# Experimental Study on Rock-Like Specimens with Single Flaw under Hydro-Mechanical Coupling
## 3 Results
### Effect of Water Pressure on Cracking Behavior  
Water pressure greatly reduced the strength of jointed specimens, so that the mechanism of water in the failure process of the jointed rock mass is worthy of attention. Table 3 shows the crack types observed in hydraulic coupled triaxial tests. When the water pressure was 1 MPa, the confining pressure had a significant impact on the crack type. Most of the specimens under 2 MPa confining pressure were fractured by tensile cracks, while specimens were fractured by shear cracks or mixed cracks under 6 MPa confining pressure. With the increase of water pressure, more tensile cracks appeared to the specimens. This suggests that water pressure promotes the initiation and propagation of tensile cracks. Figure 15 shows the heat maps of failure mode under 4 MPa and 6 MPa, respectively. When the confining pressure was 4 MPa (Figure 15a), the failure mode of the specimens changed from mixed failure to tensile failure as the water pressure increased. However, when the confining pressure was 6 MPa (Figure 15b), the shear characteristics of the specimens under 1 MPa water pressure were stronger than those of specimens in the conventional triaxial test, and the specimens changed from mixed failure to shear failure. With the further increase in water pressure, the observation results \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Specimens**} & \multirow{2}{*}{**Uniaxial**} & \multicolumn{4}{c}{**Triaxial**} \\ \cline{3-5}  & & **CP = 2 MPa** & **CP = 4 MPa** & **CP = 6 MPa** \\ \hline \multirow{2}{*}{
\begin{tabular}{c} Intact specimen \\ 0° specimen \\ 15° specimen \\ 30° specimen \\ 45° specimen \\ 60° specimen \\ 75° specimen \\ 90° specimen \\ \end{tabular} } & T3 & T3 & S2 & S2 \\  & T1, T2 & T3 & M3 & M3 \\  & T1 & T1 & M3 & M3 \\  & T1, T2 & T3 & T3, T1 & M2 \\  & T1 & T3 & M1, M2 & M1 \\  & T1 & T3, T1 & M2 & S1 \\  & T1, T2 & T3 & S2 & S1 \\  & T3 & T3 & M3 & S2 \\ \hline \multirow{4}{*}{Notes} & T1: type 1 tensile crack & & S2: type 2 shear crack & \\  & T2: type 2 tensile crack & & M1: type 1 mixed crack & \\  & T3: type 3 tensile crack & & M2: type 2 mixed crack & \\  & S1: type 1 shear crack & & M3: type 3 mixed crack & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of all crack types observed in uniaxial and triaxial compression tests. Figure 14: Failure mode of specimens under different confining pressure. showed that water pressure significantly promoted the tensile failure of the specimens. Macroscopic and microscopic observations showed that 1 MPa water pressure made the specimens more prone to shear failure. It is possible that the lubrication of water reduced the resistance of the potential sliding surface, and the effect of this lubrication exceeds the effect of hydraulic pressure expansion force under 6 MPa confining pressure. When water pressure increased, tensile cracks were observed, and the results showed that specimens were prone to tensile failure. It seems like tensile cracks were more \"easy\" to initiation and propagation under high water pressure, possibly because of the expansion force exerted by water pressure. The test results may suggest that the stress state at the crack tip and the interface friction coefficient were changed by the water pressure. The hydro-mechanical coupling may influence the fracture  
Figure 15: Failure mode of specimens under different water pressure. (**a**) Under 4 MPa confining pressure; (**b**) under 6 MPa confining pressure.