白云岩砂化是一类特殊的地质现象，砂化后的白云岩具有岩体结构松散破碎、强

Sandification of dolomite is a special geological phenomenon. The dolomite after sandification has the characteristics of loose and broken rock mass structure and deterioration of strength. In the process of tunnel excavation, it often faces problems such as difficulty in forming holes, poor forming quality, and landslides. In the Fushui tunnel section, water inrush and sand gushing are prone to occur, which seriously hinders the normal development of engineering construction and threatens the safety of tunnel construction and operation. Based on the Xiaopu Tunnel in Yuxi Section of the Central Yunnan Water Diversion Project, through on-site geological investigation, physical and mechanical test data analysis, and based on the Mohr-Coulomb criterion, this paper deduces the critical thickness prediction formula for water-rich sandy dolomite tunnel water inrush and sand inrush disaster. ;Use FLAC3D numerical analysis software to simulate the construction process of sandy dolomite tunnel, analyze the engineering geological conditions, the change of mechanical parameters of strongly sandy dolomite and severe sandy dolomite rock mass on the critical thickness of outburst prevention structure disaster, the maximum extrusion of the tunnel face Based on the influence of displacement and tunnel water inflow, a catastrophe prediction model was constructed based on the evolution law of the critical thickness of the outburst prevention structure, and the disaster indicators were determined based on the maximum extruded displacement of the tunnel face and the growth rate of water inflow. The adaptability and feasibility of the prediction model and the tunnel water gushing growth rate indicators for disaster risk are tested through actual engineering, which provides a basis for reasonably predicting the occurrence and prevention of disasters. The main achievements of the paper are as follows: <br>(1) Through the investigation of the actual engineering geological conditions on site and the analysis of the sandy dolomite rock mass structure and material structure, it is considered that the rock mass structure and material structure of the dolomite after sandification are surface water infiltration and material structure. The storage of groundwater provides conditions. The severe sandification dolomite under the condition of strong water richness and high head is the main disaster source of water inrush and sand gushing in tunnels. <br>(2) According to the research, the failure modes of inrush failure of sandy dolomite tunnels can be divided into two types: sliding type and shearing type. Insufficient anti-sliding ability of violently sanded dolomite as a filling medium or insufficient strength of strongly sanded dolomite as an anti-outburst structure are the main internal causes of inrush disasters in the cave; The change of seepage field and the insufficient thickness of anti-outburst structure are the main external causes of the inrush disaster in the water-rich sandy dolomite tunnel.<br>(3) The study believes that the slope failure mode of the outburst-prevention rock mass in the sandy dolomite cave section is the overall shear failure, and the geological model is constructed with the strongly sandy dolomite as the outburst-prevention rock mass and the water-rich severe sandy dolomite as the outburst mass. The critical thickness of the rock mass for outburst prevention in a tunnel is related to the burial depth, water head, inclination angle of the rock interface, internal friction angle of the surrounding rock, and cohesion. Based on the Mohr-Coulomb criterion, the prediction formula for the critical thickness of the water-rich sandy dolomite tunnel outburst prevention structure is derived: <br>L_c=D/(2tanφ_r )(1-√(K/(K+Ptanφ_r ))). <br>(4) Numerical simulation studies show that the critical thickness of the anti-outburst structure, the maximum extrusion displacement of the tunnel face and the water inflow are positively correlated with the water head of the tunnel, and the dip angle of the interface with the rock layer first decreases and then increases, and the buried depth of the tunnel changes. The impact on the simulation results is small; the internal friction angle and cohesion of strongly sanded dolomite are negatively correlated with the critical thickness of the outburst prevention structure, the maximum extruded displacement of the face and the water inflow volume. The influence is small; the changes of internal friction angle, cohesive force and elastic modulus parameters of severe sandification dolomite have little influence on the simulation results. <br>(5) A method based on the parametric curve curvature value method to determine the sudden change inflection point is proposed. Based on the evolution law of catastrophic critical thickness, a prediction model for water inrush and sand inrush in water-rich sandy dolomite tunnels under the comprehensive influence of multiple factors is fitted: L_c=〖0.0162H〗_w-0.0001θ^2-0.0116θ-0.1284φ_r- 1.0748c_r+4.7795. Taking the maximum extrusion displacement of the face and the growth rate of water inflow as the disaster information sources, the critical values ​​of the disaster are determined to be 0.22 and 0.18, respectively. <br>(6) Based on the Xiaopu Tunnel in Yuxi Section of the Central Yunnan Water Diversion Project, the adaptability of three catastrophe prediction formulas and critical catastrophe indicators of water inflow growth rate was analyzed. The prediction results of the three prediction formulas for the YX06+595.2 working face are 1.97m, 1.62m and 2.80m respectively. The results show that the calculation results of the theoretical prediction formula of the overall shear failure based on the Mohr-Coulomb criterion are small, and the numerical analysis fitting formula is more consistent with the engineering practice, which has a certain reference value. Suggestions and measures for preventing water inrush and sand gushing disasters in the cave during construction are given. <br>Key words: water diversion project in central Yunnan; sandy dolomite; catastrophe mechanism; critical catastrophe thickness; catastrophe prediction formula

