True Triaxial Experimental Study Of Rockburst And Related Failure In Deep Underground Tunnel

Rockburst is a typical geological hazard encountered during the excavation of deep underground tunnels.It is closely related to rock failures such as slabbing and shear slip or shear rupture that develop around the excavation boundary.It is characterized by sudden dynamic failure of rocks and violent ejection of rock fragments,which constitutes a significant safety risk for underground engineering construction.The complexity and uncertainty of rock itself,in-situ stress conditions,geological conditions and potential triggering conditions,make rockburst an extremely complex phenomenon.To date,the study of the process,mechanism and pattern of rockburst failure is still limited,which has become a major challenge that presses for solutions.Given that,this thesis presents a study of rockbursts and related failures in deep underground tunnels based on laboratory simulation tests,and the main contents are as follows:（1）Analyze the key issues usually encountered around rockbursts and related failures,and in laboratory simulation tests,and determine the strategies and methods for laboratory simulation tests and analyses.Based on the previous research,study the energy transition and loading condition correlation for rock failure and rockbursts.It is pointed out that the rockbursts and related failures of local surrounding rocks are affected and controlled by the subsequent loads and energy of the surrounding rock system that can be regarded as a kind of flexible loading system without servo control.The localized failure of the surrounding rock in the field and that of class II rock samples in the laboratory test are controlled by different mechanisms.Analyze the stress changes of the surrounding rock and the temporal and spatial distribution of rockbursts and related failures.It is pointed out that the continuous concentration of tangential stress is the main controlling factor in the development of rockbursts and related failures.Analyze the advantages and disadvantages of different laboratory test strategies,and establish the loading scheme considering the different subsequent effects of the surrounding rock system.Analyze the methods of acoustic emission（AE）monitoring and analysis,and it is pointed out that AE monitoring and analysis may be greatly affected and limited by the signal overlap and attenuation when a large number of fractures occur,and it is determined that the AE analysis in this paper is primarily focused on time sequence analysis.（2）Analyze the catastrophic failure phenomenon of hard rocks under flexible loading,and a framework for evaluating the intensity proneness of rock catastrophic failure is proposed by combining AE and energy analysis.It is pointed out that under flexible loading,catastrophic failure tends to occur in hard rocks,which combines the results of rock properties and loading conditions,and is consistent with the occurrence of rockbursts and other disasters in the field.It is proposed that the evaluation of rock catastrophic failure is significantly different from the traditional brittleness evaluation.The intensity of rock catastrophic failure is related to the ability to store energy before failure and absorb energy and quickly consume and release a large amount of energy at the moment of failure.By combining AE and energy analysis,the possible energy U_d&rd consumed and released at the moment of catastrophic failure can be quantitatively estimated,realizing the evaluation of the intensity proneness of rock catastrophic failure.（3）Conduct an experimental study on rockburst（strainburst）failure of different hard rocks.The rockburst failure of six common types of hard rocks in deep engineering is simulated and reproduced under one-free-face true triaxial loading conditions.The results show that for granodiorite,granite（or marble）and other rocks with large grain size,complex mineral composition and poor homogeneity,stress-induced micro-or meso-scale fractures are largely generated from the earlier loading states,and the rockbursts are characterized by obvious tensile splitting/slabbing,fragmentation and ejection near the free face and shear rupture away from the free face;for basalt,sandstone（or limestone）and other fine-grained,homogeneous or mineral grain strictly interlocked rocks,micro-and meso-scale fractures occur in local areas until near the peak stress,and the rockbursts（or brittle failure）are characterized by local rupture near the free face and dominated by shear rupture or faulting throughout the rock.These two modes of rockbursts correspond to slabbing and buckling-shear rupture burst and shear rupture burst respectively,which,coupled with slabbing and buckling burst,are considered as three specific failure modes of strainbursts.Different rockburst failure modes affect the strategies and methods for rockburst prediction,prevention and control and proneness evaluation.U_d&rd and the kinetic energy k have different meanings for the evaluation of rockburst proneness.（4）Carry out experimental analysis on slabbing and shear rupture failure characteristics of hard rocks and their correlation with rockbursts.Slabbing and shear rupture failures are simulated by conducting biaxial tests with variable intermediate principal stresses and true triaxial tests with different low minimum principal stresses on selected representative granite and sandstone.It is found that when compared with the free state,a small intermediate principal stress and minimum principal stress can change the spatial pattern of cracking,that is,parallel toσ₁→parallel toσ₁-σ₂→constrained in three directions and local concentration,and consequently significantly improve the strength of the rock sample.The coarse-grained granite is more sensitive to lateral stress compared to the fine-grained sandstone.Under weak flexible loading controlled by deformation,the fracturing and failure process of slabbing is gradual and generally occurs when the post-peak stress is significantly reduced,and the intensity proneness of catastrophic failure tends to be low,but the fragmentation characteristic is evident;while the shear rupture failure process occurs suddenly usually near the peak stress,coupled with obvious localization characteristics,and the intensity proneness of catastrophic failure tends to be high,but the fragmentation degree is low.Additionally,under an extremely high intermediate principal stress or strong flexible loading controlled by stress,the slabbing structure tends to rapidly break off and collapse overall,resulting in strong rockburst ejection.Lateral stress can significantly affect the rockburst ejection failure proneness of surrounding rocks.In the failure processes of slabbing and shear rupture,AE counts presents multi-and single-peak rapid accelerations,respectively.This indicates that compared to shear rupture failure,macrofracture coalescence in slabbing failure is discrete not only in space but also in time.The accelerations are similar to the power-law accelerated growth or decrease（described by Omori’s law）of fracturing before or after earthquakes and other catastrophes,therefore rockburst early warning can be carried out accordingly.（5）Conduct an experimental study on the influence of joints or flaws on rockburst failure.The experimental simulation of rockburst or brittle failure of surrounding rocks with joints or flaws rock is achieved under one-free-face true triaxial loading conditions,by prefabricating flaws of different distributions on representative samples of granite.The study shows that joints or flaws（except for the 0°distribution,parallel to the tangential direction）tend to start and expand at relatively low-stress levels and eventually cause the overall failure,which significantly reduces the rock strength and changes the original slabbing and buckling-shear rupture failure mode of the granite.With the decrease of strength and the change of failure mode,the intensity proneness of rock catastrophic failure decreases,and the degree of fragmentation decreases and the slabbing phenomenon near the free face weakens or disappears.Consequently,the joint or flaw（except for the 0°distribution）weakens the rockburst ejection failure,which is different from large-scale structural planes that can generally promote and intensify rockburst failure.