Failure Mechanism Of Rock Under Loading And Unloading Conditions And Its Application

With the development of foundation engineerings,a lot of excavation engineering problems exist,such as highand steep rock slopes and underground tunnels.Especially,the problems of rock slope excavation and the slope stability are inevitably encountered in water conservancy and hydroelectric engineering,which are directly depended on the rock's strength,deformation and failure.Excavation of rock mass slope engineering is a complex loading and unloading process.In the rock excavation process,internal damage continues to accumulate,micro-cracks are expanded and aggregated and these ultimately connect to cause rock failure.In addition,the rock's strength,deformation and failure are greatly different under loading and unloading conditions.The research on the excavation unloading of the slope by method of loading rock mass mechanics may lead to safely hazards and engineering accidents.This dissertation presents the research on the deformation and failure mechanism of rock under loading and unloading and its application in high stress area,choosing Dabenliu high slope in the Jinping hydropower station as the typical research object,analyzing the deformation mechanism of the slope based on geotechnical investigation,acquiring the mechanical parameters of rock based on the laboratorical tests,building rock damage constitutive model,determining the mechanical parameters of rock masses based on quality evaluation system of rock mass,finally analyzing the deformation mechanism and the slope stability considering the unloading and damage effects based on numerical simulation.The research output will provide theoretical guarantee to the successful construction and safe operation of similar projects and have important theoretical significance as well as engineering practical value.It is an expense issue in the slope engineering that unloading and damage effects are considered in the failure mechanism of rock.This issue relates to numerous aspects like rock mechanics test,the method to determine stress threshold of rock,rock damage constitutive model,the method to determine mechanical parameters of rock mass,the slope stability considering the unloading and damage effects,etc.This paper presents a systematic research on this issue,conbining the geotechnical investigation,laboratorial test,the building of constitution model,the simulation of microscopic test,theoretical study,numerical simulation,etc.The study and achievements are mainly as follows.(1)Triaxial compression tests and triaxial unloading tests are conducted to analyze the change laws of mechanical parameters of rock,energy and damage.Then micro simulation tests under loading and unloading conditions are conducted utilizing PFC3 D software for the purpose of discovering the change laws of microscopic energy parameters,including frictional energy,kinetic energy and strain energy.In addition,the relationships between the number of cracks and the precursors of rock failure during the rock failure process is presented.The results show that when the loading or unloading process continues to proceed,frictional energy caused by friction between particles has an increasing trend,which will influence the deformation and failure modes of rock samples.The rock sample fails by the combined action of compression and tensile-shear failure at the loading or unloading paths.The number of total cracks,the number of tensile cracks and the number of shear cracks totally increase with the increase of the axial strain,characterizing with the feature of S shape.The abrupt point of the number of cracks is in accordance with the deformation stage of the stress-strain curves.(2)Based on the damage mechanics theory,analyzing the whole process of stress-strain curve and micro damage of rock,rock damage constitutive models under loading and unloading conditions are proposed,respectively,of which the rationality are verified by case study.A novel method called the relative compression strain response(RCSR)method is proposed to determine the CI.The result indicates that the RCSR method can be employed to determine the CI of different rock types under uniaxial compression and triaxial compression based on the 227 sets of uniaxial compression test data and 222 sets of triaxial compression test data.Then utilizing the method of macroscopic and microscopic combination,a novel rock damage constitutive model considering the initial damage and residual strength by dividing the rock into three parts: undamaged material,damaged material and micro defects.Finally,introducing the unloading and energy reduction ratio and building the relationships between the unloading,energy reduction ratio and deformation parameters,a novel rock damage constitutive model under unloading condition.(3)By improving the generalized Hoek-Brown criterion and applying it to the study of the degradation of macro mechanical parameters of rock mass,a SMRU method for determining the mechanical parameters of rock mass under unloading condition is put forward.Firstly,on the basis of the excavation degradation factor(De),a novel rock mass quality evaluation system that considers the influences of damage caused by unloading and slope shapes on the degradation of rock mass quality is proposed,which has great reference value for engineering application.Combination the SMRU method and generalized Hoek-Brown croterion,the degradation law of macro mechanical parameters of rock mass in the process of unloading excavation of the slope is analysed.The results show that when the excavation of the slope continues,the mechanical parameters of rock mass are reduced with a certain extent.(4)Assuming the mechanical behavior of rock obeys a statistical damage model,the instability calculation model of stratified rock slope is established by analyzing the failure model of stratified rock slope.Based on the principle of Euler compression bar,the instability model of the vertical stratified rock slope and the model of the dip stratified rock slope are established,respectively,considering the influence of the micro defects on rock mass.Simultaneously,the instability model considering the lateral pressure without overlying layer is analyzed,and the model considering the lateral pressure with overlying layer is also analyzed.In addition,in consideration of the damage of rock mass,the method to calculate shear strength parameters based on Mohr-Coulomb criterion and the method based on Hoek-Brown criterion are derived,respectively.That can be used to evaluate the slope stability by substituting the damage strength parameters into the strength parameters.(5)Considering four conditions,including initial condition,excavationcondition without consideration of unloading,excavation condition with consideration of unloading and supporting condition,the change laws of stress,displacement,shear strain increment and the stability factor in the process of slope excavation are studied by numerical simulation.Then the formation mechanism of the slope deformation is analyzed.The numerical results show that the slope is stable under the whole conditions.In the process of slope excavation,the slope stability is gradually reduced,especially dropped off at a faster rate when considering the excavation unloading condition.It is indicated that the unloading effect has obvious influence on the slope deformation,while the stress and displacement are all within the specification requirements,which conforms to the design requirements.Actually,the deformation of excavation slope is mainly bedding fade deformation,and the compression-shear stress is concentrated at the head scarp of the slope,thus,the most likely mode of the slope deformation is buckling failure at the lower elevation of the slope or shear failure in the toe of slope.