EFFECT OF NET MEAN STRESS ON SHEAR BEHAVIOUR
Figure 7 (a) shows the stress-strain relationships on constant suction during shearing where α=45o, b =0.5, s =100kPa and different values of pnet. As the air entry value of the specimen, which was consolidated under p' larger than 200kPa, was larger than 100kPa, this series of tests was conducted under p' 200kPa or smaller than 200kPa during consolidation. It means that the specimens whose degrees of saturation are less than 100% are treated. When the net mean stress, pnet, is larger, the value of q at the failure is also larger. However, the apparent difference on the initial stiffness is not recognized in contrast with the effect of matric suction (Figs. 7, 8, 9). The strain softening is occurring, and there are the peak (failure) points at larger shear strain, εs, when the net mean stress is larger.
Figure 7 (b) shows the volumetric strain-shear strain relationships in the same case shown in Fig. 7 (a). The unsaturated specimens start dilation around the failure points. When the net stress is larger, the volumetric strain, εv , increases, that is, a more contractive response appears. It means that the soil specimen is more brittle under lower pnet, because the degree of saturation, Sr, is low.
A similar tendency concerning the strength and deformation characteristics is illustrated in Fig. 8 (s =200kPa) and Fig. 9 (s =400kPa) with the results of different matric suction. It is found that when the matric suction is large, the strain softening appears more clearly. It indicates that the soil specimen is more brittle under lager matric suction, because the degree of saturation, Sr , is low.