## 3 Test Results and Analysis  
### Seepage Result of Weakly Consolidated Sandstone  
The variation in permeability is depicted in Figure 3(a), which can be divided into three distinct stages.  
* Stage of Steady Permeability Decrease: During the initial loading phase, as the deviatoric stress increases, permeability gradually decreases. In this stage, the internal rock pores are compacted, pore sizes reduce, and the fluid's ability to flow weakens, resulting in reduced permeability. The fundamental reason for the early decrease in permeability is the compaction of weakly cemented rock's internal porosity and the reduction in pore size during triaxial compression.
* Stage of Rapid Permeability Increase: In this stage, the rock undergoes stress-induced failure, reaching its peak strength. Primary cracks widen, secondary cracks significantly extend, and they become interconnected, leading to a substantial increase in permeability.
* Stage of Equilibrium Permeability: When the axial stress on the rock specimen reaches a certain level, the changes in internal cracks become less pronounced, and permeability tends to stabilize. The evolution of permeability in weakly cemented sandstone samples exhibits distinct stages, which are crucial for understanding the rock's response to stress and its impact on fluid flow behavior.  
When deviatoric stress increases from 1 MPa to 5 MPa, the permeability of weakly cemented sandstone decreases from 8.01 \(\times\) 10\({}^{-13}\) m\({}^{2}\) to 7.85 \(\times\) 10\({}^{-13}\) m\({}^{2}\), representing a 2% decrease. Subsequently, as deviatoric stress continues to rise from 5 MPa to 9 MPa, permeability increases from 7.85 \(\times\) 10\({}^{-13}\) m\({}^{2}\) to 8.34 \(\times\) 10\({}^{-13}\) m\({}^{2}\), marking a significant 7.6% increase. Beyond 9 MPa, as deviatoric stress further increases, permeability remains nearly constant at 8.35 \(\times\) 10\({}^{-13}\) m\({}^{2}\), showing no significant changes. The trend of the permeability curve can be approximated as a \"\(\surd\)\" shape.  
It is important to note that permeability experiments for weakly cemented sandstone require prior saturation, which differs from conventional sandstone. In conventional sandstone, permeability gradually decreases during the compression process, sharply increasing after rock failure, as shown in Figure 3(b).  
For weakly cemented sandstone, the two inflection points in permeability occur during the plastic deformation and failure stages. A significant difference is the absence of a stress decrease stage after peak strength, which may be attributed to the expansive nature of weakly cemented sandstone having a more substantial impact on the stress-strain curve's  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Parameter Name**} & **Uniaxial Compressive** & **Deformation Modulus** & **Friction Angle** & **Cohesion** & **Bulk Modulus** & **Shear Modulus** \\  & **Strength \(\sigma_{c}\)/MPa** & **\(Em\)/GPa** & **\(\varphi^{\text{d}}\)** & **\(c\)/MPa** & **K/GPa** & **K/GPa** \\ \hline value & 12.3 & 15.8 & 43 & 2.32 & 9.32 & 6.55 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Estimates of mechanical parameters of weakly consolidated sandstone.  
descent. This behavior is consistent with prior findings in this study, indicating that weakly cemented sandstone undergoes a transition from brittle to ductile behavior after saturation, resulting in a gradual increase in permeability before reaching peak strength. The results suggest that deviatoric stress significantly influences permeability, gradually decreasing in the early stages, followed by a sharp increase and eventual stabilization of permeability.