Topic 4

SHEAR STRENGTH

CHAPTER 4

UPDATED DEC2020

What is Shear
Strength of Soil…

The shear strength of a soil is the
maximum internal resistance to applied
shearing forces.

A soil mass will distort when shear forces
are applied to it.

A measure of this distortion is the shear
strain.

Shear stress, is the shear force divided
by the area of the plane that it acts on

UPDATED DEC2020

Shear Strength

Factors Influencing Shear Strength:
a) Soil Composition: mineralogy, grain size and grain size distribution, shape of particles, pore fluid type and
content, ions on grain and in pore fluid.
b) Initial State: ‘State’ can be described by terms such as: loose, dense, over consolidated, normally consolidated,
stiff, soft, etc.
c) Structure: Refers to the arrangement of particles within the soil mass; the way the particles are packed or
distributed. Features such as layers, voids, pockets, cementation, etc., are part of the structure
d) Loading Conditions - drained or undrained, type of loading (magnitude)

UPDATED DEC2020

Shear Strength Parameter

COHESION (KEJELEKITAN), C ANGLE OF INTERNAL FRICTION (SUDUT
RINTANGAN RICIH), 
The sticky characteristics in soil
Its definition is derived from the Mohr-Coulomb failure
Defines as “the shear strength of soil when criterion and is used to describe the friction shear
compressive stresses equals zero” resistance of soils together with the normal effective
stress.
Cohesion is soil comes from cementation,
electrostatic and electromagnetic attraction In the stress plane of Shear stress-effective normal
between soil particles. stress, the soil friction angle is the angle of inclination
with respect to the horizontal axis of the Mohr-
Coulomb shear resistance line.

UPDATED DEC2020

Types of Soil based on Shear
Parameters

GRANULAR SOIL ( C=0) COHESIVE SOIL ( = 0)

means gravel, sand, or silt (coarse-grained soil) means clay (fine-grained soil), or soil with a high
with little or no clay content. clay content, which has cohesive strength.

Granular soil has no cohesive strength. Cohesive soil does not crumble, can be
excavated with vertical side slopes, and is plastic
Some moist granular soils exhibit when moist.
apparent cohesion.
Cohesive soil is hard to break up when dry and
Granular soil cannot be moulded when moist exhibits significant cohesion when submerged.
and crumbles easily when dry.
Cohesive soils include clayey silt, sandy clay, silty
Granular Soil includes sand, gravel clay, clay and organic clay.

UPDATED DEC2020

Determination of Shear
Strength Parameters

Coulomb’s Method

The symbol c represents shear
cohesion as a constant, which is
equal to the shear strength
when the compressive stress is
equal to zero.

UPDATED DEC2020

Determination Mohr’s - Coulomb’s Failure Criteria
of Shear
Strength The symbol c represents shear cohesion as a constant,
Parameters which is equal to the shear strength when the compressive
stress is equal to zero.

UPDATED DEC2020

Determination of Shear Parameter
Method

TOTAL STRESS METHOD EFFECTIVE TOTAL STRESS METHOD

UPDATED DEC2020

Example Normal Shear Stress
Problem #1 Stress At Failure
(kN/m2) (kN/m2)
The following results are
obtained from a Shear 50 36
Box Test under drained 100 80
condition of 0.0036m2 200 154
sample of sandy clay. 300 235
determine the shear
strength parameter

UPDATED DEC2020

Example Normal Shear Stress
Problem #1: Stress At Failure
Solution (kN/m2) (kN/m2)

1. No Need to calculate Shear 50 36
Stress & Normal Stress as it is 100 80
already given in the question. 200 154
2. Plot Shear Stress vs Normal 300 235
Stress
3. Determine C and 

UPDATED DEC2020

Example Problem #1: Solution

Normal Stress Shear Stress At Shear Envelope
(kN/m2) Failure (kN/m2)
50 250
36
100 200
80
200 Shear Stress 150
154
300 100
235

From The Shear Envelope: 50
C = 20kN/m2
 = 15o 0 100 200 300
50 Normal Stress

UPDATED DEC2020

Example Problem #2

Direct shear tests was conducted on dry sand. The Vertical Load Horizontal Load
size of the sample used for the tests was 0.06m x (kN) (kN)
0.06m. The test results obtained are given below.
Calculate the Shear Strength Parameters. 0.361 0.32
0.722 0.52
1.083 0.7
1.444 0.88

UPDATED DEC2020

Example Problem #2: Solution Plot the
graph and
Area of Sample = 0.06 x 0.06 = 0.0036m2
get the
answer

Vertical Load (kN) Shear Stress = Pv/A Horizontal Load (kN) Normal Stress = Ph/A
(kN/m2) (kN/m2)

0.361 0.361/0.0036 = 100.3 0.32 0.32/0.0036 = 100

0.722 0.722/0.0036 = 200.6 0.52 0.52/0.0036 = 144.4

1.083 1.083/0.0036 = 300.8 0.7 0.70/0.0036 = 194.4

1.444 1.444/0.0036 = 401.1 0.88 0.88/0.0036 = 244.4

From The Shear Envelope:
C = ______ kN/m2
 = ______o

UPDATED DEC2020

Example Problem #3

Triaxial compression tests on three specimens of a soil sample were performed. Each test was carried out until
the specimen experienced shear failure. The test data are tabulated as follows. Determine the soil’s cohesion
and angle of internal friction.

Specimen Minor principal stress (kN/m2) Deviator stress at failure
number (kN/m2)
115.5
1 143.5 210
2 170.1 315
3 420

UPDATED DEC2020

Example Problem #3: Solution

Determine the major stress from the table given. Use

Specimen 3 (kN/m2) Pv/A (kN/m2) 1 (kN/m2)
Number 210 115.5 + 210 = 325.5

1 115.5

2 143.5 315 143.5 + 315 = 458.5

3 170.1 420 170.1 + 420 = 590.1

Draw the Mohr’s Circle and determine the shear strength parameter. From Mohr’s Circle:
C = ______ kN/m2
 = ______o

UPDATED DEC2020

Example Problem #4

Triaxial Compression Test on three specimens of a soil sample were performed. Determine the soil’s cohesion and angle of internal friction:
A) Total Stress
B) Effective Total Stress

Specimen Shear Stress at Failure Deviator stress at Pore Water
number (kN/m2) failure (kN/m2) Pressure (kN/m2)

1 300 170 160
2 600 350 330
3 1200 380 660

UPDATED DEC2020

Example Problem #4: Solution

a) Total Stress

Specimen number 3 (kN/m2) Pv/A(kN/m2) µ(kN/m2) 1 (kN/m2)

1 300 170 160 470
2 600 350 330 950
3 1200 380 660 1580

Draw the Mohr’s Circle 3 & 1

From Mohr’s Circle:
C = ______ kN/m2
 = ______o

UPDATED DEC2020

Example Problem #4: Solution

b) Effective Total Stress

Specimen 3 (kN/m2) Pv/A(kN/m2) µ(kN/m2) 1 (kN/m2) ’1 (kN/m2) ’3 (kN/m2)
number
470 470 – 160 = 300 – 160 =
1 300 170 160 950 310 140
1580
2 600 350 330 950 – 330 = 600 – 330 =
620 270
3 1200 380 660
1580 – 660 = 1200 – 660 =
920 540

Draw the Mohr’s Circle ’3 & ’1 From Mohr’s Circle:
C’ = ______ kN/m2
’ = ______o

UPDATED DEC2020