Stress paths: typical results of triaxial tests

The explanation for the results presented can be found in the stress-strain-strength relationships of soils, such as sandy soils in the present case. Figure 5.8a shows the effective stress paths corresponding to the mobilization of active and passive limit states. These active and passive states can be obtained by performing, with specimens consolidated to the effective stress state at-rest, triaxial compression tests (with decrease in the horizontal stress, keeping the vertical stress constant), and triaxial extension tests (with increase in horizontal stress, keeping the vertical stress constant), respectively.

Figure 5.8b shows the typical curves from tests performed on those stress paths over loose or dense sand specimens. The results show the changes in the specimens’ horizontal strain against the deviator stress. It can be observed that: i) as expected, for the same type of test, the strength is greater and the strain at failure is lower in the dense sand; ii) for both sands, the strain at failure is much lower for the compression test than for the extension test; and iii) the contrast between these deformations is greater in the loose sand.

The explanation for the deformation contrast arises from two different issues. The first is due to the fact that the incremental stresses, corresponding to the transition from the at-rest

Active and passive Rankine states

Figure 5.8 Active and passive Rankine states: a) effective stress paths; b) typical shape of graphs relating maximum shear stress to horizontal strain on specimens of loose and dense sand.

stress state to the active state, are much less than those necessary to pass from the at-rest stress state to the passive state; compare, for this purpose, the magnitude of |-&L| and of h0 -cr'hP in Figure 5.5, or the length of the corresponding stress path in Figure 5.8a.

Moreover, the soil stiffness depends on the stress paths. Typically, the soils seem to show greater stiffness in the stress paths where the mean effective stress is reduced - as in the case of the active state - than in those where the mean effective stress is increased - as in the case of the change to the passive state.

The reasons for the behavior observed in the experiments is therefore understood: the mobilization of the active state needs much lower displacements since it involves much smaller incremental stresses and, for the type of loading involved, the soils have greater stiffness.

 
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