Class 6 Shear Strength - Direct Shear Test ( Geotechnical Engineering )
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CE 3121: Geotechnical Engineering Laboratory
Class 6
Shear Strength
(Direct Shear Test)
Sources:
Soil Mechanics – Laboratory Manual, B.M. DAS (Chapter 15)
Soil Properties, Testing, Measurement, and Evaluation, C. Liu, J. Evett
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Shear strength in soils
Introduction
Definitions
Direct shear test
Introduction
Procedure
Calculation
Results and Figures
Class Outlines
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Shear Strength
The strength of a material is the greatest
stress it can sustain
The safety of any geotechnical structure is
dependent on the strength of the soil
If the soil fails, the structure founded on it can
collapse
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Slope Failure in Soils
Failure due to inadequate
strength at shear interface
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Transcosna Grain Elevator Canada
(Oct. 18, 1913)
West side of foundation sank 24-ft
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Significance of Shear Strength
Engineers must understand the nature of
shearing resistance in order to analyze soil
stability problems such as;
Bearing capacity
Slope stability
Lateral earth pressure on earth-retaining
structures
Pavement
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Shear Strength in Soils
The shear strength of a soil is its resistance to
shearing stresses.
It is a measure of the soil resistance to deformation
by continuous displacement of its individual soil
particles
Shear strength in soils depends primarily on
interactions between particles
Shear failure occurs when the stresses between the
particles are such that they slide or roll past each
other
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Shear Strength in Soils (cont.)
Soil derives its shear strength from two
sources:
Cohesion between particles (stress
independent component)
Cementation between sand grains
Electrostatic attraction between clay particles
Frictional resistance between particles (stress
dependent component)
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Shear Strength of Soils; Cohesion
Cohesion (C), is a measure of the forces that
cement particles of soils
Dry sand with no cementation
Dry sand with some cementation
Soft clay
Stiff clay
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Shear Strength of Soils; Internal Friction
Internal Friction angle (f), is the measure of the
shear strength of soils due to friction
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Mohr-Coulomb Failure Criteria
This theory states that a material fails
because of a critical combination of normal
stress and shear stress, and not from their
either maximum normal or shear stress
alone.
The relationship between normal stress and
shear is given as
f tancs
frictioninternalofangle
cohesionc
strengthshears
f
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Shear
Strength,S
Normal Stress, n = = g h
C
f = f
Mohr-Coulomb Failure Criterion
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General State of Stress
σ1
σ1 major principle stress
σ3 σ3
Minor principle stress
Confining stress
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State of Stresses in Soils
σ1
Shear
stress σ3
σ3
Normal stress σn
Consider the following situation:
- A normal stress is applied vertically
and held constant
- A shear stress is then applied until
failure
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Determination of Shear Strength
Parameters
The shear strength parameters of a soil are
determined in the lab primarily with two types of tests;
Direct Shear Test
Triaxial Shear Test
Soil
Normal stress σn
Shear stress σ3
3
1
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Direct Shear Test
Direct shear test is Quick and Inexpensive
Shortcoming is that it fails the soil on a
designated plane which may not be the
weakest one
Used to determine the shear strength of both
cohesive as well as non-cohesive soils
ASTM D 3080
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Direct Shear Test (cont.)
The test equipment consists of
a metal box in which the soil
specimen is placed
The box is split horizontally
into two halves
Vertical force (normal stress)
is applied through a metal
platen
Shear force is applied by
moving one half of the box
relative to the other to cause
failure in the soil specimen
Soil
Normal stress σn
Shear stress σ3
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Direct Shear Test Data
Shearstress
Residual Strength
Peak Strength
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Direct Shear Test Data
Volume change
DH
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Direct Shear Test (Procedure)
1.Measure inner side or diameter of shear box and find the area
2.Make sure top and bottom halves of shear box are in contact and
fixed together.
3.Weigh out 150 g of sand.
4.Place the soil in three layers in the mold using the funnel.
Compact the soil with 20 blows per layer.
5.Place cover on top of sand
6.Place shear box in machine.
7.Apply normal force. The weights to use for the three runs are
2 kg, 4 kg, and 6 kg if the load is applied through a lever arm, or 10
kg, 20 kg, and 30 kg, if the load is applied directly.
Note: Lever arm loading ratio 1:10 (2kg weight = 20 kg)
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Direct Shear Test (Procedure)
8. Start the motor with selected speed (0.1 in/min) so that the
rate of shearing is at a selected constant rate
9. Take the horizontal displacement gauge, vertical displacement
gage and shear load gage readings. Record the readings on
the data sheet.
10. Continue taking readings until the horizontal shear load peaks
and then falls, or the horizontal displacement reaches 15% of
the diameter.
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Calculations
1. Determine the dry unit
weight, gd
2. Calculate the void
ratio, e
3. Calculate the normal
stress & shear stress
1
d
wGs
e
g
g
A
V
A
N
;
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Figures
Shearstress,s
Peak Stress
N1 = 10 kg
N2 = 20 kg
N3 = 30 kg
Horizontal displacement, DH
s3
s2
s1
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Figures (cont)
ShearStress,s(psf)
C
f
(1,s1)
(3,s3)
(2,s2)
Normal Stress , psf