Study On Damage And Stability Of Unloaded Rock Mass In Foundation Pit Excavtion Based On Particle Flow Simulation

For a long time,the problem of high steep rocky slope has always been one of the hot issues in geotechnical engineering.As a typical representative of underground engineering,foundation pit engineering is increasing in depth,larger in scale,more complicated in environment and more cumbersome in process.At present,a mountain of work has been carried out on the accumulation and expansion of damage during the excavation of foundation pits.But there are still shortcomings.It is of great scientific significance to study the unloading characteristics and stability of rock slopes.This paper takes the damage process and stability of rock mass during excavation and unloading of foundation pit as research objects under the background of the foundation pit engineering of Anshan Road Station of Qingdao Metro Line 4.The mesoscopic damage expansion and overall stability evolution caused by excavation and unloading of slope rock mass are studied.The main conclusions and results are as follows:(1)In this paper,the geological data and project profile of the foundation pit are collected.A stress-strain-micro-seismic monitoring system was established to obtain the laws of stress accumulation-release,rock mass deformation,damage distribution and development during the excavation of a rock foundation pit.The stress evolution characteristics in the process are as follows: the vertical stress fluctuation level is lower than the horizontal stress,and has obvious accumulation-release phase characteristics.The stress levels of each measurement point have significant differences,and the stress has a concentrated distribution inside the rock mass.The micro-seismic event has a high correlation with the excavation process of the foundation pit.The number of events usually increases significantly in a short time after blasting,At the same time,the average energy level rises,resulting in energy accumulation at the tip of the rock.The energy is released as the crack expands.During the construction process,the horizontal deformation of the retaining structure increases significantly in a short time after excavation,and gradually stabilizes with the construction advancement.The deformation curve has a bow shape: The top of the slope and the anchor are less deformed.The depth of the deformation maximum decreases as the depth of the excavation increases.(2)This paper uses the particle flow code to model and calibrate the slope rock mass.Compare the characteristics of various contact models from the perspective of model constitutive.The linear parallel bond model,smooth-joint contact model and linear contact bond model are respectively used to equilibrate the rock mass,joint and support structure respectively.Simultaneously design numerical tests such as uniaxial compression,conventional triaxial,and direct shear to calibrate the mesoscopic parameters of the model.According to the comparison of the macroscopic characteristics of the test results,it can be judged that the particle flow method can ideally realize the simulation analysis of the foundation pit slope engineering.(3)The crack propagation and rock bridge penetration mechanism of rock sample model with different initial confining pressure,unloading rate and pre-cracking form under uniaxial compression and confining pressure unloading conditions are simulated.The mechanical properties,acoustic emission characteristics and damage development forms of the rock samples during each compression test were analyzed.The peak stress and equivalent elastic modulus of a rock sample are affected by the stress path: With the increase of the initial confining pressure,the ultimate strength of the model is correspondingly increased,and the degree of influence is related to the peak stress of the model,while the equivalent modulus of the rock sample has no significant change;As the unloading rate decreases,the peak stress of the rock sample model with higher uniaxial strength will increase accordingly,and the impact degree is lower than the initial confining pressure.At the same time,the equivalent modulus of the rock sample is also reduced by a small margin.The acoustic emission characteristics of the rock sample failure process are also affected by the stress path and pre-crack characteristics of the model.According to the stress growth characteristics of the failure process,the acoustic emission phase is divided into the confining pressure loading phase,the elastic phase,the yielding phase and the destruction phase : The confining pressure loading phase is relatively stable,and the number of acoustic emissions is the least;The acoustic emission frequency begins to increase during the elastic phase,and the acoustic emission event continues;The yield stage has certain precursory characteristics on the micro-seismic characteristics,and the duration of this stage is short,and the number of events increases significantly;With continuous loading,the model enters the destruction phase,and the acoustic emission frequency is greatly reduced to the elastic phase level in a short time.According to the spatial distribution and formation characteristics of microcracks during the test,the failure characteristics of six types of crack propagation are defined: wing tension,rock bridge tension,anti-wing composite,rock bridge composite,coplanar shear and rock bridge shear crack,Class I is an airfoil crack that cracks at the crack tip and expands laterally toward the rock sample at an angle;Class II cracks at both ends of the rock bridge and expands in opposite directions;Class III cracks on the top and bottom of the rock sample and expands toward the end of the prefabricated crack.It has tensile properties at the initial stage and shear characteristics at the later stage.Class IV occurs in the rock bridge,and the expansion process has both tensile and shear properties;Class V starts to crack at the end of the crack and expands in the parallel direction of the crack;Class VI is distributed in the rock bridge and has shear characteristics.In this paper,Class I is ubiquitous in various models.Class III occurs in a model with a large rock bridge strength.Class V occurs only in models with 45° pre-cracking and 34°-61° rock bridge angles.Classes II,IV,and VI occur in models with small rock bridge lengths or large angles,which occur in turn as the rock bridge angle decreases.According to the macroscopic characteristics of the rock sample during the whole test,the conclusion is drawn: When the initial confining pressure increases or the unloading rate decreases,the rock sample failure will gradually change from the tensile characteristic to the shearing characteristic.Confining pressure has a significant inhibitory effect on the formation of microcracks,especially for tensile cracks.(4)Based on the characteristics of the site structure surface,the geological survey information,and the parameters of the numerical test calibration,The joint model of rock slope is established by using equivalent rock technology.Through simulation excavation,the deformation,damage evolution characteristics and stability index of the slope excavation process are explored.The simulation results show that: Slope excavation is a process of accumulation of damage,except for discrete damages in the field.The fracture damage is mainly concentrated in the range of 20m-30 m after the excavation face and 15m-20 m away from the surface,and it expands continuously during the excavation process.The final failure process of the slope is summarized as: initial damage-joint expansion-rock bridge penetration,The failure characteristics are mainly the tensile failure of the rock mass at the top of the slope and the slippage along the joint surface at the bottom of the slope.During the development process,it extends to the central rock bridge,and the final crack penetrates the slope.The stability of the model is analyzed by the particle flow severity increase method.The calculated safety factor is 3.45,and the safety factor is large because the blasting effect is not considered.The unloading response characteristics of slope excavation are significantly affected by different excavation sequences.The mesoscopic appearance is the different forms of damage that the rock exhibits under different unloading paths.The macroscopic realization is the different deformation characteristics reflected in different excavation sequences.The single excavation of the slope produces the largest amount of deformation.With the increase of the excavation step,the amount of deformation caused by a single excavation is correspondingly reduced,but multiple excavations will cause multiple disturbances to the slope.