Energy Adjustment Mechanism And Its Effects In Surrounding Rock Mass During Deep Rock Mass Excavation

Nowadays, deep rock mass excavation is becoming an increasing construction work in the fields of mining, strategic energy reserve and hydropower project et al. With the initiation, propagation and linking of cracks induced by blasting, the transient release of in-situ stress on the newly formed boundary often induces a series of geological disasters such as rock bursts, microseismic events and large abrupt-deformation. In nature, these geological disasters are the results driven by energy release. Therefore, research on unloading effects due to excavation from the point of energy can slip the leash of stress-strain analysis, and provide a new method and view to study the formation mechanism and evolution laws of unloading effects caused by deep rock mass excavation. The purpose of this dissertation is to study the energy accumulation, release of surrounding rock mass and its mechanical effects during the process of deep rock mass excavation with methods of theoretical analyses, numerical calculations and engineering verifications.The main research conclusions are as follows.During the excavation of deep rock mass by blasting, the transient release of in-situ stress on the excavation boundary is a dynamic mechanics process. By analyzing the energy adjustment process of surrounding rock mass induced by blasting excavation, it is found that accompanying the transient release of in-situ stress, the unloading wave propagates from the excavation boundary to the surrounding rock mass in deeper. When the unloading wave arrives, the initial state of strain energy of surrounding rock mass is broken, and appears a dynamic fluctuation like decreased first and then increased. During this process, energy transmission is in the opposite direction of unloading wave propagation, and causes the accumulation of strain energy in the surrounding rock mass. The closer excavation boundary and the shorter unloading time mean the more obvious of energy accumulation phenomenon.Considering an inelastic case, the strain energy of surrounding rock mass firstly decreases, then increases, soon reduces and stabilizes at last. Similar with the elastic case, the first decrease process of strain energy is mainly caused by unloading wave and does not induce damage of surrounding rock mass. While the second decrease process is mainly caused by that the strain energy accumulated in the surrounding rock mass exceeds the energy storage limit, and this is bound to cause damage and failure of surrounding rock mass. Further study shows that the closer excavation boundary means the larger energy rlease rate (ERR), the shorter time of energy release, and the more drastic energy release. In addition, by compared the energy release and wave velocity drop monitored in surrounding rock mass, it is found that the bigger energy release means the the larger wave velocity drop, and they have linear relationship.By analyzing the influence of confining pressure, unloading path and rock properties on the energy release process of surrounding rock mass, it is found that with the increase of confining pressure, the energy release rate decreases. This implies that the confining pressure has a strongly inhibitory effect on energy release. Accompanying the transient release of in-situ stress, energy release process of surrounding rock mass has a typically dynamic characteristic. Different unloading paths produce different energy release process. When the level of confining pressure is high, marble almost does not release energy after its peak, while granite still releases large amounts of energy and causes brittle failure. This means that compared with marble, granite which owns a very obvious brittle characteristic after its peak, has a strong role in promoting the energy release.Compared with the quasi-static unloading of in-situ stress, the higher aggregation degree of strain energy caused by dynamic characteristic of energy change of surrounding rock mass during the transient release of in-situ stress, can make it easier to exceed the damage or cracking threshold and cause the damage, cracking of rock mass. Although part of strain energy in the cracking process of surrounding rock is released in the form of friction energy and surface energy, the rest energy is still stored in the form of releasable strain energy in surrounding rock mass and presents a low?high?low hump shape distribution from the suface of excavation boundary to the surrounding rock mass in deeper. Compared with the quasi-static unloading of in-situ stress induced by TBM excavation, the transient release of in-situ stress caused by excavation enlarges the aggregation peak of releasable strain energy and magnifies the grade, intensity of immediate rock burst; while after unloading, the surrounding rock mass releases large energy during its cracking process induced by blasting excavation.This decreases the releasable strain energy and brings down the risk of time delayed rock burst.Besides the blasting load, energy release can also induce vibration (microseismic events) in surrounding rock mass during the process of blasting excavation (the transient release of in-situ stress). However, during the process of TBM excavation (the quasi-static unloading of in-situ stress), energy release is mainly used to do work against unloading stress and enlarge the potential energy of surrounding rock mass, no vibration is produced. Thus, the monitored vibration has two excitation sources, the blasting load and the transient release of in-situ stress and also has two energy sources, the energy released from explosive and the energy released from rock mass caused by excavation. These cause that the time-energy density of monitored vibration has two peak groups and the amplitude spectrums of the monitored vibration has two dominant frequency bands. Here, the low-frequency component is mainly induced by the transient release of in-situ stress, while the high-frequency component is produced by blastiung load. According to the frequency identification, vibrational signal induced by transient release of in-situ stress can be separated from the monitored vibration with a low-pass filter, where the cut-off frequency is obtained form the demarcation point between two peaks in the amplitude-frequency curve. In addition, case study shows that the proposed model based on the strain energy of excavated rock mass is the better option for predicting the amplitude of vibration induced by energy release.