EFFECTS OF STRESS PATH
The effect of stress history due to unsaturation was examined on A, B and C-type specimens through the triaxial tests. The experimental results of consolidation, shear deformation and failure properties are shown and discussed in this section.
Consolidation Properties Before Shear
Figure 11 illustrates the isotropic consolidation curves for A-type and B-type specimens. It is clear that the void ratios of A-type and B-type specimens are almost coincident during the initial stage (pnet = 50kPa) and smaller than that of the saturated specimen because of the additional compression due to matric suction. The reason for this is that the stress history of the A-type specimen is almost the same as that of the B-type specimen because pc' in Fig. 7 equals 50kPa. The void ratio of the B-type specimen is larger than that of the A-type specimen at pnet = 100 and 200kPa. The reason for this is that the specimen volume change by isotropic consolidation is small because a contact force of the soil particles is generated by applying matric suction. Though the specimen is compressed by an increase in matric suction, compressibility of the specimen against pnet becomes small. This means that even if the final stress state is the same, the void ratio varies due to the difference in stress history caused by unsaturation. The slopes of the consolidation curves for A-type and saturated specimens are almost the same because the volume change by the matric suction is nearly equal within the range of net mean stresses pnet adopted in the experiments.
Figure 12 illustrates the isotropic consolidation curves for the C-type specimen. The void ratio and the slope of the consolidation curve of this specimen are larger than those of the saturated specimen. This implies that the unsaturated specimen is able to exist in a looser state compared with the saturated specimen.
Figures 13, 14 and 15 show the A-type specimen's stress-strain, εv - εs and s - εs relationships, respectively. It can be seen that for higher pnet and s, the shear strength is larger. If pnet = 50kPa and s = 200kPa, strain softening and dilation appear clearly, and the strength approaches the steady state in every case. Matric suction decreases significantly in the initial stage of shear, and its tendency is similar to dilatancy characteristics. Matric suction for the specimen with pnet = 200kPa and s = 100kPa shows a negative value because the degree of saturation is increased to about 98% during shear.
Figures 16, 17 and 18 show the B-type specimen's stress-strain, εv - εs and s - εs relationships, respectively. Generally the tendencies of shear behaviour are similar to those of the A-type specimen. However, the differences in shear behaviour between the A-type and B-type specimens are obvious especially for the case of pnet = 200kPa and s = 200kPa, wherein the strength is smaller and the decrease of matric suction during shear is larger for the B-type specimen. It is considered that these behaviours are affected by the difference of the initial void ratio (Fig. 11).
Figures 19, 20 and 21 show the C-type specimen's stress-strain, εv - εs and s - εs relationships, respectively. The deviator stress increases and the volume decreases continuously even at 18% shear strain. At a large shear strain level, the slope of the stress-strain curve and the negative dilatancy in the case of wi = 28% are larger than those for the case of wi = 38%. The reason why the behaviour is more contractive in the case of wi = 28% is because the void ratio before triaxial compression is larger. Matric suction is kept stable in spite of the negative dilation after matric suction decreases significantly in the initial stage of shear.
The failure points (pnet - q plane) for the A and B-type specimens are plotted in Fig. 22. When a peak does not appear in the stress-strain curve, the state corresponding to εs = 18% is used. The figures beside the plotted point are the values of matric suction at failure. It is seen that when matric suction is large, the failure point is located above the CSL of the saturated soil. The strength and matric suction at failure are different for different stress paths due to unsaturation. Failure lines for constant matric suction are also shown in Fig. 22. This figure shows that failure lines for constant matric suction which are independent of the stress path exist. Figure 23 shows the relationship of e - pnet at failure. All points are plotted near the CSL, and there is a tendency for the points to be plotted a little below the CSL.
Figure 24 shows the failure point for the C-type specimen in pnet - q plane. Though all points are plotted near the CSL, it is considered that each state should finally reach the failure line of constant matric suction because q continues to increase with εs (see Fig. 19), which is indicated by the arrows in Fig. 24. The relationship of e - pnet at failure for the C-type specimen is shown in Fig. 25. Every point, except for the case of wi = 38% and pnet = 200kPa, is fairly above the CSL because volume change does not converge.