Research On Long-Term Stability Of Arch Dam And Creep Properties Of Abutment Rock Under Water-Rock Coupling

The high dam project plays an irreplaceable role in flood control safety,rational utilization of water resources,ecological environment protection,promotion of national economic development,etc.,of which,the long-term stability is not only the guarantee for the stable development of regional economy,but also related to the safety of life and property of millions of people in the downstream of hydropower station.Due to the comprehensive influence of rock weakening,creep,overload and other factors under the water-rock coupling effect in the operation process,the long-term stability problems such as structural instability,seepage failure and bank slope sliding are prone to occur,which seriously threaten the safety and long-term operation of the project.To study the strength weakening and creep of abutment rock under water-rock coupling,the rock weakening test of abutment was carried out to analyze the weakening effect of rock strength parameters,and the creep test under real water pressure was carried out to analyze the long-term deformation characteristics of fractured rock in weak interlayer zone of abutment under operation conditions and to establish the nonlinear creep damage constitutive model considering water pressure and damage variables,basing on the world’s highest concrete arch dam Jinping I project.To analyze the long-term stability of dam under water-rock coupling,the numerical simulation of typical elevation of arch dam and comparative verification of geomechanical model test were carried out basing on the weakening law of rock strength parameters and the secondary development of nonlinear creep model.The main results of the study are as follows:（1）The indexes of critical water pressure of rock weakening effect,critical damage ratio and damage index are proposed.The cohesive force of rock is weakened obviously by water pressure,and the weakening critical water pressure P_wris the water pressure when the cohesion weakens to 0 which can be used to define the influence of water pressure on rock strength as the limit value.The damage ratio corresponding to the peak stress of rock is called critical damage ratio D_er,and it can be used as a parameter to characterize whether the rock is damaged or not,and has a good linear relationship with the logarithm function of confining pressure.With the increase ofσ₃,the increasing trend of D_er slows down and tends to a certain value.The damage index C_c is proposed to be the slope of D_er-lnσ₃curve,and it can be used to judge the damage degree of rock.The larger the damage index is,the more severe the damage degree of rock is.（2）The creep law of fractured rock in weak interlayer zone of dam abutment under real water pressure environment is recognized and the nonlinear creep damage constitutive model of fractured rock considering water pressure,damage correction coefficient and initial damage is established.The creep test of rock carried out on YSL-200 rheometer can reflect the interaction of water and rock in actual engineering and it shows that:The"wedging effect"of pressure water along the fracture promotes the creep development of rock and reduces the long-term mechanical properties of rock,that the larger the water pressure is,the more obvious the"wedging effect"is,the higher the accelerated creep rate is,the shorter the duration of acceleration stage is,the smaller the load grade of creep failure is,and the lower the long-term strength is.The long-term strength of fractured rock accounts for 50%～70%of the peak strength of intact rock.It is suggested that the maximum first-order strength in stable creep stage should be taken as the long-term strength of fractured rock.Based on the creep test results of fractured rock under real water pressure,the nonlinear creep damage constitutive model of fractured rock is established by constructing the influence formula of water pressure on the viscosity coefficient,considering the weakening effect of damage variables on the viscosity coefficient and elastic modulus,and introducing the influence of damage correction coefficient and initial damage on the damage evolution of fractured rock.The secondary development and verification of the model are realized by using C++language according to the three-dimensional difference form of nonlinear creep model and the source code of creep constitutive model built in FLAC^3D.（3）The long-term stability characteristics of arch dam considering the fusion of rock weakening,creep and overload are discussed.To reveal the long-term stability of Jinping I arch dam,the comparative analysis of geomechanical model test considering comprehensive effects of rock weakening,creep and overload and numerical simulation in which the second developed nonlinear creep constitutive model is applied of arch dam is carried out.The results show that:Under the condition of design water level,the long-term deformation of right abutment tends to be stable in about 2 years,the long-term deformation of left abutment tends to be stable in about 3 years,the deformation of dam body tends to be stable in about 3years and the the deformation monitoring value is consistent with the calculated value.The long-term deformation of the arch dam is asymmetric:the left half arch is greater than the right half arch,the left abutment is greater than the right abutment,and the long-term deformation stability time of the left bank is greater than that of the right bank.The closer and the denser the weak structural plane is to the arch end and the more serious the strength weakening of the weak structural plane is,the failure of dam abutment is more prone.The abutment failure is mainly divided into three zones:tension shear zone at the upstream of arch end,compression shear zone at the downstream of arch end and shear slip zone along structural plane of left abutment.The main reason for the failure of the left abutment is the shear slip along the x-lamprophyre.The arch dam lose its stability while the overload coefficient is about 5.0,and the long-term deformation and failure process is in the order of shear sliding of weak structural zone,integral instability of dam abutment and tension crack of dam body.The results of numerical calculation and geomechanical model test show that the long-term deformation of arch dam is similar to the failure mechanism of abutment instability,and the research results confirm each other.From the overload safety factor,the dam abutment considering rock creep,weakening and overload multi factor fusion is stable.