Modeling time-dependent mechanical behavior of hard rock considering excavation-induced damage and complex 3D stress states

To investigate the long-term stability of deep rocks,a three-dimensional(3D)time-dependent model that accounts for excavation-induced damage and complex stress state is developed.This model comprises three main components:a 3D viscoplastic isotropic constitutive relation that considers excavation damage and complex stress state,a quantitative relationship between critical irreversible deformation and complex stress state,and evolution characteristics of strength parameters.The proposed model is implemented in a self-developed numerical code,i.e.CASRock.The reliability of the model is validated through experiments.It is indicated that the time-dependent fracturing potential index(xTFPI)at a given time during the attenuation creep stage shows a negative correlation with the extent of excavationinduced damage.The time-dependent fracturing process of rock demonstrates a distinct interval effect of the intermediate principal stress,thereby highlighting the 3D stress-dependent characteristic of the model.Finally,the influence of excavation-induced damage and intermediate principal stress on the time-dependent fracturing characteristics of the surrounding rocks around the tunnel is discussed.

Shear band evolution and acoustic emission characteristics of sandstone containing non-persistent flaws

Direct shear tests were conducted on sandstone specimens under different constant normal stresses to study the coalescence of cracks between non-persistent flaws and the shear sliding characteristics of the shear-formed fault.Digital image correlation and acoustic emission(AE)techniques were used to monitor the evolution of shear bands at the rock bridge area and microcracking behaviors.The experimental results revealed that the shear stresses corresponding to the peak and sub-peak in the stressdisplacement curve are significantly affected by the normal stress.Strain localization bands emerged at both the tip of joints and the rock bridge,and their extension and interaction near the peak stress caused a surge in the AE hit rate and a significant decrease in the AE b value.Short and curvilinear strain bands were detected at low normal stress,while high normal stress generally led to more microcracking events and longer coplanar cracks at the rock bridge area.Furthermore,an increase in normal stress resulted in a higher AE count rate and more energetic AE events during friction sliding along the shearformed fault.It was observed that the elastic energy released during the crack coalescence at the prepeak stage was much greater than that released during friction sliding at the post-peak stage.More than 75%of AE events were located in the low-frequency band(0e100 kHz),and this proportion continued to rise with increasing normal stress.Moreover,more AE events of low AF value and high RA value were observed in specimens subjected to high normal stress,indicating that greater normal stress led to more microcracks of shear nature.

A novel indirect optical method for rock stress measurement using microdeformation field analysis

Stress measurement plays a crucial role in geomechanics and rock engineering,especially for the design and construction of large-scale rock projects.This paper presents a novel method,based on the traditional stress relief approach,for indirectly measuring rock stress using optical techniques.The proposed method allows for the acquisition of full-field strain evolution on the borehole’s inner wall before and after disturbance,facilitating the determination of three-dimensional(3D)stress information at multiple points within a single borehole.The study focuses on presenting the method’s theoretical framework,laboratory validation results,and equipment design conception.The theoretical framework comprises three key components:the optical imaging method of the borehole wall,the digital image correlation(DIC)method,and the stress calculation procedure.Laboratory validation tests investigate strain field distribution on the borehole wall under varying stress conditions,with stress results derived from DIC strain data.Remarkably,the optical method demonstrates better measurement accuracy during the unloading stage compared to conventional strain gauge methods.At relatively high stress levels,the optical method demonstrates a relative error of less than 7%and an absolute error within 0.5 MPa.Furthermore,a comparative analysis between the optical method and the conventional contact resistance strain gauge method highlights the optical method’s enhanced accuracy and stability,particularly during the unloading stage.The proposed optical stress measurement device represents a pioneering effort in the application of DIC technology to rock engineering,highlighting its potential to advance stress measurement techniques in the field.

Rock damage and fracturing induced by high static stress and slightly dynamic disturbance with acoustic emission and digital image correlation techniques

A series of coupled static-dynamic loading tests is carried out in this study to understand the effect of slightly dynamic disturbance on the rocks under high static stress.The acoustic emission(AE)and digital image correlation(DIC)techniques are combined to quantitatively characterize the damage and fracturing behaviors of rocks.The effects of three influencing factors,i.e.initial static stress,disturbance amplitude,and disturbance frequency,on the damage and fracturing evolution are analyzed.The experimental results reveal the great differences in AE characteristics and fracturing behaviors of rocks under static loads and coupled static-dynamic loads.Both the Kaiser effect and Felicity effect are observed during the disturbance loading process.The crack initiation,stable and unstable propagation in the highly-stressed rocks can be triggered by cyclic disturbance loads,and more local tensile splitting cracks are found in the rocks subjected to coupled static-dynamic loads.The damage and fracturing evolution of rocks during cyclic disturbances can be divided into two stages,i.e.steady and accelerated stages,and the increase rate and proportion of each stage are greatly affected by these influencing factors.High initial static stress,low disturbance frequency,and high disturbance amplitude are considered to be adverse factors to the stability of the rocks,which would induce a high increase rate of the steady stage and a high proportion of the accelerated stage within the whole disturbance period.Based on the two-stage damage evolution trend,a linear-exponential damage model is employed to predict the instability of the rocks under coupled static-dynamic loads.