Mechanical analysis and evaluation method for the safe conversion position of the dual-mode shield tunneling mode

Abstract

The present invention discloses a mechanical analysis and evaluation method for the safety conversion position of a dual-mode shield based on the surrounding rock stability. First, according to relevant geological exploration report documents, a geological model diagram of the die-changing position during the construction of the dual-mode shield is determined; then mechanical analysis and evaluation parameters are defined and obtained, and the mechanical analysis and evaluation parameters include: the minimum length of the full-section hard rock in front of the cutter head of the shield machine when the TBM is converted to the EPB; the minimum length of the shield machine entering the full-section hard rock when the EPB is converted to the TBM; during the mode conversion, the minimum thickness of the overlying hard rock for the vault to maintain a stable state; during the mode conversion, when there is a cavity at the bottom of the formation, the minimum thickness of the bottom hard rock for the tunnel floor to maintain a stable state; finally, based on the above parameters, the actual rock entry length, remaining rock length, overlying hard rock thickness, and bottom hard rock thickness corresponding to this conversion position are compared, and the stability of the surrounding rock at the conversion position is analyzed, thereby realizing the mechanical analysis and evaluation of the safety of the dual-mode shield conversion position.

Description

Technical Field

[0001] The present invention relates to the technical field of tunnel construction, and particularly relates to a mechanical analysis and evaluation method for the safe conversion position of a dual-mode shield tunneling machine. Background Art

[0002] In order to adapt to variable geological conditions and achieve the safety and efficiency of construction under complex geological conditions, dual-mode (TBM-EPB) shield tunneling machines have gradually been applied to the construction of subway tunnels. For example, the invention patent with the application publication number CN111911177A discloses a method for judging the tunneling mode selection of a dual-mode tunnel boring machine in a frequency-varying stratum. Through the intelligent judgment of the rock machine sensing parameters, combined with the judgment of the slag sample discharged from the tunnel boring machine, and referring to the geological exploration data at the same time, the mode selection and the timing of mode conversion during the starting stage and the tunneling process can be judged more systematically and accurately, so that the mode selection, the conversion position and the conversion timing of the dual-mode tunnel boring machine have more standardized and more standardized operation bases. On this basis, the safety problem during the mode conversion of the dual-mode shield is also extremely important, and the most prominent one is the safety evaluation problem of the conversion position.

[0003] When the dual-mode shield tunneling machine undergoes mode conversion, the surrounding rock of the conversion position has not been supported, and there is no pressure in the soil bin of the shield tunneling machine, which cannot balance the formation water and soil pressure. If the stability of the surrounding rock at the conversion position is low, it is very easy to cause adverse effects such as the collapse of the heading face during the mode conversion process. Therefore, the primary task of analyzing and evaluating the safe conversion position of the dual-mode shield is to analyze the stability of the surrounding rock at the conversion position. If the surrounding rock is stable, the safety of the conversion position is good, and mode conversion can be carried out; if the surrounding rock is unstable, the safety of the conversion position is lacking, and in actual engineering, this conversion position should be abandoned and other conversion positions should be found to ensure the safety of personnel and mechanical equipment during the mode conversion process of the dual-mode shield tunneling machine. Summary of the Invention

[0004] Aiming at the problem that the surrounding rock of the conversion position has not been supported during the mode conversion of the dual-mode shield tunneling machine and the stability and safety problems of the conversion position, the present invention provides a mechanical analysis and evaluation method for the safe conversion position of the dual-mode shield tunneling machine to better realize the mode conversion and effectively ensure the safety of personnel and mechanical equipment during the mode conversion process of the dual-mode shield tunneling machine.

