## 4 Discussion
### Impact of Water Weakening on Mechanical Properties  
A compilation of the experimental results discussed in the above sections for tests on dry and water-saturated samples is presented in Table 7. The parameters involved in this compilation are UCS and Young's modulus derived from uniaxial compression tests, tensile strength obtained from Brazilian tests, compressive strength and [PERSON]'s modulus from triaxial tests at 1.5 MPa effective confining pressure, critical pressure derived from isotropic compression experiments, cohesion and internal friction coefficient inferred from Mohr-Coulomb analysis on the complete set of triaxial tests (only for Oboury Chalk), and finally the mode I fracture toughness measured with the semi-circular bending test. In addition to the measured parameters for the dry and water-saturated rocks, Table 7 also presents the corresponding wet-to-dry ratios (WDR values) as defined previously. Note that for some experiments like the water injection and ultrasonic monitoring tests, no WDR value can be estimated; although, as discussed above, features potentially associated with water weakening were observed in those experiments not included in Table 7. To make the interpretation more visual, Figure 16 shows a bar chart with the height of the bars corresponding to the wet-to-dry ratios. For each chalk, the WDR values are given in increasing order with the corresponding parameter on the horizontal axis. Figure 15: (**a**) Example of 3D topography on a 10 \(\times\) 10 \(\upmu\)m surface obtained on Oboury chalk. (**b**) Example of adhesion force map on a 10 \(\times\) 10 \(\upmu\)m surface for Oboury chalk. Several interesting conclusions can be derived from our compilation in Table 7 and Figure 16:  
* For both chalks, the WDR values are significantly lower than one, meaning that water definitely has a weakening effect on both Obourg and Ciply Chalks. * The WDR values for Obourg Chalk show less fluctuation than for Ciply Chalk: this can be quantified through the standard deviation, which is lower for Obourg (0.08) compared to Ciply (0.14) Chalk. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & \multicolumn{3}{c}{**Obourg Chalk**} & \multicolumn{3}{c}{**Ciply Chalk**} \\ \hline \multirow{2}{*}{**Testing Method**} & \multirow{2}{*}{**Properties**} & \multirow{2}{*}{**Dry rock**} & **Water** & \multirow{2}{*}{**Saturated**} & **Wet-to-Dry** & **Dry Rock** & **Water** & **Saturated** & **Wet-to-Dry** \\  & & & **Rock** & & **Ratio** & & **Rock** & **Ratio** \\ \hline \multirow{3}{*}{Uniaxial compression} & UCS (MPa) & 7.0 & 3.6 & 0.51 & 4.4 & 1.7 & 0.39 \\ \cline{2-8}  & Young’s Modulus (GPa) & 5.5 & 3.2 & 0.58 & 1.2 & 0.5 & 0.42 \\ \hline \multirow{3}{*}{Brazilian test} & Tensile strength (MPa) & \(-0.87\) & \(-0.44\) & 0.51 & \(-0.75\) & \(-0.56\) & 0.75 \\ \hline \multirow{3}{*}{Triaxial compression @1.5 MPa @1.5 MPa @(GPa)} & Compressive strength (MPa) & 10.7 & 6.6 & 0.62 & 9.7 & 4.0 & 0.41 \\ \cline{2-8}  & Young’s Modulus (GPa) & 5.0 & 3.1 & 0.62 & 2.3 & 1.1 & 0.48 \\ \hline \multirow{3}{*}{Isotropic compression @1.5 MPa @1.5 MPa @1. * Whatever the mechanical property involved, the WDR values for Ciply Chalk are lower than the corresponding values for Obourg Chalk, except for the tensile strength. * The average WDR value for Obourg Chalk (0.60) is significantly higher than for Ciply Chalk (0.49); therefore, the latter experiences significantly more water weakening than the former. * For both chalks, the critical pressure, UCS, and Young's modulus in uniaxial compression are the parameters that are the more sensitive to water weakening (not taking into account the low tensile stress value for Obourg corresponding to a much higher value for Ciply). * The internal friction coefficient obtained for Obourg Chalk, only, is less prone to water weakening compared to the other parameters.