Stress invariants and mechanical testing of pavement materials.

The application of results obtained using the triaxial test on the mechanical behaviour of materials to prevent pavement conditions is discussed. Assuming isotropy, the behaviour of stress-dependent materials under different stress systems can be correlated phenomenologically in terms of the three stress invariants. A set of modified invariants is derived, which is more convenient in certain respects. In the triaxial test, two principal stresses are necessarily equal and the invariants are then proportional to the stress difference. The effects of these two invariants on the mechanical behaviour are thus inextricably confused in the triaxial test. Study of stress distribution in idealised typical pavements in the light of this analysis shows that this limitation on the triaxial test impedes its sole use to study material behaviour in many stress regimes of practical importance. Some tests which might be used to supplement the triaxial test are discussed. Test results obtained at nottingham university on the elastic behaviour of an unbound base material are discussed briefly. The tentative conclusions are drawn that the material was somewhat anisotropic at low values of mean stress and that the bulk modulus was an even function (or independent) of the third invariant. (Author/publisher)