# New Perspectives on Excavation Disturbance Zones: Main Driving Forces
## 3 Influence of Humidity Factor
### Analysis of Simulation Results  
#### 3.2.1 No-Load Condition  
The processes of humidity diffusion, elastic modulus degradation, stress field change, and damage evolution within 150 days after tunnel excavation without an external load in a high-humidity environment are shown in Figure 6. At the initial stage, the humidity in the environment disturbed the surrounding rock only on the surface of the excavated space (Figure 6(a1)). Over time, the moisture at the surface gradually migrated into the interior of the surrounding rock. Subsequently, a humidity gradient was formed, which gradually decreased from the surface of the tunnel to the deep surrounding rock (Figure 6(a2,a3)). Due to the weakening effect of water on the mechanical properties of the rock, the elastic modulus and strength of the surrounding rock decreased. Moreover, with an increase in the water content, the weakening effect became more significant. Therefore, the decrease in the elastic modulus and strength of the surrounding rock exhibited a gradient effect. That is, the rock mass strength and elastic modulus of the tunnel surface were minimal, and the strength and elastic modulus of the surrounding rock gradually increased with depth. The strength and elastic modulus of the rock mass, which was not influenced by moisture diffusion, were unchanged (Figure 6(b1-b3)). Simultaneously, stress was observed in the region of diffused humidity, with an increase in the humidity, the stress value increased (Figure 6(c1-c3)). This is mainly because of the presence of a humidity gradient that generates a humidity stress field. In addition, the reduction in the elastic modulus increases the deformation capacity of the surrounding rock, whereas a reduction in strength renders the surrounding rock more vulnerable to destruction. When the humidity spreads to a certain depth in the surrounding rock, the bearing capacity of the surrounding rock is weakened due to the decrease in its strength over a large range. Subsequently, the surrounding rock is damaged under stress. Figure 6(d1-d3) presents the progressive damage process of the surrounding rock, where  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters Name** & **Rock Mass [(8)]** & **Water-Proof Layer** \\ \hline Elastic modulus (MPa) & 1500 & 30,000 \\ UCS (MPa) & 38.8 & 100 \\ Poisson’s ratio & 0.25 & 0.2 \\ density (Kg/m\({}^{3}\)) & 2610 & 2700 \\ internal friction angle (\({}^{\circ}\)) & 25 & 40 \\ Diffusion coefficient (m\({}^{2}\)/s) & \(1\times 10^{-7}\) & \(1\times 10^{-9}\) \\  
the position of the circle indicates the occurrence of micro-damage, and the size of the circle represents the amount of energy released by the micro-damage. Initially, damage mainly occurs in the sidewall and corner of the rock mass on the surface of the tunnel (Figure 6(d2)). As the humidity spreads to the deep surrounding rock, the damage gradually extends to the floor and depth of the surrounding rock; however, the overall damage is not significant (Figure 6(d3)). The mechanical property degradation and additional deformation of the surrounding rock, induced by the continuous diffusion of water into surrounding rock over time, are related to time and space. In summary, an EDZ with time-dependent properties could be formed even under no-load conditions, and humidity is a driving force for the formation of the EDZ under such conditions.  
Figure 6: Distributions of the relative humidity (**a1–a3**) (unit: 100%), elastic modulus degradation (**b1–b3**) (unit: MPa), shear stress (**c1–c3**) (unit: MPa) and damage (**d1–d3**) (unit: J) during the diffusion of humidity. The circle in the figure represents acoustic emission, and its size indicates how much energy is released.