# Strain Rate Effect on Artificially Cemented Clay with Fully Developed and Developing Structure
## 3 Results
### Rate Effect on Developed Structured Clay
Subsequently, the test was transitioned from strain control to stress control by maintaining the deviator stress constant. This undrained creep stage lasted for nearly 40 h, which was the duration required for the specimen subjected to the slower straining  
Figure 1: Test results on OACK at low and high confining stress: (**a**,**c**) deviatoric stress versus axial strain; (**b**,**d**) normalised pore pressure response. Figure 2: Illustration of local drainage and its ‘negative rate effect’ during undrained shear on OACK: (**a**) stress space; (**b**) water content and effective confining stress space. of 0.1%/h to reach its peak state. Following the creep stage, a strain rate of 5%/h was resumed, and the specimen was sheared until failure. The test performed at 5%/h with the inclusion of the creep interval reaches a higher peak strength than the test with the same strain rate without the creep stage. It is interesting to note that the peak strength achieved in the test with creep coincided with that attained in the test conducted at the slower rate of 0.1%/h. Considering that the specimens were cured for periods exceeding 28 days, it is unlikely that the 10% difference in peak strength is due to further curing. Instead, it is suggested that the duration of the test contributes as much to this apparent ageing effect, particularly when the strain rate is sufficiently low to allow for more local drainage within and outside the potential shear plane. After deceleration to the lower strain rate of 0.1%/h, the stress-strain curve is seen to rejoin the curve corresponding to that specific rate. During the post-peak stage, the response exhibits a pronounced and persistent isotach viscous pattern, regardless of the applied confining stress (both low and high). The positive rate effect on the strength of structured clay observed in this study is in good agreement with the results of previous studies. The shear plane developed and was fully completed at approximately 1% axial strain after reaching the peak. Consequently, the dominant mechanics shift entirely towards the sliding of two relatively rigid blocks. The behaviour was more like shearing in a direct shear box, but with an inclined pre-existing shear plane. However, the pore pressure remained insensitive to the strain rate, as shown in Figure 1b,d. Therefore, the persistent stress jump observed could not be caused by a change in the excess pore water pressure. The stress jumps between the stress and strain curves with a logarithmic increase in the axial strain rate were quantified using parameter \(\gamma\), where the jump in deviatoric stress, \(\Delta q\), is normalised by the value at failure, \(q_{f}\) (accounting for the initial state and confining stress level). The average rate effects show a consistent value of approximately 3%, as illustrated in Figure 4.  
\[\gamma=\frac{\Delta q/q_{f}}{log\left(\dot{e}_{after}/\dot{e}_{previous}\right)}, \tag{1}\]  
Figure 3: Additional Creep test of OACK at pre-peak shearing stage. \({}^{*}\) means test with creep stage.