Study On Characteristics Of Deep-burred Sandstone In The Coal Mine Under Loading And Unloading Conditions And Rockburst Prediction Methods

With the rapid development of national economy and society, the construction depth of engineering infrastructure and mineral exploration are gradually increasing. Rockbursts induced by high geo-stress are the constraints of the development of engineering construction, and also becoming technical difficulties as well. However, the studies of rock burst need to improve to meet the demands of preventing and controlling rockbursts disaster in the actual projects. Therefore, working out actual and feasible measures based on further research on mechanism to solve rock burst has been an important issue in underground engineering.In order to study the deformation and strength properties, energy-transfer mechanisms and failure modes of deep-buried sandstone under loading and unloading conditions, and also to understand deeply the mechanical mechanism of rock burst, the project of Xinzhuangzi coalmine in Huainan is chosen as the project background. Based on the results of triaxial unloading tests, a strength criterion and a constitutive model are proposed, which are more applicable to triaxial unloading conditions. The relationship between coefficient of brittle stress drop and rockburst tendency is proposed tentatively. On the basis of synthesizing existing rockburst vulnerability numerical indicators, a new one—RBERR (Rock Burst Energy Release Rate) is put forward and its rationality is verified by the application of rock burst samples. The above theoretical analysis and experimental achievements are applied to the project of the 62th mining area of Xinzhuagnzi coalmine, in order to forecast the possibility and magnitude of induced rockburst, and also to provide reliable basis for the engineering supports.The influence of initial damage levels and unloading rates on wall-rock's stability are summarized by analyzing the results of triaxial unloading tests, which is then applied to engineering practices of Xinzhuangzi coalmine in Huainan.(1) By making deep analysis of existing data of rock unloading tests, it is found that the key factors such as unloading paths, initial damage levels and unloading rates have a great influence on rock unloading mechanism. Therefore, a series of triaxial unloading tests for sandstone are carried out, which are set different parameters, such as unloading path, initial damage level and unloading rate. Based on the results of tests, the impact of three controlling factors on characteristics of deformation and strength, dilation evolution rules and failure modes are systematically researched, which will provide reliable basis for researches on the mechanical mechanism of rock burst.(2) Rock burst is induced by the internal factor— migration and release of energy. The energy evolution rules of sandstone unloading tests under different controlling conditions are explored based on the test results. Firstly, the energy-transfer mechanisms of three unloading modes are analyzed and the accuracy is verified by comparing with test results. Secondly, in order to study the influence of initial damage level and unloading rate on energy characteristics of sandstone under unloading conditions, a new indicator—Elastic Strain Energy Storage Rates f is put forward, which is used to measure the rockburst tendency, and then the rockburst tendency of three unloading modes are tentatively analyzed. Finally, limit energy storage theory is proposed:maximum strain energy stored in rock before failure only depends on confining pressure and unloading rate both in loading and unloading conditions, and the energy for rock failure under a certain confining pressure is a constant. This theory can provide the foundation for exploring energy criterion of rock burst.(3) The deformation and failure mechanism and constitutive model under unloading conditions are studied based on sandstone test results. It is found that Mogi-Coulomb strength criterion is more suitable for describing the failure mechanism and strength behavior of brittle rock under unloading conditions. Besides, a yield criterion built on the coupled relation between Mogi-Coulomb criterion and Griffith criterion is proposed, and a progressive damage constitutive model of sandstone under unloading conditions is established based on elasticity-plasticity theory. The paper first proposes the relationship between coefficient of brittle stress drop and rockburst tendency.(4) Based on energy release theory, a new rockburst vulnerability numerical indicator—RBERR (Rock Burst Energy Release Rate) is put forward. The contour of RBERR represents the probability of rock burst, the greater the contour, the larger the probability of rock burst. RBERR is applied to Jinping II diversion tunnel to forecast rock bust, and the predictions agree well with the real situation, which indicates that RBERR is reliable and reasonable.(5) The project of Xinzhuangzi coalmine in Huainan is chosen as engineering background to verify the accuracy of the experiment and theory. Firstly, microseismic monitoring technique is used to determine rockburst potential area of the 62th mining area. Then, RBERR is applied to rockburst potential area to analyze the location and magnitude of rock burst, which can provide reliable basis for the engineering supports. Finally, the basic laws and mechanisms of sandstone unloading tests are summarized, which is then applied to engineering practices of Xinzhuangzi coalmine in Huainan to ensure project to be carried out economically, safely and successfully.