[0005] The present invention is implemented by the following technical solutions: A mechanical analysis and evaluation method for the safe conversion position of a dual-mode shield tunneling machine, comprising the following steps:

[0006] Step A: Determine the geological model diagram of the die-changing position of the dual-mode shield tunneling machine according to relevant geological exploration report documents;

[0007] Step B: Define and obtain mechanical analysis and evaluation parameters;

[0008] (1) The mechanical analysis and evaluation parameters include: when the TBM is converted to the EPB, the minimum length L of the full-section hard rock in front of the cutter head of the shield machine 留 ; when the EPB is converted to the TBM, the minimum length L of the shield machine entering the full-section hard rock 入 ; during the mode conversion, the minimum thickness H of the overlying hard rock for the vault to maintain a stable state 覆 ; during the mode conversion, when there is a cavity at the bottom of the formation, the minimum thickness H of the bottom hard rock for the tunnel floor to maintain a stable state 底 ;

[0009] (2) Combining the rock stratum properties at the selected dual-mode conversion position of the project, the values of the above four mechanical analysis and evaluation parameters are obtained through mechanical analysis;

[0010] Step C: Based on the mechanical analysis and evaluation parameters, conduct a stability analysis of the surrounding rock at the current conversion position, and then realize the mechanical analysis and evaluation of the safety of the dual-mode shield conversion position. When L 留 , L 入 , H 覆 , H 底 All four mechanical analysis and evaluation parameters are safe, then the conversion position here is safe; otherwise, it is not a safe conversion position, and a suitable dual-mode shield conversion position needs to be reselected.

[0011] Furthermore, in the above Step B, during the mode conversion, the minimum thickness H of the key stratum for the vault to maintain a stable state 覆 is specifically obtained through the following method:

[0012] (1) The boundary of the pressure arch changes with the interface between the hard and soft rocks. Set the following working conditions:

[0013] Working condition 1: First, set the position of the interface between the hard and soft rocks and determine the outer boundary of the pressure arch;

[0014] Working condition 2: According to the trial calculation results of Working condition 1, change the position of the interface between the hard and soft rocks again and determine the outer boundary of the pressure arch;

[0015] Working condition 3: According to the trial calculation results of Working condition 2, change the position of the interface between the hard and soft rocks again and determine the outer boundary of the pressure arch;

[0016] ……;

[0017] And so on, until the outer boundary of the pressure arch at the vault part exactly coincides with the interface between the hard and soft rocks. At this time, the distance from the hard-soft interface to the tunnel vault is the minimum thickness H of the overlying hard rock for the vault to maintain a stable state 覆 .

[0018] Further, in step B, during the mode conversion, when there is a cavity at the bottom of the formation, the minimum thickness H of the hard rock at the bottom of the tunnel floor to maintain a stable state 底 is obtained specifically in the following way:

[0019] (1) Introduce basic assumptions: Assume that the rock mass is intact, homogeneous, isotropic, and continuous medium, which can bear both compressive stress and tensile stress. Simplify the rock layer at the bottom of the tunnel floor into a beam fixed at both ends, without considering the action of confined water and the pressure arch at the top of the karst cave;

[0020] (2) Simplify the calculation model for the loads acting on the beam, including the self-weight loads of the rock mass at the bottom of the tunnel and the front shield and middle shield of the shield machine;

[0021] (3) Collect the tensile and compressive strengths of the rock layer in the geological exploration report;

[0022] (4) Select the safety factor based on engineering experience and the importance of the project;

[0023] (5) Calculate the allowable tensile strength [σ t of the rock mass at the bottom of the tunnel and the allowable shear strength [τ] of the rock mass at the bottom of the tunnel;

[0024] (6) Conduct a mechanical evaluation of the rock layer's bending resistance:

[0025]

[0026] (7) Conduct a mechanical evaluation of the rock layer's shear resistance

[0027]

[0028] (8) Take H 底 = max{H 底1 , H 底2}

[0029] where γ is the unit weight of the rock at the bottom of the tunnel, B is the width of the karst cave at the bottom of the tunnel, taking the diameter of the karst cave, and G is the uniform load of the self-weight of the front shield and middle shield of the shield machine in the tunnel excavation direction H 底 is the thickness of the tunnel floor, l is the span of the karst cave, taking the diameter of the karst cave.

