Abstract
Strain burst is often considered to be a type of failure related to brittle rock material; therefore, many studies on strain burst focus on the brittleness of rock. However, the laboratory experiments show that strain burst can not only occur in hard brittle rock-like granite but also in the relatively ductile rock-like argillaceous sandstone. This result proves that behavior of rock material is not the only factor influencing the occurrence of strain burst. What must also be considered is the relative stiffness between the excavation wall/ore body and the surrounding rock mass. In order to further studying the mechanism of strain burst considering the whole system, the engineering geomechanial model and numerical model of strain burst due to excavation are built, respectively. In a series of numerical tests, the rock mass involving the excavation wall as well as roof and floor is biaxially loaded to the in situ stress state before one side of the excavation wall is unloaded abruptly to simulate the excavation in the field. With various system stiffness determined by the microproperties including the contact moduli of particles and parallel bond moduli in the models of roof and floor, the different failure characteristics are obtained. Based on the failure phenomenon, deformation, and released energy from the roof and floor, this study proves that the system stiffness is a key factor determining the violence of the failure, and strain burst is prone to happen when the system is soft. Two critical Young’s moduli ratios and stiffness ratios are identified to assess the violence of failure.
Highlights
Strain burst is often considered as a violent failure closely related to the hard brittle rock due to excavation, and many researches focus on the mechanical behavior of the rock material-like brittleness: the ratio of uniaxial compressive strength to tensile strength of rock was applied by Zhang et al [2] and Khanlari and Ghaderi-Meybodi [3], as well as Q1/(σc + σt) and Q2 sin φ have been employed by Singh [4], while Qu (u1+u2)/u1 by Tan [5] to assess the violence of rock burst, where σc and σt are the uniaxial compressive and tensile strengths, respectively; φ is the frictional angle of the rock; and u1 and u2 are the permanent and elastic axial deformation of the rock specimen in a loading-unloading cycling uniaxial test
E failure is quite violent in the scope from A to D1; buckling occurs on the unloading side of the excavation wall, and large blocks eject at high speed; from D2 to D4, fragment ejection can still be seen as well as the bulking; the failure turns to be less violent from D5 to F4, though there are still some small particle ejections and tensile cracks near the unloading face; from F5 to I, no ejection is observed and the localized shear is the main failure mode
In order to further study this relationship quanti cationally, engineering geological model, geomechanical model, and numerical model are built, respectively, and a series of strain burst tests have been carried out considering various system sti ness with the particle flow code (PFC) program
Summary
Hoek [1] mentioned that “Rockbursts are explosive failures of rock which occurs when very high stress concentrations are induced around underground openings. e problem is acute in deep level mining in hard brittle rock.” And “A characteristic of almost all rockbursts is that they occur in highly stressed, brittle rock.” Strain burst is often considered as a violent failure closely related to the hard brittle rock due to excavation, and many researches focus on the mechanical behavior of the rock material-like brittleness: the ratio of uniaxial compressive strength to tensile strength of rock was applied by Zhang et al [2] and Khanlari and Ghaderi-Meybodi [3], as well as Q1 (σc − σt)/(σc + σt) and Q2 sin φ have been employed by Singh [4], while Qu (u1+u2)/u1 by Tan [5] to assess the violence of rock burst, where σc and σt are the uniaxial compressive and tensile strengths, respectively; φ is the frictional angle of the rock; and u1 and u2 are the permanent and elastic axial deformation of the rock specimen in a loading-unloading cycling uniaxial test. Kaiser and Tang [19] used RFPA models to study the failure process, stress-strain response, seismic events, and seismic energy release during the laboratory uniaxial compression tests and field pillar failure considering different system stiffness. Kias et al [17] employed three different numerical tools to carry out uniaxial compression tests with variable platen stiffness and obtained the corresponding system response, including the stress-strain behavior of the specimen and loading system, while did not give the failure modes or process. With a series of laboratory strain burst experiments on different rock types and numerical tests (by particle flow code) considering various system stiffness of the surrounding rock mass, this paper will give some discussion on the mechanism of strain burst and assessment on the violence of strain burst in a perspective of the complete system
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