Dolomite sanding is a special geological phenomenon. After sanding, dolomite has the characteristics of loose and broken rock mass structure and strength deterioration. In the process of tunnel excavation, it often faces problems such as difficult hole formation, poor forming quality and collapse. Water and sand inrush are also easy to occur in the water rich tunnel section, which seriously hinders the normal development of engineering construction and threatens the safety of tunnel construction and operation. Based on the Xiaopu tunnel in Yuxi section of central Yunnan water diversion project, through on-site geological investigation, physical and mechanical test data analysis, and based on Mohr Coulomb criterion, this paper deduces the prediction formula of the critical thickness of water inrush and sand inrush in water rich and sandy dolomite tunnel; FLAC3D numerical analysis software is used to simulate the construction process of sandy dolomite tunnel, analyze the influence of engineering geological conditions, strong sandy dolomite and violent sandy dolomite rock mass mechanical parameters on the catastrophic critical thickness of outburst prevention structure, the maximum extrusion displacement of tunnel face and the water inflow of tunnel, and build a catastrophe prediction model based on the evolution law of catastrophic critical thickness of outburst prevention structure The growth rate of water inflow is used as the discrimination basis to determine the impending disaster index. Through the actual project, the adaptability and feasibility of the prediction model and the temporary disaster index of tunnel water inflow growth rate are tested, so as to provide a basis for reasonable prediction and prevention of disasters. The main achievements of this paper are as follows:<br>(1) Through the investigation of the actual engineering geological conditions on site and the analysis of the rock mass structure and material structure of the sandy dolomite, it is considered that the rock mass structure and material structure of the sandy dolomite provide conditions for the infiltration of surface water and the storage of groundwater. The highly sandy dolomite under the condition of strong water abundance and high water head is the main disaster source of water and sand inrush in the tunnel.<br>(2) The study shows that the surge instability failure forms of sandy dolomite tunnel can be divided into sliding type and shear type. The main internal cause of outburst disaster in the tunnel is the insufficient anti sliding ability of violent sandy dolomite as filling medium or the insufficient strength of strong sandy dolomite as outburst prevention structure; The main external causes of inrush disaster in water rich and sandy dolomite tunnel are the reduction of rock mass integrity, strength deterioration, change of seepage field and insufficient thickness of outburst prevention structure caused by construction and excavation disturbance.<br>(3) The study shows that the slope failure mode of the anti outburst rock body in the sandy dolomite tunnel section is the overall shear failure, and the geological model is constructed with the strongly sandy dolomite as the anti outburst rock body and the highly water rich sandy dolomite as the outburst body. The critical thickness of the tunnel anti outburst rock body is related to the buried depth, water head, the dip angle of the rock interface, the internal friction angle and cohesion of the surrounding rock. Based on Mohr Coulomb criterion, the prediction formula of catastrophic critical thickness of outburst prevention structure of water rich sandy dolomite tunnel is derived:<br>L_ c=D/(2tan φ_ r )(1-√(K/(K+Ptan φ_ r )))。<br>(4) The numerical simulation results show that the critical thickness of the outburst prevention structure, the maximum extrusion displacement and water inflow of the tunnel face are positively correlated with the tunnel head, and the inclination of the interface with the rock stratum decreases first and then increases. The change of the tunnel buried depth has little effect on the simulation results; The internal friction angle and cohesion of strongly sandy dolomite are negatively correlated with the critical thickness of outburst prevention structure, the maximum extrusion displacement of tunnel face and water inflow. The change of elastic modulus of strongly sandy dolomite has little effect on the simulation results; The changes of internal friction angle, cohesion and elastic modulus of severely sandy dolomite have little effect on the simulation results.<br>(5) A method to determine the abrupt inflection point based on the curvature value of parameter curve is proposed. Based on the evolution law of catastrophic critical thickness, the prediction model of water inrush and sand inflow in water rich and sandy dolomite tunnel under the comprehensive influence of multiple factors is fitted: L_ c=〖0.0162H〗_ w-0.0001 θ^ 2-0.0116 θ- zero point one two eight four φ_ r-1.0748c_ r+4.7795。 Taking the maximum extrusion displacement and water inflow growth rate of the tunnel face as the disaster information source, the disaster critical values are determined to be 0.22 and 0.18 respectively.<br>(6) Based on the Xiaopu tunnel in Yuxi section of central Yunnan water diversion project, the adaptability of three disaster prediction formulas and the critical disaster index of water inflow growth rate are analyzed. The prediction results of three prediction formulas for yx06 + 595.2 working face are 1.97M, 1.62m and 2.80M respectively. The results show that the calculation result of the overall shear failure theoretical prediction formula based on Mohr Coulomb criterion is too small, and the numerical analysis fitting formula is more consistent with the engineering practice, which has a certain reference value. The suggestions and measures to prevent water and sand inrush in the tunnel during construction are given.<br>Key words: Water Diversion Project in Central Yunnan; Sandy dolomite; Catastrophe mechanism; Critical catastrophic thickness; Catastrophe prediction formula