[0030] Further, in step B, when the TBM is converted to EPB, the minimum length L of the full-section hard rock in front of the cutterhead of the shield machine 留 and when the EPB is converted to TBM, the minimum length L of the shield machine entering the full-section hard rock 入 is obtained specifically in the following way:

[0031] (1) Collect the basic data of shield construction, including the tunnel diameter D, the water and soil pressure q acting on the face rock mass, and the tensile strength σ of the rock masst , angle of friction

[0032] (2) Calculate the diffusion angle:

[0033] (3) Substitute into

[0034] (4) L 留 , L 入 are equivalent processes in engineering and are equal, so

[0035] Furthermore, the specific evaluation method of step C is as follows:

[0036] (1) When the thickness of the overlying hard rock at the conversion point selected in the actual project is greater than the minimum thickness H of the overlying hard rock for the vault to maintain a stable state calculated 覆 , then this evaluation is safe. When the thickness of the overlying hard rock at the conversion point selected in the actual project is less than the minimum thickness H of the overlying hard rock for the vault to maintain a stable state calculated 覆 , then this evaluation is unsafe;

[0037] (2) Judge whether the thickness of the bottom hard rock in the actual project is greater than H 底 . If it is greater, then this evaluation is safe. If it is less, then this evaluation is unsafe;

[0038] (3) Judge whether the actual remaining rock and rock - entering lengths at the conversion position are correspondingly greater than L 留 , L 入 . If it is greater, then this mechanical evaluation index is safe. If it is less, then this evaluation index is unsafe.

[0039] Compared with the prior art, the advantages and positive effects of the present invention are as follows:

[0040] This solution defines mechanical analysis evaluation parameters, sets four parameters H 覆 H 底 L 留 L 入 . By comparing the actually measured rock - entering length, remaining rock length, overlying hard rock thickness, and bottom hard rock thickness with the values calculated theoretically for the four set parameters, it is judged whether the surrounding rock is stable and whether the conversion point is safe. When all four parameters are within the theoretical values, it can be determined that the surrounding rock properties here are suitable for the mode conversion of double - mode shield construction. Through the mechanical evaluation at the conversion point, this solution better ensures the mode conversion and effectively guarantees the safety of personnel and mechanical equipment during the mode conversion of the double - mode shield. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1Schematic diagram of the mechanical analysis and evaluation method for the safe conversion position in the dual-mode shield tunneling mode according to the embodiments of the present invention;

[0042] Figure 2 Schematic diagram of the mold-changing position in the dual-mode shield tunneling construction for explaining various defined parameters according to the embodiments of the present invention;

[0043] Figure 3 For the FLAC according to the embodiments of the present invention 3D Schematic diagram for determining the outer boundary of the pressure arch by changing the position of the hard and soft strata;

[0044] Figure 4 For the H according to the embodiments of the present invention 底 Schematic diagram of the engineering background for safety evaluation;

[0045] Figure 5 For the H according to the embodiments of the present invention 底 Schematic diagram of the simplified mechanical analysis and evaluation model;

[0046] Figure 6 For the FLAC in the embodiments of the present invention 3D Schematic diagram of the numerical simulation calculation model;

[0047] Figure 7 Schematic diagram of the relationship between the interface of hard and soft rocks and the outer boundary of the pressure arch according to the embodiments of the present invention; Detailed implementation manners

[0048] In order to more clearly understand the above objects, features and advantages of the present invention, the present invention will be further described below with reference to the accompanying drawings and embodiments. Many specific details are set forth in the following description in order to fully understand the present invention. However, the present invention can also be implemented in other ways different from those described herein. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0049] The embodiments of the present invention propose a mechanical analysis and evaluation method for the safe conversion position in the dual-mode shield tunneling mode, as Figure 1 shown, including the following steps:

