Geotechnical Engineering
Geotechnical Engineering is a broader term for Soil Mechanics.
Geotechnical Engineering contains:
Soil Mechanics (Soil Properties and Behavior)
Soil Dynamics (Dynamic Properties of Soils, Earthquake
Engineering, Machine Foundation)
Foundation Engineering (Deep & Shallow Foundation)
Pavement Engineering (Flexible & Rigid Pavement)
Rock Mechanics (Rock Stability and Tunneling)
Geosynthetics (Soil Improvement)
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Soil Formation
Soil material is the product of rock
The geological process that produce soil is
WEATHERING (Chemical and Physical).
Variation in particle size and shape depends on:
Weathering Process
Transportation Process
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Type of soils produced by the different weathering &
transportation process:
Boulders
Gravel
Sand
Silt
Clay
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All soils consist of a collection of:
gravel,
sand,
silt or
clay particles
with varying spaces between them which
are usually filled with water.
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Soils are usually:
cohesionless,
cohesive, or
organic.
Cohesionless soils have particles that do
not tend to stick together.
Mostly composed of sand, may be some silt.
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Cohesive soils are characterized by very
small particle sizes where surface
chemical effects predominate. They are
both "sticky" and "plastic". CLAYS.
Organic soils are typically spongy,
crumbly, and compressible. They are
undesirable for supporting structures.
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Important Physical Parameters of Soils
Soils contain three components, which
may be characterized as:
solid,
liquid, and
gas.
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The solid components of soils are weathered
rock and (sometimes) organic matter.
The liquid component of soils is almost
always water (often with dissolved matter),
and
the gas component is air.
The volume of water and gas is referred to as the void.
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SOIL PHYSICAL & INDEX PROPERTIES
1- Soil Composition
Solids
Water
Air
2- Soil Phases
Dry
Fully Saturated
Partially Saturated
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3- Analytical Representation of Soil:
For the purpose of defining the physical
and index properties of soil it is more
convenient to represent the soil
skeleton by a block diagram or phase
diagram.
4- Weight - Volume Relationships:
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The following are important relationships between
these quantities.
The notation will follow:
•Total volume of soil V= Vv + Vs = Va + Vw + Vs
•Volume of air, Va,
•Volume of water, Vw
•Volume of solid, Vs and
•Volume of void, Vv= Va + Vw
The same notation is used for W (weight) and M
(mass).
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A graphical presentation of soil properties
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Mass
Total mass of soil, M = Mw + Ms
Mass of solid, Ms
Mass of water, Mw
Weight
Total weight of soil, W = Ww + Ws
Weight of solid, Ws
Weight of water, Ww
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1. Density of solids, 
 = Ms/Vs or  = Gs . 
s
s
s
w
2. Specific Gravity, Gs
Ratio of the density of solids to the density
of water.
Gs= s/w
Gs= Ms/Vs

w
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Specific Gravity, Gs
For Sand, Gs=
2.6 – 2.7
Clay, Gs = 2.65 – 2.80
Organic soils, Gs= 2.5
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The degree of saturation can range between
the limits of zero for a completely dry soil
and 1 (or 100%) for a fully saturated soil.
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Relative Density, Dr
emax: max. void ratio for the loosest possible
state
emin: min. void ratio for the densest possible
state
efield: void ratio of the soil for which relative
density is defined.
Relative Density, Dr is a parameter for granular
soils.
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Relative Density
(%)
0-15
15-35
35-65
65-85
85-100
Packing
Very loose
Loose
Medium
Dense
Very dense
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8. Unit Weight
Also known as Bulk Unit Weight
9. Dry Unit Weight, d= Weight of solid/Total volume
10. Air content, A= Volume of air, Va/ Volume of soil, V
Va = Vv-Vw
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Name
Equation
Description
Void Ratio
Ratio of the void volume to
the solid volume.
Porosity
Percent of the total volume
that is taken up by the void.
Degree of Saturation
Percent of the void that is
taken up by the water.
Water Content
Percent of the weight of
water to the weight of solids.
Unit Weight
Total density of the soil.
Includes solids and the void.
Dry Unit Weight
Density of the soil when it is
completely dried out.
Unit Mass, Dry Unit Mass
Density of the soil and the
dry density of the soil.
Ratio of the density of solids
to the density of water.
Note:
Specific Gravity of Solids
ρw
Other Important
Relationships:
W=Ws+Ww
V=V +V =V +V +V
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