Download - Lecture 17
NPTEL- Advanced Geotechnical Engineering
Dept. of Civil Engg. Indian Institute of Technology, Kanpur 1
Module 4
PORE WATER PRESSURE AND SHEAR STRENGTH
(Lectures 17 to 26)
Topics
1.1 MOHR-COULOMB FAILURE CRITERIA
1.2 SHEARING STRENGTH OF GRANULAR SOILS
1.2.1 Direct Shear Test
1.2.2 Triaxial Test
1.2.3 Axial compression tests
1.2.4 Axial extension tests
1.2.5 Critical Void Ratio
1.2.6 Curvature of the Failure Envelope
Effect of angularity of soil particles
Effect of rate of loading during the test
1.2.7 Shear strength of Granular Soils under Plane Strain Condition
1.3 SHEAR STRENGTH OF COHESIVE SOILS
1.3.1 Triaxial Testing in Clays
Consolidated drained test
Consolidated undrained test
Unconsolidated undrained test
1.3.2 Unconfined Compression Test
1.3.3 Relation of Undrained Shear Strength and Effective Overburden
Pressure
1.3.4 Effect of Rate of Strain on the Undrained Shear Strength
1.3.5 Effect of Temperature on Shear Strength of Clay
1.3.6 Relationship between Water Content and Strength
1.3.7 Unique Effective Stress Failure Envelope
1.3.8 Unique Relationship between Water Content and Effective Stress
NPTEL- Advanced Geotechnical Engineering
Dept. of Civil Engg. Indian Institute of Technology, Kanpur 2
1.3.9 Vane Shear Test
1.3.10 Undrained Shear Strength of Anisotropic Clay
1.3.11 Applicability of Drained ) and Undrained ( ) Shear Strength
Parameters for Foundation Design
1.3.12 Hvorslev’s Parameters
1.3.13 Creep in Soils
NPTEL- Advanced Geotechnical Engineering
Dept. of Civil Engg. Indian Institute of Technology, Kanpur 3
Module 4
Lecture 17
Pore water pressure and shear strength -1
Topics
The shear strength of soils is an important aspect in many geotechnical engineering engineering problems
such as the bearing capacity of foundations, the stability of the slopes of dams and embankments, and lateral
earth pressure on retaining walls.
1.1 MOHR-COULOMB FAILURE CRITERIA
In 1910, Mohr presented a theory for rupture in materials. The failure along a plane in a material occurs by a
critical combination of normal and shear stresses, and not by normal or shear stress alone. The functional
relation between normal and shear stress on the failure plane can be given by
(1)
Where s is the shear stress at failure and is the normal stress on the failure plane. The failure envelope
defined by equation (1) is shown in Figure 4. 1.
In 1776, Coulomb defined the function as
(2)
Where c is cohesion and is the angle of friction of the soil.
Equation (2) is generally referred to as the Mohr-Coulomb failure criteria. If the normal and shear stresses
on a plane in a soil mass are such that they plot as point A, shear failure will not occur along that plane.
Figure 4. 1 Mohr-Coulomb failure criteria
NPTEL- Advanced Geotechnical Engineering
Dept. of Civil Engg. Indian Institute of Technology, Kanpur 4
Shear failure along a plane will occur if the stresses plot as point B, which falls on the failure envelope. A
state of stress plotting as point C cannot exits, since this falls above the failure envelope; shear failure would
have occurred before this condition was reached.
In saturated soils, the stress carried by the soil solids is the effective stress and so equation (2) must be
modified:
(3)
Where u is the pore water pressure and is the effective stress on the plane.
The term is also referred to as the drained friction angle. For sand, inorganic silts, and normally
consolidated clays, . The value of c is greater than zero for over consolidated clays.