[0050] Step A: Make a geological model diagram of the mold-changing position in the dual-mode shield tunneling construction according to documents such as geological exploration reports;

[0051] Step B: Define and calculate the mechanical analysis and evaluation parameters;

[0052] (1) As Figure 2 shown, it is the geological model diagram of the mold-changing position in the embodiments of the present invention, Figure 2 and the definitions of each parameter in it are as follows:

[0053] Since all mode conversions are required to be carried out within the hard rock section, in order to match the shield machine mode with the rock formation state, it is required to perform the mode conversion as close as possible to the hard-soft interface. However, the closer to the hard-soft interface, the worse the surrounding rock stability is compared to the hard rock section, which creates a contradiction. Therefore, the following parameters are defined:

[0054] Horizontal direction:

[0055] L 留 Represents the minimum length of the full-section hard rock in front of the cutterhead of the shield machine when the TBM is converted to EPB;

[0056] L 入 Represents the minimum length of the shield machine entering the full-section hard rock when the EPB is converted to TBM.

[0057] Vertical direction:

[0058] H 覆 Represents the minimum thickness of the key layer (hard rock) that keeps the crown stable during the mode conversion;

[0059] H 底 Represents the minimum thickness of the hard rock that keeps the tunnel floor stable during the mode conversion when there are cavities at the bottom of the formation.

[0060] (2) The minimum thickness H of the key layer (hard rock) that keeps the crown stable during the mode conversion 覆 The calculation method is as follows:

[0061] Principle: Tunnel construction destroys the original equilibrium state of the surrounding rock, changes the load transfer path, and under the action of gravity and the initial stress field, the surrounding rock undergoes new deformations. At the same time, the stress, strain, and energy of the surrounding rock reach a new equilibrium state. Due to the non-uniformity of the deformations in the surrounding rock, a tangential compressing effect similar to that of an arch structure occurs in the surrounding rock within a certain range around the tunnel perimeter, namely the pressure arch effect. The surrounding rock in the pressure arch bears its own weight and the pressure of the surrounding rock and soil mass. The existence of the pressure arch can ensure the stability of the tunnel chamber and reduce the stress on the lining. If the surrounding rock is regarded as a structure, the pressure arch is a structure with the mechanical characteristics of an arch that ensures that the surrounding rock of the tunnel will not collapse.

[0062] 1) Judgment criterion: When the outer boundary of the pressure arch is less than the overburden thickness, the surrounding rock is on the safe side at this time. When the outer boundary of the pressure arch is greater than the overburden thickness, the surrounding rock is on the dangerous side at this time.

[0063] 2) Use FLAC 3D software to establish an analysis model as Figure 3 shown. During numerical simulation analysis, the tunnel burial depth remains unchanged. By changing the position of the hard-soft rock interface, the stress distribution characteristics at the crown are analyzed to study the minimum safety thickness of the hard rock at the crown.

[0064] (3) The boundary of the pressure arch changes with the interface between hard and soft rocks. The following working conditions are set: In Working Condition 1, first set the position of the interface between hard and soft rocks to determine the outer boundary of the pressure arch; in Working Condition 2, according to the calculation results of Working Condition 1, change the position of the interface between hard and soft rocks again to determine the outer boundary of the pressure arch; in Working Condition 3... and so on, until the outer boundary of the pressure arch at the arch crown exactly coincides with the interface between hard and soft rocks, which is the minimum thickness H of the overlying hard rock when the arch crown maintains a stable state. 覆 。

[0065] (3) During the mode conversion, when there is a cavity at the bottom of the formation, the minimum thickness H of the hard rock for the tunnel floor to maintain a stable state 底 The calculation method is as follows:

