Study On The Creep Failure Mechanism Of Sandy Mudstone Based On Micro-Mesoscopic Mechanics

Essentially,the creep behavior of rock is the result of constant damage,degradation,refinement and sliding of its microscopic component under the constant loading.Currently,most of researches focus on the macroscopic deformation characteristics and regular patterns of rock in the creep action.And the studies on the micro-mesoscopic mechanisms of rock creep behavior are far from enough.With the problems of geotechnical engineering being increasingly complex,it will be of great significance to deepen the understandings of the inherent mechanisms of rock creep behavior.In this study,the sandy mudstone samples from KouZiDong Coal Mine were taken as the main research object.Mechanisms of sandy mudstone creep behavior was investigated based on micro-mesoscopic mechanics.The main research contents and conclusions are listed as below:(1)The micro-mesoscopic structures of typical sandy mudstone,sandstone and coal from KouZiDong Coal Mine were analysed in order to reveal the mechanisms of different rocks' deformation behavior.The porosity of 3 kinds of rocks showed that coal > sandy mudstone > sandstone.And the cementing(bonding)status showed that sandstone > sandy mudstone > coal.The sandy mudstone was mainly composed of clay minerals.The micro-mesoscopic structure of sandy mudstone plays a key role in its deformation behavior.(2)Micro-mesoscopic rheological tests were conducted on sandy mudstone samples based on AE and DICM technologies.The test results showed that: 1)The AE versus time curve of sandy mudstone samples in the creep action was consistent with the axial rheological strain versus time curve of sandy mudstone samples.Three stages were identified: decelerated stage,steady stage and accelerated(tertiary)stage.2)The AE spatial distribution of sandy mudstone samples in the creep action revealed that the ageing action of constant load was to make the damage in sandy mudstone more homogeneous and more dispersed.3)The surface deformation field of sandy mudstone samples showed symmetrical isoline when the sandy mudstone sample was in the stable creep stage.But this could not be observed when the sandy mudstone sample was in the unstable creep stage and consequently,shear sliding failure occurred.4)Based on the method of variable-order fractional calculus,the creep characteristics of sandy mudstone sample was analysed.(3)SEM and 3D Scanning technologies were adopted to study the creep-induced fracture surfaces' meso-micromorphology of sandy mudstone samples and the box fractal dimensions of micromorphology was calculated.The relation between mesoscopic asperities and micro-mesoscopic shear dilation effect was investigated.The hardening effect observed in the step-loading uniaxial creep test was well explained as the result of internal clay minerals' compaction based on the SEM images.The fracture surfaces of sandy mudstone samples from creep tests were rougher and more dispersed than that from uniaxial compression tests at the micro-mesoscopic level.Hence,micro-mesoscopic shear dilation effect of sandy mudstone fracture surfaces was more obvious.This is an important reason for creep-induced larger deformation.(4)Based on the time-dependent Parallel-Bonded Stress Corrosion Model(PSC)presented by Potyondy,the secondary development of PFC creep contact model was accomplished and numerical simulations on sandy mudstone creep behavior was conducted in this study.Based on the large-scale parallel computing capability of “Tianhe-2” supercomputer system,relative molecular dynamics simulation was conducted.The above numerical simulations reproduced the micro-mesocopic damage evolution and failure mechanisms of sandy mudstone in the creep action.The energy curves of the particle system in PFC revealed that: the work done by applied constant force input the energy into the sandy mudstone samples and some energy were dissipated by the cracking processes and damage evolution of sandy mudstone samples.The input and dissipation processes were in the relation of mutual promotion and mutual transformation.The atomic level simulations numerically reproduced the phenomenon of instantaneous elasticity and stress corrosion in the interfaces of quartz contact.It revealed the mechanisms of stress corrosion and stress relaxation of quartz contact interfaces,that is,the high potential energy and contact stress in the quartz contact interfaces significantly increase the atoms' abilities to run out of the quartz crystal.And time increases the probability of ultimate escape.The quartz crystal is destroyed when the atoms run out of it.Hence,the initial contact pressure is decreased constantly.The stress corrosion and relaxation occurr.At the same time,this numerical simulation proved that the mechanism of stress relaxation is the dissipation of strain energy(potential energy)of quartz crystal,which is somewhat different from that of creep.