# Study on the Damage Process and Numerical Simulation of Tunnel Excavation in Water-Rich Soft Rock
## 2 Staged Softening Process of Water-Rich Soft Surrounding Rock Based on Tunneling
### Continuous Water-Softening Stage
#### 2.4.2 Deterioration Law of Mechanical Parameters of Water-Encountered Rock  
The typical unloading stress-strain curve of rock during water-rock interaction is illustrated in Figure 5 [49]. \(\sigma_{1},\sigma_{3},\epsilon_{1}\) denote the axial stress, confining pressure, and axial strain, respectively. From Figure 5, it can be seen that the unloading stress-strain curves of the rocks are mostly the same at different water-rock interaction times and can be roughly divided into compaction phase, elastic phase, yield phase, damage phase, and residual phase. With the increase of the times of water-rock interaction, the stress-strain curves of rock samples have some obvious trends: the compaction phase gradually grows, the slope of the elastic phase gradually decreases, the yield phase gradually becomes apparent, the peak strength gradually decreases, the axial strain corresponding to the peak strength gradually increases, the drop of stress-strain curve slows down gradually, the  
Figure 4: The deformation characteristics of surrounding rock in Xujiadi tunnel: (**a**) Before water-softening; (**b**) After water-softening. Figure 3: Sprinkling water seepage in tunnel vault. brittle damage characteristic gradually weaken, and the residual strength also gradually decreases. It concludes that the rock sample tends to become softer under the water-rock interaction. According to the results of laboratory tests such as water-saturation test, wet-dry cycle test, and field observation, the strength, elastic modulus, cohesion, and internal friction angle of saturated limestone specimens are reduced, and the degree of reduction is related to many parameters such as initial confining pressure, water immersion time of rock samples, osmotic pressure, and internal fractures. In the saturated state, the increase of the confining pressure makes the internal microcracks close, which leads to the strengthening of the deformation resistance of the rock, so the larger parameters such as compressive strength and elastic modulus lead to the smaller deterioration degree. When the osmotic pressure increases, the overall stress-strain curve moves downward, the peak strength, residual strength, and elastic modulus all decrease to some extent, and the degree of deterioration increases. The longer the immersion time is, the smaller the compressive strength, tensile strength, shear strength, and elastic modulus of the rock sample are, and the greater the parameter deterioration, which will reach stability in about 90 days. The higher the number of wet-dry cycles, the lower the compressive strength, elastic modulus and other parameters of the rock sample, and the greater the deterioration degree. Moreover, the more developed the internal fissures and the lower the initial strength lead to the greater deterioration of mechanical parameters of the rock sample in the saturated state. The deterioration of peak compressive strength, tensile strength, elastic modulus, cohesion, internal friction angle of the limestone sample in the saturated state compared with the natural state is 19.0-31.4%, 23.9-38.0%, 26-33.6%, 25.2-32.4%, 7.1-14.8%, respectively [50; 51; 52; 53; 54; 55; 56; 57; 58]. The above parameters are taken as large values when the initial confining pressure is small, the osmotic pressure is large, the fracture is adequately developed and the immersion time is prolonged, otherwise, they are taken as small values. Damage Evolution Equation of Soft Surrounding Rock Considering Dual Softening of Unloading and Water Encountering  
Most of the simulations of underground engineering in water-rich geological sections in literature mainly favor the fluid-solid coupling of stress and seepage fields, but the effects of excavation-induced unloading and water-softening on the stress and seepage fields of surrounding rocks are less studied.