CE 2113
Chapter Five
Soil Mechanics I
Shear Strength of Soil
Instructor: Dr. Hany El Naggar
The Direct shear Test
The soil is sheared along a predetermined plane by placing it in a box and then moving the top
half of the box relative to the bottom half.
T
CE 2113
Chapter Two
Soil Mechanics I
Seepage and Flow nets
Instructor: Dr. Hany El Naggar
LABORATORY DETERMINATION OF PERMEABILITY
Permeability of a coarse grained soil can be determined by the constant head permeability test
(ASTM D2434), and in a fine g
CE 2113
Chapter One
Soil Mechanics I
Basic Characteristics of Soils
Instructor: Dr. Hany El Naggar
=
=
+
=
+
=
+
=
=
+
+
=
=
+
+
Divide both numerator and denominator by
+
+1
=
=
=
=
( +
1+
)
(
+
1+
)
=1
=0
1+
or
=
Note that in discussing soils that are s
CE 2113
Chapter Two
Soil Mechanics I
Seepage and Flow nets
Instructor: Dr. Hany El Naggar
From the figure
=4
N = 2.5 and N = 8 respectively.
The seepage loss of the dam per meter length of dam is
=
=
4 10
= 4.42 10
2 10
5 2.5 8 = 4.42 10
/ 3600 24 = 3.8
CE 2113
Chapter One
Soil Mechanics I
Basic Characteristics of Soils
Instructor: Dr. Hany El Naggar
Phase relationships
Soil is a three phase material which consists of solid particles which make up the soil skeleton
and voids which may be full of water if
CE 2113
Chapter One
Soil Mechanics I
Instructor: Dr. Hany El Naggar
Basic Characteristics of Soils
Plasticity of Fine Soils
Plasticity is an important characteristic in the case of fine soils, the term plasticity describing the
ability of a soil to underg
CE 2113
Chapter Two
Soil Mechanics I
Seepage and Flow nets
Instructor: Dr. Hany El Naggar
Solution:
Lets take the datum at the drain level. This makes the total head at the top of the ground surface
= 4.0 m. and the head loss from the surface to the drain
CE 2113
Chapter Two
Soil Mechanics I
Seepage and Flow nets
Instructor: Dr. Hany El Naggar
SEEPAGE
Darcys law can be used only to calculate the one directional flow of water through soil. In
many instances, the flow of water through soil is not in one dire
6. Unit weights
W
V
2845 10 6 kN
157.1 10 6 m 3
Ws
V
2327.9 10 6
157.1 10 6
bulk
dry
. kN / m 3
181
14.8 kN / m 3
If the sample were saturated there would need to be an additional 14.9 106 m3 of water. This
would weigh 146.2 106 kN and thus the saturate
CE 2113
Chapter One
Soil Mechanics I
Instructor: Dr. Hany El Naggar
Basic Characteristics of Soils
Example 5:
A sample of soil is taken using a thin walled sampling tube from a soil deposit. After the soil is
extruded from the sampling tube a sample of di
CE 2113
Chapter Two
Soil Mechanics I
Seepage and Flow nets
Instructor: Dr. Hany El Naggar
Some important rules for drawing flow nets:
Draw scaled crosssection of problem, defining all boundaries due to the site, structure etc.
Drawing a flow net takes se
CE 2113
Chapter One
Soil Mechanics I
Basic Characteristics of Soils
Instructor: Dr. Hany El Naggar
Grain Size Distribution
In any soil mass the sizes of the grains varies to a great extent. To classify the soil we need to
know its grain size distribution.