[0066] If there is a karst cave at the bottom of the tunnel, after the tunnel is excavated, the thickness of the hard rock at the bottom of the tunnel decreases. Under the influence of excavation disturbance and other factors, the rock stratum at the tunnel floor may be damaged, resulting in accidents such as tunnel floor collapse, seriously affecting the safety of tunnel construction machinery and operating personnel. During the mode conversion of the TBM-EPB dual-mode shield, the mode conversion is carried out in the hard rock section. There is no segment support for the surrounding rock within about 7.5 m behind the cutterhead of the shield machine at the conversion position. If there is a karst cave at the bottom of the tunnel, it will directly affect the stability of the surrounding rock during the mode conversion. In this embodiment, the minimum thickness of the hard rock at the bottom of the tunnel is studied through theoretical analysis to ensure that the tunnel floor meets the safety requirements during the dual-mode shield mode conversion. Enrich the scientific connotation of the mechanical analysis and evaluation method for the safe conversion position of the dual-mode (TBM-EPB) shield mode.

[0067] H 底 The specific calculation method is as follows:

[0068] 1) Introduce basic assumptions: The rock mass is intact, homogeneous, isotropic continuous medium, which can bear both compressive stress and tensile stress. Simplify the rock stratum at the tunnel floor into a beam fixed at both ends, without considering the action of confined water and the action of the pressure arch at the top of the karst cave, as Figure 4 shown.

[0069] 2) The loads acting on the beam include the rock mass at the bottom of the tunnel and the self-weight loads of the front shield and middle shield of the shield machine. The simplified calculation model is as Figure 5 shown.

[0070] 3) Collect the tensile and compressive strengths of the rock stratum in the geological exploration report

[0071] 4) Select a safety factor of 2 or 2.5 or 3, and select it according to engineering experience and the importance of the project

[0072] 5) Calculate the allowable tensile strength [σ t of the rock mass at the bottom of the tunnel and the allowable shear strength [τ] of the rock mass at the bottom of the tunnel.

[0073] 6) Conduct the flexural mechanical evaluation of the rock stratum:

[0074]

[0075] 7) Conduct the shear mechanical evaluation of the rock stratum

[0076]

[0077] 8) Take H 底 = max{H 底1 , H 底2}.

[0078] (4) The minimum length L of the full-section hard rock in front of the cutterhead of the shield machine when the TBM is converted to EPB 留 and the minimum length L of the shield machine entering the full-section hard rock when the EPB is converted to TBM 入 The calculation method is as follows;

[0079] 1) Collect the basic data of shield construction, including the tunnel diameter D, the water and soil pressure q acting on the rock face of the heading face, the tensile strength σ t of the rock mass, and the friction angle

[0080] 2) Calculate the diffusion angle:

[0081] 3) Substitute

[0082] 4) L 留 , L 入 are equivalent processes in the project and are taken to be equal, then

[0083] Step D: Based on the above four mechanical analysis and evaluation parameters, conduct a stability analysis of the surrounding rock at the conversion position, and then realize the mechanical analysis and evaluation of the safety of the conversion position of the double-mode shield. When L 留 , L 入 , H 覆 , H 底 all four mechanical analysis and evaluation parameters are safe, then the conversion position here is safe; otherwise, it is not a safe conversion position, and a suitable conversion position of the double-mode shield should be reselected. Specifically, during the evaluation:

[0084] Obtain the thickness of the overlying hard rock, the thickness of the bottom hard rock, the actual remaining rock and the entering-rock length at the selected conversion position in the actual project, and compare them with the corresponding four evaluation indexes calculated;

[0085] (1) When the thickness of the overlying hard rock at the selected conversion point position in the actual project is greater than the minimum thickness H 覆If so, this evaluation is safe. When the thickness of the overlying hard rock at the position of the conversion point selected in the actual project is less than the minimum thickness H of the overlying hard rock for the vault to maintain a stable state calculated 覆 If so, this evaluation is unsafe;

[0086] (2) Judge whether the thickness of the bottom hard rock in the actual project is greater than H 底 If it is greater, this evaluation is safe; if it is less, this evaluation is unsafe;

[0087] (3) Judge whether the actual remaining rock and rock entry lengths at the conversion position are correspondingly greater than L 留 and L 入 If they are greater, this mechanical evaluation index is safe; if they are less, this mechanical evaluation index is unsafe.

