An evaluation method for the formation stability under the condition of normal pressure opening of a rectangular tunnel

Abstract

The present invention discloses a method for evaluating the formation stability under the condition of normal pressure opening of a rectangular tunnel. According to the force characteristics that the support force in the middle area of the heading face is lost after the opening of the warehouse, a structural model is established with a sliding block with a wedge-shaped body on the upper part and a quadrangular prism-shaped block on the lower part; the strip method and the limit equilibrium principle are adopted to determine the calculation formula of the stability safety factor, and the tunnel burial depth, working face size and other data are obtained according to the construction information or on-site actual measurement, and the relevant parameters of the soil body are obtained from the geological exploration data. The obtained optimal value is used to evaluate the formation stability under the condition of normal pressure opening of the rectangular tunnel, so as to ensure the safety of the actual project and achieve risk avoidance.

Description

Technical Field

[0001] The present invention relates to a method for evaluating the formation stability and safety of a rectangular tunnel boring machine, and particularly to a calculation method for determining the formation stability safety factor under the condition of opening the chamber for obstacle clearance under normal pressure. Background Art

[0002] Due to its advantages such as improved space utilization rate and less interference with ground traffic, rectangular tunnels are widely used in the construction of urban underground space projects such as subways. In urban construction, trenchless methods are mostly adopted, using a rectangular tunneling machine for excavation and combined with pipe jacking method for propulsion. The cross-section of a rectangular tunnel is relatively large, and the support pressure distribution is uneven, which easily leads to formation instability. Due to the complexity of the working conditions, the unpredictability of the geology, and the limitations of equipment such as the cutter head, when problems such as boulders and other obstacles are encountered during the tunneling process, the tunneling machine cannot continue to work and needs to open the chamber for obstacle clearance before construction can continue. For most tunneling machines, the chamber can be opened for obstacle clearance by removing the screw conveyor. At this time, it is under normal pressure to open the chamber, which greatly increases the risk of formation instability.

[0003] The formation stability safety factor is an important part of tunnel safety analysis. However, the current research does not consider the possible removal of the screw conveyor for chamber opening and obstacle clearance during the construction process, and the loss of support force in the middle area of the working face will pose a greater safety risk. Therefore, it is necessary to control the formation stability. Determining the calculation method of the formation stability safety factor under the condition of opening the chamber for obstacle clearance under normal pressure of a rectangular tunnel boring machine has great practical significance for tunnel construction safety. Summary of the Invention

[0004] In view of the deficiencies of the existing technology, the present invention provides a scientific and reliable method for evaluating the formation stability and safety of a rectangular tunnel boring machine, which can effectively avoid engineering risks.

[0005] The technical solution for achieving the purpose of the present invention is to provide a method for evaluating the formation stability under the condition of opening a rectangular tunnel under normal pressure, including the following steps:

[0006] (1) Establish a model under the condition of opening a rectangular tunnel boring machine under normal pressure. The model includes a lower model and an upper model; the lower model corresponds to the soil area in front of the middle working face where the tunnel loses support force after opening the chamber, and the upper model corresponds to the overlying soil of the soil area of the lower model; the lower model is a sliding block in the shape of a wedge, and the width of the wedge corresponds to the width of the tunnel chamber opening area B and the height of the wedge corresponds to the height of the working face D， The angle between the bottom sliding surface of the sliding block and the horizontal plane in the tunnel advancing direction is , which is 30° to 80°; the upper model is a prismatic block, and the height of the prismatic block corresponds to the tunnel burial depthC ;

[0007] (2) Obtain the following data from construction information or on-site actual measurements: the working face height of the tunnel D , the tunnel burial depth C , the width of the opening area B ; Obtain the following parameters from geological exploration data: the unit weight of soil , the effective internal friction angle of soil shear resistance , the effective cohesion of soil ;

[0008] (3) According to the model established in step (1) and the data provided in step (2), define the stability safety factor according to Equation (1) :

[0009] , (1)

[0010] where is the maximum shear strength of the soil, is the actually exerted shear strength;

[0011] Calculate the optimal value of the stability safety factor according to Equation (2):

[0012] (2)

[0013] where

[0014] (3)

[0015] (4)

[0016] (5)

[0017] is the load applied by the overlying soil, and is calculated according to Equation (6):

[0018] (6)

[0019] where is the lateral pressure coefficient, ; R is the ratio of the volume of the prismatic block of the upper model to its surrounding lateral area, ; is the surface load, is 0;

[0020] (4) The optimal value of the calculated safety factor For evaluating the stratum stability under the condition of normal pressure opening of a rectangular tunnel, when [condition 1], the stratum stability under the condition of normal pressure opening of the rectangular tunnel is in a safe state; when [condition 2], the stratum stability is in a limit state, or a state of instability and failure occurs.

