## 3 Results
### Strength and Elastic Weakening Evidenced through Standard Mechanical Testing
#### 3.1.3 Triaxial Compression Tests  
Figure 9 shows typical results obtained in triaxial tests on Obourg and Ciply chalk at the same low effective confining pressure (1.5 MPa) defined as \(\text{P}_{\text{c,eff}}=(\sigma_{3}-\text{p}_{\text{f}})\), with \(\sigma_{3}\) the confining pressure and \(\text{p}_{\text{f}}\) the pore fluid pressure. While for dry tests \(\text{p}_{\text{f}}\) corresponds to the atmospheric pressure, for the water and brine-saturated sample it is equal to 1 MPa. Several tests were conducted with different pore saturating fluids. The compressive strength is defined as the differential stress \(\text{Q}_{\text{f}}=\sigma_{1}-\sigma_{3}\) at failure where \(\sigma_{1}\) is the axial stress. Whereas for most of the samples, failure is manifested by a peak on the loading curve, no peak is  
\begin{table}
\begin{tabular}{c c c c c c c} \hline  & & **Obourg Chalk** & & & **Ciply Chalk** & \\ \hline \multirow{3}{*}{**Pore Fluid**} & **Number of Samples** & **TENSILE Strength (MPa)** & **Standard** & **Number of** & **Tensile** & **Standard** \\  & **TENSILE Strength (MPa)** & **Deviation** & **Samples** & **Strength (MPa)** & **Deviation** \\ \hline Air (dry) & 4 & \(-0.87\) & 0.14 & 1 & \(-0.75\) & - \\ \hline Distilled water & 2 & \(-0.44\) & 0.07 & 2 & \(-0.56\) & 0.15 \\ \hline \end{tabular}
\end{table}
Table 3: Tensile strength values for both chalks for dry and water-saturated samples.  
observed for the water-saturated Ciply Chalk sample (Figure 9); for that particular sample, we took the stress value at the kink correlated with the onset of strain hardening. Let us first compare the behavior of the dry samples to that of the water-saturated samples. For both chalks, the compressive strength is significantly reduced for the water-saturated samples compared to the dry ones. The same is true for Young's modulus measured in the linear part of the loading curves (Table 4).  
Therefore, both chalks exhibit a strong strength and elastic weakening induced by water, in good agreement with our results obtained under uniaxial loading. Note that despite the low value of the effective confining pressure, there is a significant enhancement of the compressive strength with respect to the UCS. The wet-to-dry ratio is WDR(Q\({}_{\ell}\)) = 0.62 for Obourg Chalk and 0.41 for Ciply Chalk, meaning that the latter experienced more water weakening.  
For Obourg chalk, we found again that when the rock sample is saturated with NaCl brine, the behavior is comparable to the one obtained for the water-saturated sample, whereas using a MgCl\({}_{2}\) brine slightly inhibits the water weakening. These results are consistent with our observations on UCS variations (Figure 8).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Obourg Chalk**} & \multicolumn{2}{c}{**Ciply Chalk**} \\ \hline \multirow{2}{*}{**Pore Fluid**} & **Compressive Strength** & **Young’s Modulus** & **Compressive Strength** & **Young’s Modulus** \\  & **(MPa)** & **(GPa)** & **(MPa)** & **(GPa)** \\ \hline \multirow{2}{*}{Air (dry sample)} & \(11.1\) & 5.0 & & \\ \cline{2-5}  & 10.3 & 4.9 & 9.7 & 2.3 \\ \hline \multirow{2}{*}{Deionized water} & 6.5 & 3.1 & & \\ \cline{2-5}  & 6.7 & 3.1 & & \\ \hline Brine 35 g/L NaCl & 6.7 & 3.2 & X & X \\ \hline Brine 35 g/L MgCl\({}_{2}\) & 8.8 & 3.8 & X & X \\ \hline \hline \end{tabular}
\end{table}
Table 4: Compressive strengths and elastic moduli for both chalks tested under dry and water-saturated conditions at 1.5 MPa effective confining pressure. Two Obourg chalk samples were tested in order to check the reproducibility of the experiments. Additional experiments were conducted on Obourg Chalk with two different brines saturating the pores.  
Figure 9: Triaxial tests at 1.5 MPa effective confining pressure for Obourg and Ciply chalks with different fluids saturating the pore space. The \(y\)-axis scale is the same for both plots.