# Behaviour of Uniform Drava River Sand in Drained Condition--A Critical State Approach
## 5 Discussion  
A series of drained triaxial tests under three different initial states were conducted on Drava River sand. The results consist of simple stress-strain relation, change in volume behaviour and plastic dilatancy relations.  
Basic stress-strain relation with volume behaviour was presented in Figures 5-7, for loose, medium dense and dense samples, respectively. The results for densely prepared sand samples show an expected behaviour. There is a significant difference in peak and residual deviatoric stress (\(q\)) as can be depicted form Figure 7b. The approximated isotropic consolidation curve fits well on consolidated data for both extension and compression test. As it is mentioned before, extension tests cannot reach the same critical state as compression tests, but the results show the tendency toward the dashed critical state curve, Figure 7c. From Figure 7d it can be concluded that all six samples have reached the critical state.  
Critical state values of DrOS018 sand form a relatively narrow range of specific volume variation, as presented in Figure 8. The average difference in specific volume is from 0.0395, for large confining stress, to 0.0365 for low confining stress. This is very similar to the data presented by [PERSON] and [PERSON] [28]. As mentioned before, the data of specific volume were plotted against normalised mean effective stress with atmospheric pressure, Figure 9. The type of plot presented in Figure 9 ensured the smallest error in the critical state range and was used to propose the best fit parameters for critical state of DrOS018 sand, Figure 10. Comparing the Figures 8 and 10 it is quite noticeable that the proposed critical state is very close to the isotropic consolidation curve for the samples with 50% initial relative density. This is one of the reasons why the results in that area show versatile results. In order to define a critical state parameter \(M\), and thus the critical friction angel \(\phi_{cs}^{\prime}\), plastic dilatancy was calculated, which are plotted for loose, medium dense and dense samples on Figures 11-13, respectively. Figures 12 and 13 indicate that the value of critical state parameter is somewhere between 1.3 and 1.4. This was confirmed by plotting the maximum values of stress ratio \(\eta_{max}\) against the minimum plastic dilatancy \(D_{min}^{p}\), Figure 14. The data were approximated with the line whose slope represents the critical state parameter \(M\), that equals 1.364. This indicates that the critical state friction angle for DrOS018 is 33.75\({}^{\circ}\). Figure 14 also shows the data compared with the test presented by [PERSON] and [PERSON] [60] and [PERSON] [65]. The data obtained from this research are in the range of [PERSON] and [PERSON]'s data for negative plastic dilatancy but with smaller slope. Further investigations will be made with the focus on the slope of plastic dilatancy to maximum stress ratio. Data for Cornforth [65] were modified according to [64]. Values of peak friction angle (\(\phi_{peak}\)) and state parameter (\(\psi_{0}\)) were plotted together and compared with the range of various tests, Figure 15. Although there is a small discrepancy in data, it is visibly a clear and valid trend of how the friction angle changes with the change of state parameter. The concerning part is that the tested data are in the upper bound of all documented data, according to [PERSON] [64]. Further investigations will be performed to refine those points. The values of state parameter compared to the volumetric strain at peak friction angle, as presented on Figure 16, are in good agreement with the previously tested sands [64].