# Strain Rate Effect on Artificially Cemented Clay with Fully Developed and Developing Structure
## 3 Results  
### Rate Effect on Developed Structured Clay  
Figure 1a shows the general stress-strain behaviour of old artificially cemented kaolin (OACK) specimens prepared with the same developed bond structures, and tested under the same low confining pressure of 100 kPa, applying different constant strain rates (0.01, 0.1, and 1%/h) during the pre-peak stage. The stress-strain behaviour exhibits a highly brittle nature, with coinciding curves up to the peak, while the excess pore pressure shows minor differences. The rate sensitivity of the peak strength and the strain at the peak strength are insignificant. This trend is similar to the previous results for cemented kaolin conducted by [PERSON] et al. (2016), which suggests that 4% cement kaolin shows an insignificant effect of fast shearing on the peak and residual strength. However, this outcome deviates from the rate effect on natural cemented soils ([PERSON] et al., 2016), where an obvious 'positive' rate effect on the peak strength was found. This discrepancy may arise from differences in bond composition: artificial bonds in ACK are significantly stronger than those in the base clay, whereas natural clay bonds exhibit similar strength to the base material. Consequently, the behaviour of Leda clay was predominantly influenced by the deformation of both the bonds and the base clay, whereas in ACK, the pre-peak stage behaviour was primarily governed by the bonds alone, whose strength and deformation were insensitive to the strain rate. In order to investigate the rate effect on clay with ruptured bond structures, two specimens were tested under a higher confining stress of 600 kPa. In contrast to the response observed in pure kaolin (PK) and OACK under low confining stress, a \"negative\" rate effect on the peak strength was observed (Figure 1c), while the pore water pressure response shows minimal sensitivity to the strain rate (Figure 1d). This may be attributed to local drainage from the shear plane to the outside area, and the contractive behaviour results in a drier (denser) state on the shear plane. This localised drainage-induced increase in shear resistance outweighs the positive viscous effect, resulting in a \"negative\" rate effect. This phenomenon is further elucidated through the stress path and v-lnp' spaces shown in Figure 2. Local drainage in non-structured PK results in a new stress path from B to F rather than the traditional path from B to E, while local drainage in the structured ACK results in a stress path from G to H to F rather than from G to I to F. The mechanics based on strain \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Test** & **Curing \&** & & & **Pre-Peak** & **Post-Peak** \\
**Name** & **Consolidation** & **Stress (kPa)** & **(Days)** & **Strain Rate (\%/h)** & **Strain Rate (\%/h)** \\ \hline OACK-P100-R0.01 & & & & 0.01 & 0.1/5 \\ OACK-P100-R0.1 & & & 100 & 0.1 & 0.1/5 \\ OACK-P100-R1 & 100 & 30 & & 1 & 0.1/5 \\ OACK-P600-R0.1 & & & & 0.1 & 0.1/5 \\ OACK-P600-R5 & & & 600 & 5 & 0.1/5 \\ OACK-P600-R5 Cr - & & & & 5 + creep & 0.1/5 \\ \hline YACK-P100-R0.01 & & & & 0.01 & 0.1/5 \\ YACK-P100-R1 & & & 100 & 1 & 0.1/5 \\ YACK-P100-R5 & 100 & 2 & & 5 & 0.1/5 \\ YACK-P600-R0.1 & & & 600 & 0.1 & 0.1/5 \\ YACK-P600-R5 & & & 5 & 0.1/5 \\ \hline \hline \end{tabular} Notes: OACK means Old ACK cured for 30 days, YACK means Young ACK cured for 2 days; P100 means the confining stress equal to 100 kPa; R0.01 means the strain rate is 0.01%/h during the pre-peak shearing stage. * Test with undrained creep shear during pre-peak stage.  
\end{table} 
Table 1: Summary of triaxial tests on ACK specimens. localisation and local drainage can also explain the more 'ductile' behaviour under high strain rates, as shown in Figure 1c. An additional test was carried out to investigate the negative rate effect during the pre-peak stage, as shown in Figure 3. In this test, the specimen was initially subjected to a strain rate of 5%/h until the deviator stress reached approximately 90% of the peak strength.