Research On The Influence Of Dacite Unloading Damage Evolution On The Stability Of Underground Caverns Under Complex Structural Conditions

To reveal the mechanical damage properties of rock in the unloading process,as well as analyze the stability of underground cavern with complex geological conditions,in this research,the geological background,topography,stratum lithologic,geological structure and the natural ground stress of LCRM Hydropower Station were investigated.Then,a triaxial unloading test with acoustic emission(AE)monitoring and a triaxial compression test as a control group were conducted under initial confining pressures of 5MPa,10MPa,20MPa,and 30MPa.So that rock deformation and fracture mechanism,stress-strain curve relationship,dynamic damage law of mechanical parameters and AE activity characteristics were discussed.Furthermore,the evolution equation of unloading damage was derived theoretically,developed in FLAC software,and applied to the stability analysis of underground caverns.The following main research results were obtained:(1)The neotectonic movement is active and there are historical records of earthquakes due to the location of a junction on the Qinghai Tibet Plateau where are many blocks and suture zones and the effect of Zhuka fault.The overall terrain is high in the north and low in the south,which is a typical high mountain canyon deep cutting area.There are commonly dacites gray white and gray black colors which belong to Zhuka Formation of the Middle Triassic(T₂z).Meanwhile,the study area formed a strong weathering and strong unloading transformation environment,complex geotechnical structure environment,complex hydrogeological environment,and deep overburden riverbed environment under the action of long-term erosion and undercutting in the valley.And the natural geostress here is high,with the maximum principal stress of 23-28.3 MPa,the intermediate principal stress of17.1-18.3 MPa,and the minimum principal stress of 8-9.6 MPa.(2)Results of laboratory test showed that the X type elastic-plastic shear failure mainly occurred in rock samples under loading conditions,while the rock showed strong expansion characteristics under unloading conditions.The stress dropped rapidly after failure,and the brittle characteristics were significant.The lateral strain and volumetric strain were large,and the failure rock samples were broken.The cohesion and deformation modulus decreased during unloading confining pressure,and the internal friction angle and Poisson's ratio increased.With the increased of confining pressure,the peak stress and axial strain of rock samples under loading and unloading conditions increased.However,under the same initial confining pressure,the peak stress under unloading conditions was less than that of loading test,indicating that rock samples under unloading conditions were more prone to failure.In addition,the AE activity characteristics well reflected the rock deformation and fracture process,and the number of AE ringing and the cumulative number of AE ringing under unloading conditions were also more than those under loading conditions.(3)Based on the generalized elastic-plastic theory,Mohr-Coulomb,Griffith and other failure criteria,it was assumed that the rock sample was uniform and complete in texture,and its mechanical properties conformed to the isotropic assumption.The deformation and failure satisfied the basic material mechanics assumptions such as continuity,small deformation,and plane section.Taking the cumulative number of AE ringing as the intermediate variable of damage variables,combined with the rock micro-element hypothesis and Weibull distribution function,the functional relationship between basic mechanical parameters and strain deterioration was established.The deformation and failure process was divided into elastic deformation stage,unloading yield stage,brittle failure stage and residual strength stage,and the unloading damage constitutive model of rock was established only from the corresponding relationship between stress and strain in stages.(4)With the help of Microsoft Visual Studio 2010 compiler C++development language and the Mohr-Coulomb model as the blueprint,the basic categories,membership functions,constitutive registration,information exchange between constitutive and FLAC^3D main program,constitutive state indicator and other information were rewritten,and the secondary development of unloading damage constitutive model was realized.The standard cylinder model with a diameter of 50mm and a height of 100 mm was established in FLAC^3D,and the triaxial unloading numerical simulation test was carried out and compared with the laboratory test results to verify the rationality of unloading damage constitutive model.(5)Taking LCRM Hydropower Station in Tibet as an example,Rhino 6.0,Griddle and FLAC^3D software were used to establish the model of underground cavern.The results of Mohr-Coulomb model and unloading damage model showed that the maximum principal stress was mainly concentrated around the diversion tunnel and tailrace tunnel.The maximum principal stress concentration value of unloading damage model was relatively small,but the distribution range was significantly larger than that of Mohr-Coulomb model.The minimum principal stress had a certain stress concentration in the side wall of the main plant,the main transformer tunnel and the surge chamber,and the concentration degree of the right side wall of the main transformer tunnel reached the maximum.The stress relaxation zone of different degrees and ranges appeared in the disturbance range of the vault,the bottom plate and the tailrace tunnel of the tunnel,and the local tensile stress appeared.The stress relaxation zone range and the maximum tensile stress value under the unloading damage model were larger than those under the Mohr-Coulomb model.The shear stress showed obvious stress concentration near the cavern,and takes the center of the cavern as the symmetry point.The shear stress distribution was conjugate along the diagonal of the top arch and the bottom plate in the upstream and downstream of the cavern.The shear stress value was larger at the intersection of the top arch and the side wall,the side wall and the bottom plate,the traffic corridor,the tailrace tunnel and the cavern.The maximum shear stress value in the unloading damage model was less than that in the conventional Mohr-Coulomb model.The vault subsidence,floor heave and side wall horizontal extrusion were caused by the excavation of the tunnel,and the calculation results of unloading damage model were 3.5 times that of Mohr-Coulomb model.The deformation and failure of the two models,the deformation and failure of the underground cavern were mainly manifested as shear yield.The plastic failure zone first appeared at the top arch position of the cavern,and then expanded to the turning cross position of the side wall,the floor and the traffic corridor,and the tailrace tunnel,and gradually extended to the deep part of the rock mass around the cavern.Finally,a complete plastic deformation failure zone was formed around the cavern,indicating that these positions were the weak links that may occur shear failure after the excavation of the underground cavern,and they should be supported in time to ensure the stability of the cavern.