# New Perspectives on Excavation Disturbance Zones: Main Driving Forces
## 5 Discussion
### EDZ and Its Actual Depth under Low Confining Pressure  
In 1985, [PERSON] et al. confirmed the existence of an EDZ from the results of a similar simulation test; however, the simulated working conditions did not reflect the actual working conditions. The uniaxial compressive strength of the simulated material was 1.0 MPa, the maximum load applied during the test was 6.8 MPa, and the ratio of the maximum load to strength reached 6.8. In a different set of similar simulation tests conducted by [PERSON], the ratio of maximum load to strength ranged between 4.4 and 6.2, which is a significantly high confining pressure condition. This indicates that the depth of the tunnel reached several thousands of meters. For example, the ground stress condition corresponding to the model test was a maximum of 68 MPa when the coal rock strength was 10 MPa, which is different from the well-depth conditions in which the EDZ phenomenon was observed. However, the EDZ was observed at significantly smaller depths in underground tunnels under actual working conditions. Meanwhile, with reference to the available literature, there are no reports on the laboratory simulation results of the EDZ under actual working conditions. Therefore, assuming that an EDZ under actual working conditions cannot be simulated using mechanical loading under low confining pressure conditions or the actual depth of the EDZ cannot be obtained, there should be another formation mechanism for the EDZ observed under the actual on-site working conditions. As the results of this study show, when humidity or temperature was introduced into the model, the formation process of the EDZ under low confining pressure was accurately simulated, and the EDZ was formed without the application of confining pressure. This is because the deterioration of rock material properties caused by the propagation of humidity or heat in the rock mass led to a decrease in the rock mass strength, and then a weakened zone was gradually formed. Moreover, a humidity stress field or thermal stress field was generated in the rock mass, which led to an increase in the load acting on the rock mass. Thus, the rock mass was destroyed, and a failure zone was formed gradually when the load exceeded the strength of the rock mass. The weakened and failure zones both specifically demonstrated the formation of the EDZ. In other words, this phenomenon can be summarized in two points. First, environmental factors deteriorate the mechanical properties of the surrounding rocks, which reduces the load required for the damage to the surrounding rocks. Second, the stress field generated by the environmental factors increases the load acting on the surrounding rock. When the strength of the surrounding rock is continuously reduced and the load acting on it is increased, the results obtained are consistent with the EDZ in the on-site low-confining pressure conditions. Although the confining pressure conditions exert an influence on the propagation speed of humidity (heat) in the rock mass, it cannot be determined whether the humidity (heat) can propagate in the rock mass. Therefore, regardless of the confining pressure conditions, an EDZ can be formed in the rock mass around the tunnel. The main differences are that the depth of the EDZ and the damage extent of the surrounding rock under the condition of low confining pressure are both smaller than those under the condition of high confining pressure. However, under the same circumstances, a relatively high confining pressure is required to form an EDZ without considering environmental factors in laboratory simulation tests. This is the essential reason why laboratory simulation results cannot obtain an EDZ and its actual depth under on-site working conditions. The propagation of environmental factors, such as humidity (temperature) in the rock mass, leads to a weakened zone of the material properties of the surrounding rock, which expands with time.