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Thermohydromechanical behaviour of soils and soil structure interfaces 235

Figure 5.20 Effect of thermal cycles on the undrained shear strength of a normally consolidated
Tody clay. Redrawn after Burghignoli, A., Desideri, A., Miliziano, S., 2000. A laboratory study on the
thermomechanical behaviour of clayey soils. Can. Geotech. J. 37 (4), 764 780.

ends up being OC (at an ambient temperature T 0 ) because of the thermally induced over-
consolidation and is characterised by an increase in shear strength. Experimental results
that corroborate the previous phenomenon have been provided by Burghignoli et al.
(2000) and are reported in Fig. 5.20. If a sample under initial NC conditions is heated and
cooled under drained conditions, and then sheared, itsundrained shearstrengthishigher
than that of an equivalent sample tested at constant ambient temperature.

5.5.3 Flow rule

Upon distortional yielding, the flow rule defines the ratio between volumetric plastic
strain increment dε and deviatoric plastic strain increment dε . The associated flow
ν
p
p
q
rule postulates that, irrespective of the stress increment vector, on the yield limit the
corresponding plastic strain increment vector should be normal to the yield limit
(Schofield and Wroth, 1968). This normality assumption is essential to the validity of
the limit (or bound) theorems, which provide the framework for many plastic mecha-
nism analyses. Yet, it is often incorporated in critical state models (Schofield and
Wroth, 1968). An analysis of the yield limit and plastic strain increment can be exam-
0
ined together by aligning dε with p and dε with q. If normality is observed, then
p
p
ν
q
the yield surface also becomes the plastic potential surface (Roscoe and Burland,
1968). Most soils do not appear to obey to an associated flow rule (Wong and
Mitchell, 1975).
Considerations about the flow rule of fine-grained soils and the potential influence
of temperature on the deviation of the plastic strain increment vectors with respect to
the normal to the yield limit have been reported, for example by Cekerevac and