thevolumeof water (Vw) in a soil can vary
between zero (i.e. a dry soil). This can be
expressed as thedegreeof saturation (S) in
percentage.
Degreeof saturation is theratio of thevolume
of water to the volume of voids.
𝑆 100% =
𝑉w
× 100%
𝑉𝑣
(1-9)
38. 38
Degree of saturation (S)
The degree of saturation tell us what percentage
of the volume of voids contains water .
For fully saturated soil, VV = VW, S =1 or 100%
For a dry soil, S = 0 and
For partially saturated soil 1<S<0
𝑆 =
𝑉w ×
𝑉𝑆 1 ww
×
𝛾𝑠
=
w𝑐 × 𝐺𝑠
𝑉𝑣 𝑉𝑆
=
e
×
𝛾w w𝑠 e
(1-10)
39. IUST 39
Air content (a)
The air content, a, is the ratio of air volume to
total volume .
Theair- voids, V
a , is that part of thevoids
spacenot occupied by water
For a perfectly dry soil : a = n
For a saturated soil : a = 0
𝑎 100% =
𝑉𝑎
× 100%
𝑉𝑡
(1-11)
𝑎 100% = 𝑛 1 − 𝑆 (1-12)
40. Weight –volume relationship
Unit weight (𝛾)
Density is a measureof thequantity of mass in a
unit volume of material. Unit weight is a measure
of the weight of a unit volume of material.
Both can beused interchangeably. Theunits of
density areton/ m³, kg/ m³ or g/ cm³.
The unit of unit weight is kN/m³.
IUST 40
41. IUST 41
Unit weight (𝛾)
Theunit weight of a soil is theratio of theweight
of soil to the total volume.
𝛾 =
w𝑡
(1-13)
𝑉𝑡
In natural soils the magnitude of the total unit
weight will depend on how much water happens to
be in the voids as will as the unit weight of the
mineral grains themselves.
42. IUST 42
Dry unit weight (𝛾𝑑)
Thedry unit weight of a soil is theratio of the
weight of solids to the total volume.
(1-14)
𝛾𝑑 =
w𝑠
𝑉𝑡
𝑑
𝛾 = =
w𝑠 𝛾𝑠
=
𝛾w 𝐺𝑠
𝑉𝑠 1 + e 1 + e 1 + e
(1-15)
𝛾 =
w𝑡
𝑉𝑡
=
w𝑠 1 + ww
w𝑠
𝑉𝑡
= 𝛾𝑑 1 + w𝑐 (1-16)
The dry unit weight can also be determined as
43. IUST 43
Saturated unit weight (𝛾𝑠𝑎𝑡)
For a saturated soil, the unit weight becomes
(1-17)
(1-18)
𝛾𝑠𝑎
𝑡
=
w𝑡
𝑉𝑡
𝛾𝑠𝑎𝑡 =
w𝑠 1 +
ww
w𝑠
𝑉𝑠1 + e
=
𝛾𝑠 1 + e
𝐺𝑠
1 + e
=
𝛾w 𝐺𝑠 1 + e
𝐺𝑠
1 + e
𝛾𝑠𝑎
𝑡
=
𝛾w 𝐺𝑠 + e
1 + e
44. IUST 44
Submerged unit weight (𝛾𝑠𝑢𝑏 )
Thesubmerged unit weight of thesoil is
given as
(1-19)
𝛾𝑠𝑎𝑡 = 𝛾𝑠𝑢𝑏 + 𝛾w 𝛾𝑠𝑢𝑏 = 𝛾′ = 𝛾𝑠𝑎𝑡 − 𝛾w
G.W.T
Ground Surface
S = 0
S =( 0 to 1)
S = 1
𝛾𝑑
𝛾
𝛾𝑠𝑎𝑡
𝛾𝑠𝑢𝑏
46. Use
IUST 46
In summary, for the easy solution of phase
problem, you don’
t haveto memorizelots of
complicated formulas. Most of them can easily be
derived from the phase diagram. Just remember
the following simple rules:
1. Remember the basic definitions of properties
2. Draw a phasediagram
3. Assumeeither VS = 1 or VT = 1.
47. IUST 47
Example 1
An undisturbed sampleof saturated clay has been
found to havea moisturecontent of 24 %. The
specific gravity of the solid particles was
determined as 2.7. By deriving any
relationships needed using thebasicdefinitions
and a phase diagram for this soil, determine the
void ratio and the bulk unit weight.
48. IUST 48
Solution of example1
Vt =1+e
Volume
Solid
Water e
Vs =1
Weight
GS γw
e γw
(GS +e) γw
49. IUST 49
Solution of example1
γ = (2.7 + 0.648) 9.81/(1+0.648)
γ =19.93 kN/m3
𝑆 = 1 =
w𝑐 × 𝐺𝑠
e
e = 0.24 * 2.7 = 0.648
𝛾𝑠𝑎
𝑡
=
𝛾w 𝐺𝑠 + e
1 + e
50. Use
IUST 50
Example 2
Prove the following relationships:
𝛾𝑑 = 1 − 𝑛 𝛾w 𝐺𝑠
w𝑐(𝑠𝑎𝑡) =
𝑛 𝛾w
𝑠
𝐺 =
𝛾𝑠𝑎𝑡 − 𝑛 𝛾w
𝛾𝑠𝑎𝑡
𝛾w − w𝑐 𝛾𝑠𝑎𝑡 − 𝛾w
a)
b) 𝛾𝑠𝑎𝑡 = 𝐺𝑠 − 𝑛 𝐺𝑠 − 1 𝛾w
c)
d)
51. Use
IUST 51
Example 3
A soil has void ratio = 0.72, moisture content = 12%
and Gs= 2.72. Determine its
(a) Dry unit weight
(b) Moist unit weight, and the
(c)Amount of water to beadded per m3 to makethe
soil saturated.
52. IUST 52
Example 4
The dry unit weight of a sand with porosity
of 0.387 is 15.6 𝑘𝑁/𝑚3
Find the void ratio of the soil and the specific
gravity of the soil solids
53. IUST 53
Example 5
A cubic meter of soil in its natural state weighs
17.75 kN; after being dried it weighs 15.08
kN. The specific gravity of the solids is 2.70.
(a)Determinethewater content, void ratio,
porosity and degree of saturation for the soil as
it existed in its natural state.
(b)What would bethebulk unit weight and
water content if the soil were fully saturated at
the same void ratio as in its natural state ?
54. IUST 54
Example 6
For a given soil , the following aregiven :GS = 2.67;
wet unit weight ;γ = 16.8 kN/m³ moisture content
WC = 10.8 % . Determine :
1. Dry unit weight
2. V
oid ratio
3. Porosity
4. Degree of saturation
55. IUST 55
Example 7
For a soil ;given γd = 16.8 kN/m3 ;e= 0.51,
determine:
1. Specific gravity
2. Saturated unit weight
3. Unit weight when the degree of saturation is 45%.
4. Saturated water content
5. Porosity.
56. IUST 56
Example 11
For a saturated soil;given
γd = 15.3 kN/ m^3 ;and WC = 27 %;Determine:
1. Saturation unit weight
2. V
oid ratio
3. Specificgravity
4. Wet unit eight when thedegreeof saturation
is 50 %.
57. IUST 57
Example 12
A soil sample has a unit weight of 16.62 kN/m³
and a saturation of 50%. When its saturation
is increased to 75%, its unit weight raises to
17.72 kN/m³
Determine the voids ratio eand the specific
gravity Gs of this soil.
