TRIAXIAL STRESS AND STRAIN - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 94

invoke the reduction of volume necessary to increase normal stresses and strength.
For undrained clay, parameter k in the Tresca model corresponds to undrained
strength s u or apparent cohesion c u .

B M OHR -C OULOMB MODEL
In the Mohr-Coulomb model, soil behaviour is elasto-plastic. Loading causes
elastic deformations, but once stresses reach a yield envelope, indefinite plastic
deformation occurs and the soil collapses. This collapse is assumed to occur only in
some preferential shear planes, where the local friction resistance is exceeded,
similarly as in the friction model of Coulomb.

A

2

T r

DC 1

90 o

1

90 o -

B

1

2

c

ii

s

1

s

c

s

r 2

B'

r

T s

s

45 o

/2

s

A'

s

1

2

Figure 7.3 Mohr-Coulomb model for a landslide, related stress orientation and
equilibrium

These failure shear planes can be shown with the stress Mohr circle as it touches
the failure envelope ABB'A', shown in Fig 7.3. Here, in contradiction to Tresca's
model, the yield envelope depends on the local isotropic stress level. In the contact
points T s or T r a critical stress state occurs in a plane parallel to s or r . The critical
stress state is formulated by the so-called Mohr-Coulomb failure criterion 33

| s | = c + ' s tan (7.2)

33

This expression was originally proposed by Terzaghi, see Fig 3.4.