| With the gradual development of mineral resources mining and underground space engineering,the rock mass storage environment is extremely complicated.Under the deep high in-situ stress,the rock mass undergoing stress adjustment may induce rockbursts in surrounding rocks.Due to the severity,suddenness and uncertainty of rockbursts,it poses a great threat to the construction of underground projects and the safety of construction personnel and equipment.In this paper,rockburst simulation tests were conducted using the true-triaxial testing system,the micro-camera monitoring system and the self-developed rock drilling unloading test device to study the influence of drilling unloading,stress state,loading rate,maximum principal stress direction,tunnel cross-section shape,and bedding angle on the failure process and characteristics of surrounding rock.In addition,the stress distribution was calculated theoretically,and the occurrence mechanism and inducing conditions of rockburst were analyzed and investigated.The main research contents and conclusions are as follows:(1)Using the true-triaxial testing system,the micro-camera monitoring system and the self-developed rock drilling unloading test device,the indoor simulation tests of loading first and then drilling(LFDT),and drilling first and then loading(DFTL)were carried out.By comparison,in the LFDT test,the initial failure stress of the sidewall is lower,the sidewall is more prone to failure,and the severity of rockburst is stronger,the rock fragments are larger in size,and the total mass of rock fragments is higher.It indicates that the internal unloading effect causes a certain degree and scope of damage to the surrounding rock,which causes the strength-strengthening effect of the surrounding rock.As the confining pressure increases,the stability of the surrounding rock increases,the initial failure stress increases,and the strength-weakening effect caused by unloading is more obvious.(2)By conducting the rockburst simulation test under different loading rates,the influence of loading rate on the surrounding rock failure is analyzed.With increasing loading rate,the initial failure stress of the circular hole sidewall increases.After the sidewall failure,the failure evolution process accelerates in the same period.However,the failure severity decreases.The proportion of small-sized rock fragments to the total mass of rock fragments increases,the fragmentation degree of rock fragments increases,and the fractal dimension increases.(3)The influence of the maximum principal stress direction on the failure characteristics of a D-shaped tunnel is analyzed.With the increase of maximum principal stress direction θ,the size of the rock fragments first decreases and then increases.The fractal dimension of rock fragments first increases and then decreases with the increase in θ,and the fractal dimension is the largest when θ=45°.With an increase in θfrom 0° to 90°,the left failure position transformed from the sidewall to the corner and finally to the floor;the right failure position transformed from the sidewall to the spandrel and finally to the roof,which was consistent with the failure position in the underground engineering.In addition,the initial failure vertical stress first decreased and then increased,showing a U-shaped change trend.By contrast,the failure severities under different maximum principal stress directions were from strong to weak: 90° > 60° > 30° > 45° > 0°.(4)The influence of tunnel section-section shape on surrounding rock failure is studied.The failure range of the arc-shaped surrounding rock is smaller than that of the D-shaped surrounding rock.The greater the height of the straight-wall,the wider the failure range,the larger the generated rock fragments,and the wider the range of the V-shaped notch.Because the size of the rock fragments produced by the straight-wall-shaped surrounding rock is relatively large,but its length-towidth ratio is relatively small,and the rock fragments are thin slices.The size of the rock fragments produced by the failure of arc-shaped surrounding rock is relatively small,and its length-to-width ratio is relatively large,and rock fragments are long strip.(5)Uniaxial compression and circular tunnel rockburst simulation tests were carried out on phyllite samples,and the influence of bedding angle on rock mechanical properties and tunnel surrounding rock failure characteristics is summarized.The P-wave velocity of the phyllite sample gradually decreases with increasing the bedding angle.In the uniaxial compression test,the peak stress,peak strain and cumulative acoustic emission counts show U-shaped trend.Compared with the tunnel layout in which the bedding plane is parallel to the hole axis,when the bedding plane is perpendicular to the hole axis,vertical stress at the initial failure of the hole is much higher.The arrangement of the tunnel axis perpendicular to the bedding plane is conducive to improving the stability of the surrounding rock and reducing the risk of rockburst.(6)Using the complex variable function and conformal transformation theory in elastic theory,the stress distribution and stress concentration factor in the surrounding rock of circular,rectangle and straight-wall-arched holes are calculated.The influence of the lateral pressure coefficient and the bedding angle on the stress distribution in the surrounding rock are analyzed,and the conditions for rockbursts in deep rock masses are determined: surrounding rock masses have the ability to accumulate higher elastic deformation energy;the stress concentration coefficients in the regions where rockbursts occur in surrounding rocks are higher,and higher energy accumulates in this location;static stress adjustment or dynamic disturbance occurs in the surrounding rock. |
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