## 4 Discussion
### Study on Damage Regularity of Weakly Consolidated Sandstone  
The previous chapters provided an in-depth examination of the permeability characteristics of weakly cemented sandstone under various stress conditions. These studies have revealed a close relationship between rock permeability and stress states, offering valuable insights for rock engineering design and practice. However, changes in permeability are just one aspect of understanding rock behavior. To gain a comprehensive understanding of how weakly cemented sandstone behaves under stress, it is essential to further investigate its damage patterns. As rocks experience stress, they undergo internal damage and fracture, significantly impacting their mechanical properties and permeability. Therefore, studying the influence of stress states on the damage evolution of weakly cemented sandstone becomes imperative. The subsequent chapters will delve into the influence of stress states on the damage patterns of weakly cemented sandstone. This paper employs a combined approach of experiments and numerical simulations to systematically study features like crack propagation and alterations in rock strength under different stress conditions. Through a deeper understanding of damage evolution patterns, it will be possible to uncover the intrinsic behavior of weakly cemented sandstone under stress states, offering a more reliable foundation for reliability assessment and prediction in rock engineering. Through this study, a valuable contribution is made to the advancement of rock mechanics and rock engineering, providing a scientific basis and guidance for practical engineering applications to ensure safety, reliability, and sustainable development of engineering projects. In UDEC, the definition of the damage variable typically depends on the specific damage model and the research context. For instance, in certain models, the damage variable can be defined as a parameter associated with factors like fracturing, microcracks, and porosity, reflecting a reduction in the damage zone elements or a decrease in the material stiffness. This variable's value changes as new cracks appear during the simulation process. In this paper, damage variable D is defined, and its value increases with the occurrence of new cracks during the simulation. The extent of material damage can be measured by comparing D values at the beginning and the end of the simulation. D is defined as the total number of new cracks that occur during the simulation. To track and record new cracks during the simulation process, a custom FISH function (FISH being a built-in programming language in UDEC) needs to be developed to monitor crack generation, and then these functions are used to calculate the value of D.  
During the uniaxial compression of weakly cemented sandstone under different confining pressures, the expansion of micro-crack numbers in the specimen is illustrated in Figure 15. Throughout the compression process, shear cracks predominate while tensile cracks play a secondary role. It is evident that the development of micro-cracks in weakly cemented sandstone is closely related to variations in confining pressure. It can be seen from Figure 15 that the number of fractures in the weakly consolidated sandstones with different confining pressures during uniaxial compression can be roughly divided into three stages as the calculation steps increase: slow increase stage, rapid increase stage, and stable development stage after failure. The three stages correspond to the microfracture development stage of microfracture change, the fracture instability development stage, and the post-failure stage, respectively. Figure 16 illustrates the total number of cracks formed under various confining pressures. Within the range from 0 MPa to 4 MPa confining pressure, there is a gradual decrease in the total number of cracks as the confining pressure increases. However, with a 6 MPa increase in the confining pressure, the total number of cracks surpasses that observed under 0-5 MPa confining pressures.