Study On Deformation Localization And Catastrophic Rupture Precursor Information Of Flawed Sandstone

With the development of mineral resources exploitation,energy production,underground space,and hydropower transportation engineering,rock mass’s stability and fracture failure have become increasingly prominent.Under the action of external load and gravity,many discontinuities such as holes,fissures,and joints gradually evolve into localized damage,leading to coalescence and penetration.It is challenging to predict and prevent the fracture instability of rock mass.Therefore,this paper focuses on the theme of“study on the deformation localization and catastrophic rupture precursor information.”This study systematically analyzes the characteristics of deformation localization and fracture failure modes of flawed rock.It explores the macro-and micro-cracks evolution mechanism of the flawed sandstone during the loading process.The application of acoustic emission（AE）signal to identify the failure precursor information of flawed rock is analyzed in detail.Finally,based on the backpropagation neural network modeling（BPNN）,the empirical relationship of instability rupture time is obtained.It considers the input variables,such as test loading conditions,filling conditions,flaw inclination,rock ligament angle,peak strength,elastic modulus,localized zone inclination,and localized zone thickness.The following research contents and conclusions can be drawn:（1）A series of uniaxial compressive tests on non-filled and gypsum-infilled flawed sandstone with different flaw geometries were conducted using MTS815 rock mechanics testing system,three-dimensional digital imaging（3D-DIC）technology,and PCI-ⅡAE system.The characteristics of deformation localization and crack coalescence modes are analyzed.The results show that the peak strength and elastic modulus of the non-filled and gypsum-infilled flawed sandstone present an‘inverted’Gaussian with the increase of the ligament angle and reach the minimum at the ligament angle of 60°.A total of ten crack types and four coalescence modes are obtained.The penetration failure mode changes from indirect penetration,which is approximately parallel to the axial tension-shear mixed failure,to direct penetration of the oblique shear failure.In addition,three types of crack stress levels are identified under non-filling and infilling conditions using AE technology.The filling increases the threshold of the crack stress level,and the percentage of crack initiation stress is more significant than those of coalescence and peak stresses.Furthermore,whether the cracks are infilled or not,the tensile cracks nucleate and initiate at low stress levels.By contrast,the shear cracks start to initiate and develop when approaching the peak stress.（2）A series of biaxial loading tests on gypsum-infilled flawed sandstone are conducted using WDAJ-600 dual-axis electro-hydraulic servo control system,3D-DIC system,AE system,and electron microscope system（SEM）,which is used to study the effect of lateral pressure on the deformation localization characteristics,crack penetration mode and meso-flaws characteristics of flawed sandstone.The results show that the macroscopic crack connection and penetration are restrained to some extent with lateral pressure.Also,the initiation stress level of shear cracks gradually decreases.On the contrary,the initiation stress level of tensile cracks gradually increases.In addition,from the micro-scale of cracks for the typical sandstone samples,it can be found that the phenomenon of angular crystal,transgranular fracture and more debris gradually evolves into shear sliding along the grain,intergranular fracture,and more debris as the lateral pressure increases.（3）Based on the constructed discrete element numerical modeling,considering the mineral composition,the macroscopic deformation and fracture phenomena generated in the loading are further explained from mesoscale.The results show that the cracking level mainly includes the microcrack closure stage,crack initiation stage,slow crack growth stage,uniform crack growth stage,and accelerated crack growth stage.With the increase of the ligament angle,the crack coalescence mode still changes from direct penetration to indirect penetration.The initiation stress levels of micro-tensile and shear cracks increase with the increase of the flaw inclination.Meanwhile,the number of micro-cracks increases with lateral pressure,and the proportion of tensile cracks gradually decreases.In contrast,the proportion of shear cracks gradually increases.In addition,based on the measuring circle method,the stress and displacement vector fields of flawed sandstone around the pre-existing flaws are carried out to further confirm the stress transmission and transferring mechanism due to the existence of infilling from the perspective of mesco-scale.（4）Based on the R/S statistical analysis method,the characteristics of nonlinear time series of AE signal and the evolution mechanism of micro-crack of non-filled and gypsum-infilled flawed sandstone under different loading conditions were quantitatively characterized.The results show that under the same flaw geometric parameters,the fractal dimension of the non-filled sample is more significant than that of the gypsum-infilled one,and the fractal dimension has a negative correlation with the lateral pressure.Subsequently,based on the multifractal method,the fractal geometric characteristics of the AE signal for flawed sandstone under different stress stages are characterized in detail.The results show that the average bandwidth（Δα）shows a trend of the first decrease and then increase.The fractal dimension value decreases obviously when the sample approaches the instability rupture with the increasing stress level.When the stress level is less than 0.8σ_c,the bandwidth difference（Δα₀）is greater than zero.When the stress level is more significant than 0.8σ_c,Δα₀ gradually changes from a positive value to a negative one.The fractal spectrum parametersΔf andΔα₀ show the opposite trend.Finally,based on the time-frequency characteristics of the original AE waveforms,the frequency spectrum behavior of flawed sandstone is analyzed in detail.The fracture mechanism is dominated by micro-tensile and shear cracks,irrespectively of uniaxial and biaxial loading conditions.With the increase of lateral pressure,the dominated microcrack mechanism is gradually transferred from micro-tensile fracture to micro-shear fracture.（5）Based on the characteristics of AE b value in the loading and rupture process,it is shown that this parameter is reliable as the identification of precursor information.Also,based on the Omori-Utsu time inversion law and critical slowing down theory,the characteristic points,including the precursory warning signal,local instability rupture,and final instability failure in the cracking process,have been identified.Finally,using the BPNN modeling,the empirical relationship of the instability rupture time of flawed rock is obtained.The weight of the input variables on the prediction modeling is discussed.It is found that the loading condition has the most significant influence on its relative importance.