the potential of the simple triaxial test for investigating the non-linear mechanical behaviour of bitumen-bound and unbound granular pavement materials and soils is discussed in relation to the stress regimes encountered in practice. the discussion is based on the use of the stress invariants as measures of the magnitude and nature of the stress system. the triaxial test imposes severe limitations on the possible values of the invariants, which make it impossible to use the test to simulate many of the stress regimes encountered in pavements. in general, the limitations imply a unique relation between the invariants, independent choice of which is generally impossible. within a limited region, however, the relation is not unique, and some scope exists for their independent variation. a test programme taking advantage of these opportunities is proposed, which should assist in identifying the separate effects of the three invariants on the mechanical behaviour. because of the severe limitations on the nature of the possible stress regimes inherent in the simple triaxial test, the potential of other systems, in particular a torsion + triaxial system, is considered along the same lines. it is shown to be much more flexible, and the invariants can be chosen independently over a wide region. (a)

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