[0088] The following combines specific cases to elaborate on the solution of the present invention in detail:

[0089] Taking a certain (Shenzhen) Urban Rail Transit Line 13 as an example, by using the solution of the present invention, the safety of the conversion position of the dual-mode shield is analyzed and evaluated to guide the specific construction as follows:

[0090] The dual-mode shield section between Liuxiandong Station and Baimang Station is the section with the largest span of Shenzhen Metro Line 13. The full length of its right line is 4610.855 m, and the full length of its left line is 4606.182 m. The tunnel diameter is about 6.7 m, the excavation diameter of the shield machine cutter head is 6.98 m, and the outer diameter of the shield is 6.95 m; the outer diameter of the reinforced concrete segment is 6700 mm, the inner diameter of the segment is 6000 mm, the segment thickness is 350 mm, the ring width is 1500 mm, and 4 EPB-TBM dual-mode shield machines are used in the section. They start from the large mileage end of Liuxiandong Station and the small mileage end of Baimang Station respectively, and are hoisted out by the intermediate ventilation shaft. The line spacing between the left and right lines is 11.0 m to 17.3 m, and the buried depth of the tunnel vault is about 10.6 m to 49.6 m. This project has the following characteristics: The interval tunnel passes through uneven hard and soft strata. The main strata passed through by the interval tunnel are medium- to slightly weathered biotite granite and medium- to slightly weathered migmatitic granite, and locally passes through hard plastic gravelly clay, completely- to strongly weathered biotite granite, and completely- to strongly weathered migmatitic granite. Among them, the highest strength of slightly weathered granite can reach 177 MPa, and the completely- to strongly weathered granite has the characteristics of softening, disintegrating when encountering water, and a sharp decrease in strength; the interval tunnel passes through the F3 and F4 fault zones, and there are often spherical weathered bodies distributed; the groundwater in the interval is rich, and the stable groundwater level is buried at a depth of 2.30 to 16.20 m.

[0091] (1) Conduct an H 覆 Safety evaluation:

[0092] Use FLAC 3DThe software establishes a three-dimensional model. The dimensions of the model are 45 m in length, 60 m in width, and 60 m in height. Taking the center of the tunnel section as the origin, horizontal constraints are applied to the sides of the model, vertical constraints are applied to the bottom of the model, and the ground surface is a free surface. The Mohr-Coulomb constitutive model is adopted for the rock and soil mass. The tunnel excavation process is simulated using the null model. The excavation footage is taken as 1.5 m, which is the width of the segment. The formation parameters are shown in Table 1, and the numerical simulation model is as shown in Figure 6 shown below.

[0093] Table 1 Physical and mechanical parameters of the formation

[0094]

[0095] Based on the calculation results, a relationship diagram of the interface between hard and soft rocks - the outer boundary of the pressure arch as shown in Figure 7 is drawn. The intersection point of the two lines indicates that the distance from the interface between hard and soft rocks to the tunnel crown is exactly equal to the distance from the outer boundary of the pressure arch to the tunnel crown, that is, the minimum safety thickness H of the slightly weathered granite at the tunnel crown at the mode conversion position sought by the present invention 覆 is 4.5 m.