Sanding of dolomite is a special geological phenomenon. Sanded dolomite is characterized by loose and broken rock mass structure and strength deterioration. In the process of tunnel excavation, it is often faced with problems such as difficult hole formation, poor forming quality and collapse. Water inrush and sand gushing are also easy to occur in the water-rich tunnel section, which seriously hinders the normal development of engineering construction and threatens the safety of tunnel construction and operation. In this paper, based on Xiaopu tunnel in Yuxi section of water diversion project in central Yunnan, through on-the-spot geological investigation and physical and mechanical test data analysis, and based on Mohr-Coulomb criterion, the critical thickness prediction formula of water inrush and sand inrush disaster in water-rich sandy dolomite tunnel is derived. The FLAC3D numerical analysis software is used to simulate the construction process of sandy dolomite tunnel, and the influence of engineering geological conditions, changes of mechanical parameters of strongly sandy dolomite and strongly sandy dolomite rock mass on the critical thickness of outburst prevention structure, the maximum extrusion displacement of the face and the water inflow of the tunnel is analyzed. Based on the evolution law of critical thickness of outburst prevention structure, the disaster prediction model is built, and the impending disaster index is determined by taking the maximum extrusion displacement of the face and the water inflow growth rate as the discrimination basis. Through practical engineering, the adaptability and feasibility of prediction model and tunnel water gushing growth rate impending disaster index are tested, which provides the basis for reasonable prediction of disaster occurrence and prevention. The main achievements of this paper are as follows: (1) Based on the investigation of actual engineering geological conditions, the analysis of the rock structure and material structure of sandy dolomite, it is considered that the rock structure and material structure of sandy dolomite provide conditions for the infiltration of surface water and the storage of groundwater, and the severely sandy dolomite under the condition of strong water-rich and high water head is the main disaster source of water inrush and sand gushing in tunnels. (2) According to the research, the failure modes of sudden outburst instability of sandy dolomite tunnel can be divided into sliding type and shearing type. The main internal cause of outburst disaster in the cave is the lack of anti-sliding ability of severely sanded dolomite as filling medium or the lack of strength of strongly sanded dolomite as outburst prevention structure. The main external causes of outburst disaster in water-rich sandy dolomite tunnel are the decrease of rock integrity, strength deterioration, change of seepage field and insufficient thickness of outburst prevention structure caused by excavation disturbance. (3) According to the research, the slope failure mode of outburst prevention rock mass in the sandy Baiyun cave section is the whole shear failure, and the geological model is constructed by taking strongly sandy dolomite as outburst prevention rock mass and water-rich severely sandy dolomite as outburst body. The critical thickness of tunnel outburst prevention rock mass disaster is related to the buried depth, water head, inclination angle of rock stratum interface, internal friction angle and cohesion of surrounding rock. Based on Mohr-Coulomb criterion, the prediction formula of critical thickness of outburst prevention structure in water-rich sandy dolomite tunnel is derived: L_c=D/(2tanφ_r )(1-√(K/(K+Ptanφ_r )))。 (4) The numerical simulation study shows that the critical thickness of outburst prevention structure, the maximum extrusion displacement of the face and the water inflow are positively correlated with the tunnel head, and the dip angle of the interface with rock strata first decreases and then increases, while the change of the tunnel buried depth has little influence on the simulation results. The internal friction angle and cohesion of strongly sandy dolomite are negatively correlated with critical thickness of outburst prevention structure, maximum extrusion displacement of face and water inflow, and the change of elastic modulus of strongly sandy dolomite has little influence on simulation results. The changes of internal friction angle, cohesion and elastic modulus parameters of severely sandy dolomite have little influence on the simulation results. (5) A method of judging abrupt inflection point based on curvature value of parametric curve is proposed. According to the evolution law of catastrophic critical thickness, the prediction model of water inrush and sand gushing in water-rich sandy dolomite tunnel under the comprehensive influence of many factors is fitted: L _ C = [0.0162h] _ W-0.0001θ 2-0.0116θ-0.1284φ _ r-1.0748C _ r+4.7795. Taking the maximum extrusion displacement and the water inflow growth rate of the working face as the disaster information sources, the disaster critical values are determined to be 0.22 and 0.18 respectively. (6) Based on Xiaopu Tunnel in Yuxi section of Central Yunnan Water Diversion Project, the adaptability of three catastrophe prediction formulas and critical catastrophe index of water inflow growth rate is analyzed. The prediction results of three prediction formulas for YX06+595.2 working face are 1.97m, 1.62m and 2.80m respectively. The results show that the calculation results of the theoretical prediction formula of overall shear failure based on Mohr-Coulomb criterion are small, and the fitting formula of numerical analysis is in good agreement with the engineering practice, which has certain reference value. Suggestions and measures to prevent water inrush and sand gushing in the tunnel during construction are given. Key words: Central Yunnan Water Diversion Project; Sand dolomite; Catastrophic mechanism; Critical catastrophic thickness; Catastrophe prediction formula