The uniqueness of the proposed elastic boundary was examined by changing the material in this study. Kaolin clay was chosen as a new material to achieve this objective.

Figure 10(a) shows the stress-strain relationships where αs = 45o, qh=60kPa, p'=100kPa and αh has different values. Stress conditions of bh=bs=0.5 were chosen in these tests. Although the yield point is not as clear as that of Yoneyama sandy silt, when the difference between αh and αs is larger, the deviator stress q, which can maintain elasticity during the drained shear process, decreases. Figure 10(b) shows the volumetric strain-shear strain relationships in the same case given in Fig. 10(a). When the difference between αh and αs is larger, a more contractive response is observed.

Figure 11(a) shows the stress-strain relationships of bs=0.5 where αhs=45o, qh60kPa, p'=100kPa and bh has different values. When bh is coincident with bs, the linear part before yielding is maintained at the larger deviator stress q. The relationship of volumetric strain-shear strain in the same case given in Fig. 11(a) is shown in Fig. 11(b). When the difference of b between the shear history and the shear process is larger, more contractive volumetric strain occurs.

The relationships between αε and εs, taking into account the effect of αh, are shown in Fig. 12. Although convergence is not good because of measurement error, the non-coaxiality between stress and strain does not occur when the directions of the shear history and shear are coincident or are at right angles to each other. However, the direction of shear strain tends to converge with the direction of shear stress as shear strain increases.

The yield points are determined by the same method as in Toyota et al. (2001). The linear part of the stress-strain curve is moved in parallel with a offset of εs=0.27% as the qemax of the tests, which are subjected to qh=60kPa, becomes approximately 60kPa using this method(Fig. 13).

Figure 14 shows the relationships between ry, α' and b' in three dimensional space. The elastic boundary was calculated in equation 1. The contour lines of ry on the α' - b' plane are shown in Fig. 15. Although these are limited experimental cases, the results obtained from Kaolin clay correlate well with the proposed elastic boundary obtained from Yoneyama sandy silt. It means that the proposed elastic boundary is identical irrespective of the type of cohesive soil. Therefore, universality of the elastic boundary with regard to type of cohesive soil can be deduced.