Class 6 Shear Strength - Direct Shear Test ( Geotechnical Engineering )

Civil Engineering - Texas Tech University
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

 Shear strength in soils
 Introduction
 Definitions
 Direct shear test
 Introduction
 Procedure
 Calculation
 Results and Figures
Class Outlines

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

Slope Failure in Soils
Failure due to inadequate
strength at shear interface

Shear Failure in Soils

Bearing Capacity Failure

Transcosna Grain Elevator Canada
(Oct. 18, 1913)
West side of foundation sank 24-ft

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

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

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)

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

Shear Strength of Soils; Internal Friction
 Internal Friction angle (f), is the measure of the
shear strength of soils due to friction

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

Shear
Strength,S
Normal Stress, n =  = g h
C
f = f
Mohr-Coulomb Failure Criterion

General State of Stress
σ1
σ1 major principle stress
σ3 σ3
Minor principle stress
Confining stress

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

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

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

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

Direct Shear Test

Direct Shear Test Data
Shearstress
Residual Strength
Peak Strength

Direct Shear Test Data
Volume change
DH

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)

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.

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
  ;

Figures
Shearstress,s
Peak Stress
N1 = 10 kg
N2 = 20 kg
N3 = 30 kg
Horizontal displacement, DH
s3
s2
s1

Figures (cont)
ShearStress,s(psf)
C
f
(1,s1)
(3,s3)
(2,s2)
Normal Stress , psf

Figures (cont)
Verticaldisplacement
Horizontal displacement

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