# Strain Rate Effect on Artificially Cemented Clay with Fully Developed and Developing Structure
## 3 Results
### Rate Effect on Developing Structured Clay  
Figure 5a shows the general stress-strain behaviour of young ACK specimens (curing for 2 days) under three different constant strain rates and a low confining stress. During the pre-peak shear stage, it is evident that slower strain rates result in higher peak strengths due to the longer curing time. Additionally, the axial strain at peak strength was slightly delayed with higher strain rates, specifically speak = 0.48%, 0.53%, 0.9% under 0.01%/h, 1%/h, and 5%/h, respectively. Notably, a lower strain rate resulted in a higher shear strength and more brittle behaviour, which has been found in OACK and was attributed to local drainage.  
Similar results were obtained under high confining stress, as shown in Figure 5c. Comparing these results to tests conducted on OACK, it appears that the additional curing  
Figure 4: Positive rate effect of OACK during the post-peak shearing stage.  
Figure 5: Test results on YACK at low and high confining stresses: (**a,c**) deviatoric stress versus axial strain; (**b,d**) normalised pore pressure response.  
time effect due to lower strain rates is not as pronounced under such a high confining stress. Again, the volumetric change in soil dominates the behaviour of the young ACK specimen, rather than any bond structure. Furthermore, the additional strength value obtained was approximately 37.3 kPa at a slower strain rate of 0.1%/h, which closely aligns with the value observed in OACK at 37 kPa.  
Figure 5b,d show the development of excess pore pressure during undrained shearing at different strain rates. In the case of OACK specimens whose structure was much stronger before peak, the pore pressure development appeared to be independent of the strain rate. Conversely, in young ACK specimens, a lower strain rate led to higher shear resistance and greater excess pore pressure. However, after reaching the peak, a relatively stable pore pressure was obtained. Furthermore, the axial strain at the peak excess pore pressure exhibits a slight delay compared to the axial strain at the peak strength.  
In the post-peak shear stage, a negative rate effect was observed, accompanied by a more pronounced and regular \"stick-slip\" pattern [33] when the shear plane is well formed. It is important to note that the mechanism behind the negative rate effects differs significantly from that of the pre-peak shear stage: the immediate stress jump was still positive due to the rate acceleration, but it was almost overshadowed by the subsequent negative effect resulting from a combination of curing and stick-slip behaviour. Throughout the undrained shear process, the young ACK specimens continued curing and strengthening. Therefore, a slower strain rate allowed for more curing time, which led to increased shear resistance. Simultaneously, as the shear plane had already been initiated, the roughness of the surface was higher than that of the shear plane formed in OACK. This increased roughness was a consequence of clay aggregate breakage, resulting in intensified \"stick-slip\" behaviour associated with higher shear resistance. The rate effects were also quantified using the same method mentioned above, and the average value was around \(-8\%\), as shown in Figure 6. It is worth mentioning that since the roughness of the shear plane was determined by the entire history of post-peak shear stages, this negative rate effect was more random than the positive rate effect.