## 4 Creep Strain Fields And Discussions
### Evolution of Average Strains  
DVC was applied to the X-ray micro-tomographic images obtained at the different steps of creep tests. This allowed computing three-dimensional strain fields of all tested samples. From these fields, the macroscopic strains of each sample were calculated as volumetric averages of full strain field. In Figure 6, the evolutions of both axial and radial strains with time are presented for three samples with different orientations. It is noticed that the step-like variations in these figures corresponded to the instantaneous strains induced by the increase of axial stress at the change of loading step. At first sight, the strains evolutions were clearly different between three samples, indicating a strong anisotropy of creep behavior of the COx claystone. Comparing the parallel and perpendicular samples (see Figure 6a,c, despite the very similar peak strength of these two samples, the axial strain of the perpendicular sample (\(\perp\)) was almost two times larger than that of the parallel one (\(\parallel\)), both for the instantaneous strain when the axial stress was increased and the creep strain under the constant stress. This difference was clearly related to the progressive compaction of bedding planes under the axial stress in the perpendicular sample. It is also interesting to compare the radial strain between these two samples. Under low stress levels, the radial strain of the perpendicular sample was smaller than that of the parallel one. It is even compressive (positive value) during the first step of loading. This means that due to the important compaction of bedding planes, the application of axial stress did not induce extensive radial strain. The sample exhibited a compaction in both the axial and radial direction. When the prescribed axial stress became higher, the radial strain became extensive and \begin{table}
\begin{tabular}{l||c c c c c c c c} \hline \hline \multirow{2}{*}{**Rock Core**} & \multirow{2}{*}{**Direction**} & \multirow{2}{*}{\begin{tabular}{c} **Diameter** \\ **(mm)** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Height** \\ **(mm)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Creep** \\ **Steps** \\ **(MPa)** \\ \end{tabular} } & \multicolumn{2}{c}{**Failure**} & \multicolumn{2}{c}{**Stress Ratios for**} & \multicolumn{2}{c}{**Axial Elastic**} & \multicolumn{2}{c}{**Axial**} \\  & & & & & & **C creep Tests** & **Modulus** & **Poisson’s** \\ \multicolumn{1}{c}{} & & & & & **(MPa)** & \(q_{c}\)\(l\)\(l\)\(q_{peak}\)\((\%)\) & **E/(GPa)** & **Ratio**\(\
u\) \\ \hline EST58125 & \(\parallel\) & 4.87 & 10.65 & 4 & 35.0 & 41\%, 60\%, 77\%, 94\% & \(\sim\)10.0 & 0.18–0.20 \\ EST58125 & \(\parallel\) & 4.84 & 10.88 & 3 & 27.7 & 40\%, 67\%, 90\% & \(\sim\)8.0 & 0.15–0.24 \\ EST58125 & \(\perp\) & 4.96 & 9.54 & 4 & 36.0 & 40\%, 60\%, 70\%, 90\% & \(\sim\)6.3 & 0.26–0.30 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Elastic and strength properties of the Callovo-Oxfordian (COx) claystone and stress steps for creep tests. progressively larger. During the third and fourth loading steps, the radial strain of the perpendicular sample became larger than that of the parallel one, due to its higher creep deformation. As mentioned above, only three loading steps were realized on the inclined sample due to the sample failure at the third step. Its axial strain was larger than the parallel sample and smaller than the perpendicular one. For this sample, the compressive axial stress could generate the time-dependent sliding of bedding planes, enhancing the creep deformation of sample. As a consequence of inclination of bedding planes, the radial strain was also amplified. Therefore, the radial strain of the inclined sample was larger than those of two other samples. From these results, one can remark that both the instantaneous and creep strains of the COx claystone were significantly dependent on the loading orientation and the motion of bedding planes (compaction or sliding) played an essential role in the deformation process of material.