NPTEL- Advanced Geotechnical Engineering
Module 4
Lecture 19
Pore water pressure and shear strength -3
Topics
1.2.3 Axial compression tests
1.2.4 Axial extension tests
1.2.5 Critical Void Ratio
1.2.3 Axial compression tests
1. Radial confining stress
constant and axial stress
increased. This is the test procedure described
above.
2. Axial stress
constant and radial confining stress decreased.
3. Mean principal stress constant and radial stress decreased.
For drained compression tests,
is equal to the major effective principal stress
is equal to the
minor effective principal stress
which is equal to the intermediate effective principal stress
. For the
test listed under item 3, the mean principal stress
, is kept constant. Or, in other
words,
is kept constant by increasing
and decreasing .
1.2.4 Axial extension tests
1. Radial stress kept constant and axial stress
decreased.
2. Axial stress
constant and radial stress increased.
3. Mean principal stress constant and radial stress increased.
For all drained extension tests at failure,
is equal to the minor effective principal stress
, and
is
equal to the major effective principal stress
which is equal to the intermediate effective principal stress
.
1.2.5 Critical Void Ratio
An increase or decrease of volume means change in the void ratio of soil. The nature of the change of the
void ratio with strain for loose and dense sands is shown in Figure 4.9.
Dept. of Civil Engg. Indian Institute of Technology, Kanpur
1
NPTEL- Advanced Geotechnical Engineering
Figure 4.9 Definition of critical void ratio
The concept of critical void ratio was first introduced in 1938 by A. Casagrande to study liquefaction of
granular soils. When a natural deposit of saturated sand that has a void ratio greater than the critical void
ratio is subjected to a sudden shearing stress (due to an earthquake or to blasting, for example), the sand will
undergo a decrease in volume. This will result in an increase of pore water pressure u. at a given depth, the
effective stress is given by the relation
. If
(i.e., the total stress) remains constant and u
increases, the result will be a decrease in . This, in turn, will reduce the shear strength of the soil. If we
shear strength is reduced to a value which is less than the applied shear stress, the soil will fall. This is called
soil liquefaction.
Dept. of Civil Engg. Indian Institute of Technology, Kanpur
2