Study On Splitting Failure Mechanism And Engineering Application Of High Sidewall Cavern Under High In-situ Stress

With the rapid development of the national economy,China demand for resources and energy has increased sharply.Many high sidewall cavern under construction and planning,inter basin water transfer projects and diversion tunnels continue to move deep.With the increasing excavation depth of these underground projects,a series of new characteristic scientific phenomena have appeared in deep rock mass engineering.Among them,splitting failure of high sidewall cavern is a special engineering failure phenomenon caused by cavern excavation and unloading under the condition of true three-dimensional high stress,This phenomenon has become an important factor affecting the safety and long-term operation stability of underground cavern construction and excavation,and has attracted the general attention of scholars.Now there is no clear understanding of the formation mechanism of the splitting failure of the high side wall cavern,and there is no effective solution to how to control the splitting failure of the cavern.Only by thoroughly clarifying splitting failure mechanism induced by unloading of high side wall cavern excavation under the condition of true three-dimensional high in-situ stress,can we formulate scientific support and control measures to ensure the safety of construction,excavation and operation of the underground powerhouse.Taking the high sidewall cavern of the main powerhouse of Pubugou Hydropower Station as a case study,the true three-dimensional geomechanical model test of excavation and unloading of high sidewall cavern under high in-situ stress is carried out,and the generation conditions and influencing factors of splitting failure phenomenon are systematically studied;Based on the geomechanical model test,a elastoplastic damage-softening model of splitting failure of underground cavern considering plastic dissipation work is established by theoretical method.The analytical analysis method of splitting failure is constructed according to the complex function,and the calculation and analysis program of splitting failure is developed based on ABAQUS platform.Through model test,theoretical research and numerical simulation,the generation conditions and failure mechanism of splitting failure are effectively revealed.The main research results are as follows:(1)Through the true three-dimensional physical model test of splitting failure of high side wall cavern under high ground stress,the phenomenon of parallel layered splitting failure due to excavation-induced unloading is finely simulated,the generation conditions and influencing factors of splitting failure are revealed,and the displacement The strain and stress show the law of oscillatory attenuation with the interval distribution of wave crest and wave trough.(2)By analyzing the size effect and strain gradient characteristics of rock failure process,a yield function containing strain gradient which can describe rock plastic hardening and strain softening is established.Based on strain gradient and plastic damage theory,an elastoplastic damage softening model of deep cavern splitting failure is established.(3)Through the conformal transformation method of complex function,the mapping function from the outer domain of high side wall cavern to the outer domain of unit circle is obtained,the complex expression of relevant displacement field and stress field is deduced,the elastic-plastic analytical analysis method of splitting failure of high side wall cavern is proposed,and the analytical variation law of displacement and stress of high side wall cavern under high ground stress is obtained.(4)According to the established elastic-plastic damage softening model of splitting failure,the implicit time integration scheme is proposed,the high-order hexahedron element is constructed,the shape function and stiffness matrix of the high-order element are deduced,the numerical analysis method of splitting failure of high side wall cavern is proposed,and the corresponding calculation and analysis program is developed based on ABAQUS platform.(5)Through model test,theoretical analysis and numerical simulation,the splitting failure mechanism of high side wall cavern is revealed,that is,the important condition for splitting failure is that the initial maximum principal stress is parallel to the tunnel axis and reaches a certain degree(0.21～0.7 times the uniaxial compressive strength of surrounding rock).When the initial maximum principal stress parallel to the tunnel axis exceeds 0.7 times the uniaxial compressive strength of the rock,the high side wall cavern will transform from splitting failure to zoning failure.The fundamental mechanical cause of splitting failure is that the wall stress of excavation unloading high side wall oscillates alternately between peak and trough.