## 3 Test Results and Discussion
### Change of the Sample State during the Shearing Process and the Shear Steady State  
The tests showed that the water content has a significant impact on the shear deformation characteristics of the soils [25; 26; 27]. In the saturated state, the soil particles are dispersed and completely wrapped by water. During the shearing process, the particles are nearly sliding on the same plane, and there is almost no macroscopic volume change. As the water begins to drain, specifically when the water content falls below the plastic limit, the cohesive particles agglomerate, the pore-water shrinks and wraps in the agglomerate, or it forms a liquid meniscus at the contact point between the agglomerates. During the shearing process, the relatively large agglomerates or sand particles roll over each other, and the soil exhibits a relatively strong dilatancy macroscopically. The soil begins to exhibit properties of coarse-grained soils [28], such as when the water content is 36%. As the water content continues to decrease, this phenomenon tends to be more evident, such as when the water content is 16%. Thereafter that, as the water content continues to drop, the agglomerates begin to lose the adsorbed water and disperse. Subsequently, the agglomerates tend to return to the original grain size of kaolin (completely dried). In this case, the shear behaviour is similar to the fully saturated state; that is, the soil is sheared under its own particle size. However, because it is difficult to reach a completely dry state (water content of 0), the soil only exhibits characteristics that approximate the saturated state, such as when the water content is 4%. During the shearing process, the mass water content of the soil remains unchanged, but the compaction state (specific volume) continues to change. Figure 6 shows the changing processes of the sample states on the \(e-\ln p^{\prime}\) plane during the shearing process at three water contents of 4, 16, and 36%, in which \(e\) represents the void ratio of the sample. The control state points of the shearing process are clearly labelled in the figure: the maximum volume shrinkage state point, namely the phase transition state point; the peak state point, namely the maximum deviatoric stress point; and the strain localisation state point, which can be referred to as the shear steady state point. At the shear steady state point, an obvious shear plane is formed, and the shearing of the sample tends to be stable. In other words, the stress tends to be constant, and the deformation continues to steadily develop. By comparing with the critical state definition, changes in the sample states during shearing indicate that the shear development is approaching the critical state. However, due to the occurrence of strain localisation, the sample deformation does not meet the critical state in the end; instead, it reaches a shear steady state. Moreover, due to the non-uniformity of the sample deformation, it is difficult to reach the strict critical state in the test. The shear steady state is chosen herein to approximate the critical state. Figure 5: Tensile strength characteristic curve as a function of mass water content. Figure 7 shows the critical state test points and the fitted critical state lines on \(q\)-\(p^{\prime}\) and \(e-\ln p^{\prime}\) planes for the different mass water contents. The results show that the \(q\)-\(p^{\prime}\) and \(e-\ln p^{\prime}\) relationships are almost parallel for the different mass water content states. Therefore, the critical state lines of the unsaturated soils with different mass water contents on the \(q\)-\(p^{\prime}\) and \(e-\ln p^{\prime}\) planes have the same slopes of \(M\) and \(\lambda\) as the soil in the saturated state.