## EFFECTS OF DRAINAGE CONDITIONS

The effect of the drainage conditions on the shear process was examined on A-type specimens through torsional shear tests. The experimental results of shear deformation and failure properties are shown and discussed in this section.

**Shear Properties**

Figures 26, 27 and 28 show the stress-strain, ε_{v} - ε_{s} and *s* - ε_{s} relationships in constant water content tests, respectively. Looking at the stress-strain relationships, there are small peaks and it seems that the ultimate (residual) state is reached at larger strain. The peak appears at a smaller ε_{s} when *p*_{net} is smaller. Volume change shows a contractive behaviour except for the case of *p*_{net} = 50kPa. Matric suction decreases significantly in the initial stage of shear, which is similar to that observed in triaxial compression tests.

Figures 29 and 30 shows the stress-strain and ε_{v} - ε_{s} relationships in consolidated drained (constant suction) tests, respectively. Small peaks also appear and the ultimate (residual) strength is approximately equal for the same *p*_{net}. The stress-strain and ε_{v} - ε_{s} relationships show that the peak in stress-strain relationship tends to appear at smaller ε_{s} and that the dilation occurs at smaller *p*_{net} and larger *s*. There is a variation in the volume change behaviour for the case of *p*_{net} = 50kPa and *s* = 100kPa because of local failure, which means that the appearance of a shear band occurs in the specimen.

**Final State**

The relationship between *p*_{net} and *q* corresponding to the maximum strength is shown in Fig. 31. The failure line at constant matric suction is parallel to the CSL of a saturated soil and plots higher for a larger matric suction. This means that any increment in strength due to matric suction is constant and independent of *p*_{net}. Figure 32 shows the *e* - *p*_{net} relationship at maximum strength. The failure line of unsaturated soil is below the CSL and is not affected by matric suction. Moreover, the *p*_{net} - *q* relationship at the ultimate (residual) state is shown as USL (ultimate state line) in Fig. 33. It can be seen that the effect of matric suction does not appear, unlike in the case of the maximum strength state. This line (USL) is a little below the failure line at *s* = 40kPa. This is a boundary, which distinguishes strain softening and hardening of the stress-strain curve.

Figure 34 shows the relation between strength increment *f*_{s}(s) due to matric suction and matric suction *s*. The value of *f*_{s}(s) is calculated from *q* at a certain *s* minus *q* of saturated soil, and this corresponds to the intercept of the failure line at a certain suction in the *q* and *p*_{net} relationship. Figure 34 shows that there is a non-linear relationship between *f*_{s}(s) and *s*. Furthermore, it is considered that there is some effect of the stress state on *f*_{s}(s) because there is a large difference between the *f*_{s}(s) of the triaxial compression test (α = 0^{o} and *b* = 0) and that of torsional shear test (α = 45^{o} and *b* = 0.5). However, more experimental data with controlled α and *b* are necessary to demonstrate and prove the effect of the stress state.

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