[0096] 2. Conduct the safety evaluation of H 底 :

[0097] According to the geological exploration report, the unit weight of the slightly weathered granite is 27 kN / m 3 , and the tensile strength is 2.3 MPa. Taking the safety factor as 2.0, then [σ t = 1.15 MPa. Calculated according to the flexural strength, the minimum safety thickness H of the rock layer at the tunnel floor 底min1 ; After calculation, the shear strength of the slightly weathered granite is 5.722 MPa. Taking the safety factor as 2.0, then [τ] = 2.86 MPa. Calculated according to the shear strength, the minimum safety thickness H of the rock layer at the tunnel floor 底min2 . According to the above analysis, calculate the minimum thicknesses H 底min1 and H 底min2 of the tunnel floor under different karst cave diameter conditions according to the flexural strength and shear strength, as shown in Table 2:

[0098] Table 2 Minimum thicknesses of the tunnel floor under different karst cave sizes

[0099]

[0100] 3. Conduct the safety evaluation of L 留 and L 入 :

[0101] According to the on-site geological exploration report, the buried depth of the water table is taken as 10 m, the buried depth of the tunnel crown is 34 m, the water pressure at the tunnel axis is about 275 kPa, the earth pressure is calculated as the at-rest earth pressure, the buoyant unit weight is taken as about 11 kN / m3, the at-rest lateral pressure coefficient is taken as 0.35, and the earth pressure at the tunnel axis is about 144 kPa. Therefore, q is the resultant force of the water and earth pressures, which is 420 kPa; P is the support force of the shield machine on the heading face. There is no pressure in the soil chamber during the mode conversion process, and P is 0; σ t is the tensile strength of the rock mass, taken as 1.15 MPa; θ is the diffusion angle, taken as the friction angle of slightly weathered granite, which is 68°; the minimum safety thickness of the slightly weathered granite on the heading face calculated according to the tensile strength is 2.6 m.

[0102] Through the above analysis and comparison with the specific project of Shenzhen Metro Line 13, the safety of this conversion position is evaluated.

[0103] The above are only the preferred embodiments of the present invention and do not limit the present invention in other forms. Any person skilled in the art may use the disclosed technical content to make changes or modifications into equivalent embodiments with equivalent changes and apply them to other fields. However, any simple modification, equivalent change, and modification made to the above embodiments based on the technical essence of the present invention without departing from the technical solution content of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A mechanical analysis and evaluation method for the safe conversion position of the dual-mode shield tunneling mode, characterized in that, it includes the following steps: Step A: Determine the geological model diagram of the tunneling mode conversion position of the dual-mode shield according to relevant geological exploration report documents; Step B: Define and obtain mechanical analysis and evaluation parameters; (1) The mechanical analysis and evaluation parameters include: when the TBM is converted to the EPB, the minimum length L of the full-section hard rock in front of the cutterhead of the shield machine 留 ; when the EPB is converted to the TBM, the minimum length L of the shield machine entering the full-section hard rock 入 ; during the mode conversion, the minimum thickness H of the overlying hard rock for the vault to maintain a stable state 覆 ; during the mode conversion, when there is a cavity at the bottom of the formation, the minimum thickness H of the bottom hard rock for the tunnel floor to maintain a stable state 底 ; (2) Combine the rock formation properties of the selected dual-mode conversion position of the project, and obtain the values of the above four mechanical analysis and evaluation parameters through mechanical analysis; Step C: Based on the mechanical analysis and evaluation parameters, conduct a stability analysis of the surrounding rock at the current conversion position, and then realize the mechanical analysis and evaluation of the safety of the conversion position of the dual-mode shield. When L 留 、L 入 、H 覆 、H 底 All four mechanical analysis and evaluation parameters are safe, then the conversion position here is safe; otherwise, it is not a safe conversion position, and a suitable conversion position of the dual-mode shield needs to be reselected.

