Applied laboratory stress paths
Composite stress parameters such as deviator stress and mean effective stress are widely used in soil mechanics. Two sets of these parameters are in common use, one having been popularized by the Massachusetts Institute of Technology (MIT) and the other by Cambridge University. Unfortunately, the two groups use the same symbols, q and p, but for different parameters, and it is intended here to adopt the Cambridge usage for q, p and to substitute t, s for q, p respectively, as used by MIT. In terms of the principal stress parameters σ1, σ3, and their effective stress equivalents:
The Cambridge parameters q, p are the stress invariants
Under triaxial conditions, in compression, σ2 = σ3 and
Some difﬁculties arise in distinguishing between triaxial compression and triaxial extension where the directions of major and minor principal stresses interchange, and these can be overcome by the use of parameters σa, σr, for the axial and radial stresses respectively, thus:
The use of these parameters gives q positive for compression tests and negative for extension tests, thus allowing distinction between the two types of tests in plotting stress paths.