CE 2113
Chapter Two
Soil Mechanics I
Seepage and Flow nets
Instructor: Dr. Hany El Naggar
Example 1:
A long horizontal drain is located at 4 m below the ground surface. The drain collects the
ground water in a lowlying area. The free water table coincide
CE 2113
Chapter One
Soil Mechanics I
Basic Characteristics of Soils
Instructor: Dr. Hany El Naggar
kN/m3
Unit Weight
To sufficient accuracy the density of water rw is given by
w
= 1000 kg/m3= 1 g/cm3
In most applications it is not the mass that is importa
CE 2113
Chapter One
Soil Mechanics I
Basic Characteristics of Soils
Instructor: Dr. Hany El Naggar
In general, Soil Mechanics is one of the youngest disciplines of Civil Engineering involving
the study of soil, its behavior and application as an engineeri
CE 2113
Chapter One
Soil Mechanics I
Basic Characteristics of Soils
Instructor: Dr. Hany El Naggar
Moisture content
The moisture content, w, is a very useful quantity because it is simple to measure. It is defined as
the ratio of the weight of water to th
CE 2113
Chapter One
Soil Mechanics I
Basic Characteristics of Soils
Instructor: Dr. Hany El Naggar
We can use Gs to calculate the density or unit weight of the solid particles
s
= Gs
w
s
= Gs
w
and hence the volume of the solid particles if the mass or we
CE 2113
Chapter Two
Soil Mechanics I
Seepage and Flow nets
Instructor: Dr. Hany El Naggar
Hydraulic gradient
Hydraulic gradient is the total head loss per unit length. When water flows from point A to point
B as shown in Figure 3, the total head at A has
CE 2113
Chapter Two
Soil Mechanics I
Seepage and Flow nets
Instructor: Dr. Hany El Naggar
Introduction
Permeability is a measure of how easily a fluid (Water) can flow through a porous medium
(soil). Generally, coarser soils have larger voids and conseque
CE 2113
Chapter Five
Soil Mechanics I
Shear Strength of Soil
Instructor: Dr. Hany El Naggar
Cohesive Strength
Some types of soil have shear strength regardless the effective stress, , is present or not
(e.g., clays and silts). This strength is called the
CE 2113
Chapter Two
Soil Mechanics I
Seepage and Flow nets
Instructor: Dr. Hany El Naggar
In a flow net, the equipotential lines are drawn such that the total head difference between two
adjacent ones is the same (=
drops in a flow net,
=
) throughout the
CE 2113
Chapter Two
Soil Mechanics I
Seepage and Flow nets
Instructor: Dr. Hany El Naggar
Darcys Law
Darcy (1856) found that the flow rate (laminar flow) in a porous medium (e.g. soil) is:
i. Proportional to the total head difference
ii. Proportional to t
CE 2113
Soil Mechanics I
Chapter Four
Instructor: Dr. Hany El Naggar
Consolidation
The solution of the governing differential equation is written in terms of
2
=
Where
(2
as:
sin
is the initial excess PWP and d is the length of the longest drainage path a
CE 2113
Chapter Five
Soil Mechanics I
Shear Strength of Soil
Instructor: Dr. Hany El Naggar
o
Frictional resistance between particles (stress dependent component)

This component is dependent of normal stress on shear plane

The larger the normal stress
CE 2113
Chapter One
Soil Mechanics I
Basic Characteristics of Soils
Instructor: Dr. Hany El Naggar
Laboratory Compaction tests
There are several types of test which can be used to study the compaction properties of soils.
Because of the importance of comp
CE 2113
Chapter One
Soil Mechanics I
Basic Characteristics of Soils
Instructor: Dr. Hany El Naggar
Plasticity index
The plasticity index (PI) is a measure of the plasticity of a soil. The plasticity index is the size of
the range of water contents where t
CE 2113
Chapter Two
Soil Mechanics I
Seepage and Flow nets
Instructor: Dr. Hany El Naggar
Solution:
The impermeable boundaries: along sheet piling BCD and along the impermeable stratum GG;
thus, BCD and GG are therefore streamlines.
Lets take the datum at
CE 2113
Chapter One
Soil Mechanics I
Basic Characteristics of Soils
Instructor: Dr. Hany El Naggar
hence a relationship between
=
dry
and w for a given value of A can be derived as follows
1+
( +
+
=
(1 + )
( +
=
Ws
Ww
Vw
Gs w
w
Now Vs
)(1
100)
)(1 + )
w
CE 2113
Soil Mechanics I
Chapter Three
Instructor: Dr. Hany El Naggar
Effective Stress
Effective Stress in the Ground
Ground movements and instabilities can be caused by changes in total stress (such as loading
due to foundations or unloading due to excav
CE 2113
Chapter Two
Soil Mechanics I
Seepage and Flow nets
Instructor: Dr. Hany El Naggar
Example 2:
A sheet pile is driven 7 m into a homogeneous sandy soil 12 m thick. The sand is underlain by an
impermeable stratum. The water level was initially 5.5 m
Numerical Analysis:
Plaxis 2D (P2D) was used to analyze the four arches of different shapes. The project properties were
defined as a Plane Strain model with 15Noded Triangular Elements. The 15Noded Triangular Element
option is chosen to get an accurate