Research On Zonal Disintegration Around Chamber Based On Three-dimensional Photoelastic Test

As the buried depth of underground chambers in the mineral resources exploitation,traffic construction and hydropower development has exceeded 1000 m,the safety and stability control of underground chambers has become the focuses of attention all over the world.The complex environment of deep underground chamber and the appeared zonal disintegration with nonlinear characteristics in the surrounding rock led to the deterioration of rock strength,and reduced the stability of surrounding rock.Aiming to control zonal disintegration and keep the stability of underground chamber,the 3D photoelastic stress freezing method was applied to visually and quantitatively characterize the stress field inside of the surrounding rock of the chamber.Furthermore,the formation mechanism of zonal disintegration was revealed by the distribution of stress field based on the direction and value of maximum principal stress as well as the section shape of chamber.Finally,the bolt support measures were proposed to effectively control the zonal disintegration.The 3D photoelastic stress freezing test loading device has been developed to realize the intuitive and quantitative characterization of the stress field of surrounding rock.In order to ensure the constant and non-eccentric load in the process of stress freezing,a three-dimensional photoelastic stress freezing test loading device was designed to realize the stress freezing of the test model under the constant pressure.Meanwhile,the stress field in the surrounding rock of the chamber could be characterized combined with the digital photoelastic instrument.The influence mechanism of the maximum principal stress direction on the zonal disintegration in the surrounding rock was revealed.The stress distribution of surrounding rock was first investigated in the case of that the maximum principal stress was perpendicular or parallel to the axial direction of the chamber,also the full-field shear stress and the maximum shear stress around the chamber were calculated using the ten-step phase-shift method and the photoelastic stress-optics law respectively.Furthermore,the fracture morphology of the surrounding rock was evaluated based on the Mohr-Coulumb yield criterion and strain localization principle.The direction of the maximum principal stress in surrounding rock of the chamber was parallel to the axial direction of chamber was the prerequisite for zonal disintegration.When the maximum principal stress was perpendicular to the axial direction of the chamber,large wedge cracks occurred at the four corners near the wall of the chamber,while no zonal disintegration formed;when the maximum principal stress was parallel to the axial direction of the chamber,zonal disintegration occurred in the surrounding rock of the chamber.The fracture degree of zonal disintegration and its formation mechanism in the surrounding rock were investigated under different maximum principal stress.According to the stress distribution of surrounding rock with different maximum principal stress values,the maximum shear stress was calculated,and the radius of the first plastic zone of surrounding rock was determined based on Mohr-Coulumb yield criterion.Furthermore,combining with the theory of elastic-plastic mechanics and the strength criterion of deep rock mass,the area and width of fracture zone and the number of fracture circles of surrounding rock mass were quantitatively characterized.The results showed that when the maximum principal stress exceeded 1.1 times the compressive strength of the rock mass,the surrounding rock of the chamber began to generate zonal disintegration.With the increase of the maximum principal stress,the area and width of fracture zone and the number of rupture circles increased.In addition,multiple stress redistribution and elastic-plastic transformation led to the formation of zonal disintegration.As the zonal disintegration extended to the deep surrounding rock,the width of the fracture zone decreases,and the width of the integrated zone increases.The effect of the shape of chamber section on the fracture morphology of zonal disintegration was studied.The full-field shear stress and the maximum shear stress around the chamber were calculated from the stress distribution with circular,horseshoe-shaped,three-core arch-shaped and gate-shaped sections.Combined with Mohr-Coulumb yield criterion and strain localization principle,the fracture morpholiogy of surrounding rock was determined.The results showed that the fracture of the chamber wall with larger curvature was prone to zonal disintegration.The fracture morphology of surrounding rock was a complete circular ring concentric with the chamber when the section of the chamber was circular and horseshoe-shaped.When the section of the chamber was three-core arch-shaped and gate-shaped,the upper surrounding rock of the chamber appeared incomplete circular fracture zone and the two bottom corners of the chamber were broken because of stress concentration,the zonal disintegration was not observed.Based on the formation mechanism of zonal disintegration,the bolt support scheme of ‘supporting the first rupture zone' was proposed,and its effectiveness was verified by numerical simulation and theoretical calculation.The formation mechanism of zonal disintegration was the results of stress redistribution after the formation of the first fracture zone,and the multiple fracture zones appeared as the time development.Therefore,the control of the first fracture zone by bolt supporting was the key to control the occurrence and evolution of subsequent zonal disintegration.From the numerical simulation and theoretical analysis,the bolt supporting could effectively reduce the area and width of fracture zone and the number of fracture cycles,which verified the effectiveness and rationality of the bolt support scheme to reduce and restrain the zonal disintegration.3D photoelastic stress freezing test could visually present the distribution of stress field around the chamber,thus revealing the influence effect of maximum principal stress direction,magnitude and chamber section shape on zonal disintegration.The bolt support strategy based on the formation mechanism of zonal disintegration was the basis of formulating the support scheme;the above results had a good guiding significance for the construction design and safety maintenance of deep chamber.