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.