Dilation and Stability of Sand in Triaxial Tests

Abstract

Possible links between certain aspects of pre-failure instabilities of sand (instability line) and plastic dilation are studied. The starting point is experimental results obtained from triaxial investigations, which are approximated separately by analytical formulae for initially contractive and dilative sands. The irreversible strains are deduced from the condition that plastic work (dissipation) should be positive. Then, analytical formulae for plastic dilation are derived and presented in graphical form. In the case of initially contractive sand, a possible link between the instability line and maximum of the plastic dilatancy function is demonstrated. This condition is equivalent to the minimum dilation parameter or maximum of Rowe’s function. In the case of initially dilative sand, it is shown that the negative work done by a mean stress on expanding (dilating) soil should not be treated as dissipative because of thermodynamical requirements. Consequently,the plastic dilation function is zero for shearing stress exceeding the instability line.