2. The mechanical analysis and evaluation method for the safe conversion position of the dual-mode shield tunneling mode according to claim 1, characterized in that: During step B, during the mode conversion, the minimum thickness H of the key layer for maintaining the stable state of the vault 覆 is obtained specifically in the following manner: (1) The boundary of the pressure arch changes with the change of the interface between hard and soft rocks. The following working conditions are set: Working condition 1: First, set the position of the interface between hard and soft rocks and determine the outer boundary of the pressure arch; Working condition 2: According to the trial calculation results of working condition 1, change the position of the interface between hard and soft rocks again and determine the outer boundary of the pressure arch; Working condition 3: According to the trial calculation results of working condition 2, change the position of the interface between hard and soft rocks again and determine the outer boundary of the pressure arch; And so on until the outer boundary of the pressure arch at the vault exactly coincides with the interface between hard and soft rocks. At this time, the distance from the interface between hard and soft rocks to the tunnel vault is the minimum thickness H of the overlying hard rock for the vault to maintain a stable state. 覆 .

3. The mechanical analysis and evaluation method for the safe conversion position of the dual-mode shield tunneling mode according to claim 1, characterized in that: During step B, when there is a cavity at the bottom of the formation during mode conversion, the minimum thickness H of the hard rock at the bottom to keep the tunnel floor in a stable state 底 It is obtained specifically in the following way: (1) Introduce basic assumptions: Assume that the rock mass is complete, homogeneous, isotropic continuous medium, which can bear both compressive stress and tensile stress. Simplify the rock formation at the tunnel floor as a beam fixed at both ends, without considering the effect of confined water and the effect of the pressure arch at the top of the karst cave; (2) Simplify the calculation model for the loads acting on the beam, including the self-weight loads of the rock mass at the tunnel bottom and the front shield and middle shield of the shield machine; (3) Collect the tensile and compressive strengths of the rock formation in the geological exploration report; (4) Select the safety factor based on engineering experience and the importance of the project; (5) Calculate the allowable tensile strength [σ t and the allowable shear strength [τ] of the rock mass at the bottom of the tunnel; (6) Conduct a mechanical evaluation of the rock formation's bending resistance; (7) Conduct a mechanical evaluation of the rock formation's shear resistance (8) Take H 底 = max{H 底1 , H 底2} Among them, γ is the unit weight of the rock mass at the bottom of the tunnel, B is the width of the karst cave at the bottom of the tunnel, taking the diameter of the karst cave, and G is the uniformly distributed load of the self-weight of the front shield and the middle shield of the shield machine in the tunnel excavation direction. H 底 The thickness of the tunnel floor slab, and l is the span of the karst cave, taking the diameter of the karst cave.

4. The mechanical analysis and evaluation method for the safe conversion position of the dual-mode shield tunneling mode according to claim 1, characterized in that: In the said step B, when the TBM is converted to the EPB, the minimum length L of the full-face hard rock in front of the cutterhead of the shield machine 留 and when the EPB is converted to the TBM, the minimum length L for the shield machine to enter the full-face hard rock 入 is obtained specifically by the following method: (1)Collect basic shield tunneling data, including tunnel diameter D, water and soil pressure q acting on the face rock mass, and tensile strength σ of the rock mass t , friction angle (2) Calculate the divergence angle: (3) Bring in (4)L 留 、L 入 In engineering, it is an equivalent process, and the two are equal, so 5. The mechanical analysis and evaluation method for the safe conversion position of the dual-mode shield tunneling mode according to claim 1, characterized in that: The specific evaluation of step C is carried out in the following way: (1) When the thickness of the overlying hard rock at the conversion point selected in the actual project is greater than the minimum thickness H of the overlying hard rock for the vault to maintain a stable state calculated 覆 then this evaluation is safe. When the thickness of the overlying hard rock at the conversion point selected in the actual project is less than the minimum thickness H of the overlying hard rock for the vault to maintain a stable state calculated 覆 then this evaluation is unsafe; (2) Determine whether the thickness of the underlying hard rock in the actual project is greater than H 底 , if it is greater, this evaluation is safe; if it is less, this evaluation is unsafe; (3) Determine whether the actual remaining rock and rock penetration lengths at the conversion position are correspondingly greater than L 留 and L 入 . If it is greater, this mechanical evaluation index is safe; if it is less, this mechanical evaluation index is unsafe.

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