| In actual engineering,in order to effectively reduce the engineering problems caused by the excavation unloading damage effect,the damaged rock mass will be reinforced during the excavation process to improve its strength and stability.Based on this,this paper selects the excavation limestone in a hydropower station project as the research object,combined with the indoor test and theoretical analysis,and establishes the PFC2D particle flow discrete element model to study the restraint effect of the reinforcement on the unloading damage of the limestone and its microscopic mechanism,The main research content and results are as follows:(1)In order to study the influence of the unloading stress path on the mechanical properties and energy evolution of limestone,the unloading tests of limestone under three different unloading stress paths with constant bias pressure unloading confining pressure,constant axial pressure unloading confining pressure,and axial pressure unloading confining pressure are designed.And after unloading to the corresponding unloading amount,load failure again.The research results show that under the constant bias pressure unloading confining pressure unloading stress path,the strength and deformation parameters of the rock sample deteriorate more significantly when the unloading amount is greater than 85%.The degree of degradation of the lateral and axial strain confining pressure compliance,deformation modulus,and compressive strength from the initial values are 365.19%,313.10%,17.32%,and 20.00%.;The greater the initial confining pressure,the more significant the deterioration of deformation parameters during unloading.When the initial confining pressure is 30MPa,the lateral and axial strain confining pressure compliance,Poisson's ratio,increase by 0.750MPa-1,0.470MPa-1 and0.058,and deformation modulus decrease by 7.841GPa;Comparing the three kinds of unloading stress paths,the deformation parameters of the rock sample under the unloading stress path under axial pressure unloading confining pressure have the most obvious degradation.The compressive strength of the rock sample after reloading is lower than that of triaxial compression,and the maximum under the stress path of confining pressure unloading is 14.15%;With the increase of unloading amount,the failure mode of the rock sample transitions from shear failure to shear and tension failure,and finally changes to tension and shear failure;On this basis,the energy evolution law and damage variable changes of rock samples are studied,and it is found that under constant bias pressure unloading confining pressure unloading stress path,the dissipated energy and damage variables are the largest overall,with an average of 3.126MJ·m-3 and 0.826.(2)On the basis of different stress path unloading tests,5 kinds of unloading amounts of70%,75%,80%,85%,90%and 4 kinds of reinforcement level of 50%,70%,90%,100%are considered,and the confining pressure is applied to simulate the reinforcement effect after unloading to the corresponding unloading amount,and the unloading-damaged limestone reinforcement and reloading test are carried out.The study found that when the unloading amount is 70%,the compressive strength of the rock sample under the unloading stress path of constant bias unloading confining pressure is increased by 23.92%compared with the unloading failure strength,and the axial and lateral cumulative strain is reduced by 82.47%and91.52%,the unloading damage suppression effect is the most obvious;With the increase of the initial confining pressure,the compressive strength,cumulative strain and peak strain of the rock sample show a non-linear increase trend.The larger the reinforcement level,the better the reinforcement effect.When the initial unloading amount is 70%and the reinforcement level is100%,the compressive strength is increased by 23.92%,and the cumulative axial and lateral strains are reduced by 81.61%and 91.54%;Small unloading damage and reinforcement process have little effect on the failure mode of the rock sample.The failure mode of the rock sample under the unloading stress path of constant bias pressure unloading confining pressure takes85%of the unloading amount as the turning point from shear failure to shear tension.The effect of failure transformation and unloading amount on the failure mode of rock samples is smaller under the stress path of constant axial pressure unloading confining pressure and axial pressure unloading confining pressure unloading.(3)In the reinforcement and reloading test of unloading damaged limestone,according to the theory of loading and unloading response ratio and the calculation formula of energy density,the energy evolution law of the rock sample and the changes of loading and unloading response ratio and damage variables before and after reinforcement are obtained.,From the energy point of view,study and analyze the restraint mechanism of reinforcement on the unloading damage of limestone.The results show that the loading and unloading response ratio shows a non-linear decrease trend with the unloading amount 85%as the turning point.When the unloading amount is 90%,the loading and unloading response ratio decreases the largest by 85.34%compared with the initial value.The dissipated energy and damage variables in the unloading stage of the rock sample also increase nonlinearly with the increase of the unloading amount,and the maximum increase is 0.282MJ·m-3 and 82.76%when the unloading amount is 90%.Compared with the end point of unloading,the damage variable of the reinforcement end point is reduced,and the maximum reduction is 27.92%when the unloading amount is 70%;Comparing the three unloading stress paths,the loading and unloading response ratio under the stress path of axial pressure unloading confining pressure decreases by 45.13%at the minimum.The maximum reduction of dissipated energy in the reinforcement stage is 28.67%,and the damage variables of each stage under the constant bias unloading confining pressure unloading stress path are the largest overall,and the maximum unloading end point is 0.53 when the initial unloading amount is 90%.(4)Use the PFC2D particle flow discrete element program to establish a numerical model of the rock sample,simulate the triaxial unloading failure of the rock sample under three unloading stress paths and the unloading-reinforcement-reloading failure test.Analyze the propagation and evolution of micro-cracks in the development process of unloading damage,and the restraint effect of reinforcement on the micro-cracks developed due to unloading damage.The research results show that when the unloading amount is less than 70%,the total number of microscopic cracks in the unloading damage development process is the largest under the constant bias unloading confining pressure unloading stress path,which is an increase of 7331 from the unloading starting point.The total number of cracks during unloading failure is also the largest,which is 83.10%more than when the unloading amount is 90%,and the distribution is the most dense.After reinforcement,the number of micro-cracks is the least reduced,and only 1.13%is reduced when the initial unloading amount is 90%;Under the constant axial pressure unloading confining pressure unloading stress path,when the unloading amount is greater than 80%,the rate of increase of the number of cracks begins to gradually increase.After reinforcement,the number of cracks in the rock sample decreases more significantly than the end of unloading.When the amount is 70%,it is reduced by 27.29%;Under the stress path of unloading confining pressure with axial compression,the formation and growth rate of micro-cracks are the slowest during the development of unloading damage,which only increases by 14.58%when unloading to 90%of the unloading amount.The reinforcement effect has the most obvious inhibitory effect on crack development and propagation.When the initial unloading amount is 70%,the reinforcement end point is reduced by up to 40.66%compared to the unloading end point.The distribution of cracks in the numerical simulation test at the time of the final failure under the three unloading stress paths is consistent with that in the laboratory test;The fracture degree and damage suppression degree of the rock sample are defined.It is found that the maximum fracture degree under the constant bias unloading confining pressure unloading stress path is 0.467,and the damage inhibition degree is the maximum 0.412 under the axial pressure unloading confining pressure unloading stress path;The reinforcement effect reduced the proportion of the total number of cracks in the rock sample from 34.47%to 32.73%,and the proportion of the number of tension cracks decreased by 6.49%,which transformed the sample to shear failure. |
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