[0021] The principle on which the model constructed in the present invention is based is as follows: The cross-section of a rectangular tunnel is relatively large, and multi-cutter heads are often used in tunneling machines, with a relatively large opening ratio, and the soil chamber pressure is prone to fluctuations and uneven distribution. Based on the characteristics of equipment such as cutter heads and screw conveyors, the cut soil and obstacles after opening the chamber must be discharged from the bottom conveyor. As the soil in the area near the discharge port is discharged, the soil in the upper and surrounding areas collapses accordingly. Therefore, the middle area of the soil chamber is most significantly affected, and an empty chamber area is correspondingly formed; the influence on the other two sides is much smaller, and the residual soil serves as the support for the working face, forming a residual support area. The working face is correspondingly divided into three parts. The irregular empty chamber area in the middle is simplified into a rectangle, and the areas on both sides are also rectangular areas that are still subject to residual support pressure. Using the limit equilibrium and slice method for model force analysis, with the model constructed in the present invention, the safety factor can be determined by analyzing the forces on the model using the limit equilibrium and slice method.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention combines the situation of normal pressure opening encountered in engineering construction with the analysis method of the slice method, provides a method for calculating the safety factor and evaluating the stability of the tunnel excavation face under special circumstances, and can effectively avoid engineering risks. Description of the Drawings

[0023] Figure 1 is a schematic diagram of the soil chamber pressure distribution characteristics under opening the chamber provided by an embodiment of the present invention;

[0024] Figure 2 、 3 are respectively schematic diagrams of the wedge-shaped body model under the condition of opening the chamber provided by an embodiment of the present invention, Figure 2 indicating the actual situation of the rectangular tunnel tunneling in the stratum, Figure 3 indicating the corresponding simplified model;

[0025] Figure 4 、 5 are respectively the force analysis diagrams of the slice method adopted by an embodiment of the present invention, Figure 4 indicating horizontal slicing of the wedge-shaped body, Figure 5 indicating a micro-element taken after slicing;

[0026] In the figure, 1 is the empty chamber area, not subject to support force; 2 is the residual soil support area, subject to uniform support force; 3 is the discharge port of the double-screw conveyor; I and III are the tunnel working faces bearing residual support force The area of Ⅱ; Ⅱ is the area without support force at the tunnel working face and is the research object for calculating the safety factor; Ⅳ and Ⅵ are the overlying soils of the tunnel working face areas Ⅰ and Ⅲ, which correspond to triangular prism-shaped blocks in the model; Ⅴ is the overlying soil of the tunnel working face area Ⅱ, which corresponds to a quadrangular prism-shaped block in the model. Detailed implementation manners

[0027] Example 1

[0028] See the appendix Figure 1 , which is a schematic diagram of the earth pressure distribution characteristics under the condition of opening the silo for the rectangular tunnel provided in this embodiment, and is used to calculate the earth pressure distribution of the rectangular tunnel boring machine under normal pressure when opening the silo; as Figure 1 can be seen, the silo consists of three rectangular areas. The middle is the empty silo area 1, which does not bear the support pressure, and the width is B ; the two sides are the residual soil support areas 2, which are subject to uniform support force; the discharge ports 3 of the double-screw soil extractor are located on both sides of the bottom of the empty silo area; the width of the tunnel working face is , and the height of the working face is D。

[0029] The method steps for calculating the residual support pressure of the working face of the rectangular tunnel boring machine in this embodiment are as follows:

[0030] Step 1:

[0031] See the appendix Figure 2 and 3 , which are respectively schematic diagrams of the model structure under the condition of opening the silo provided in this embodiment. Among them, Figure 2 represents the actual situation of the rectangular tunnel boring in the stratum, that is, the rectangular tunnel boring machine boring in the uniform soil layer, Figure 3 represents the corresponding simplified model. The model under the condition of opening the silo of the rectangular tunnel boring machine is a wedge-shaped body plus a silo structure, that is, a wedge-shaped body model and its upper silo, and the lower model corresponds to the soil body area in front of the empty silo area; as Figure 3 can be seen, Ⅰ and Ⅲ are the areas of the tunnel working face that bear the residual support force ; Ⅱ is the area without support force at the tunnel working face and is the research object for calculating the safety factor; Ⅳ and Ⅵ are the overlying soils of the tunnel working face areas Ⅰ and Ⅲ, which correspond to triangular prism-shaped blocks in the model; Ⅴ is the overlying soil of the tunnel working face area Ⅱ, which corresponds to a quadrangular prism-shaped block in the model. The upper model corresponds to the overlying soil of the lower model soil body area. Through the limit equilibrium analysis of the wedge-shaped body model, the block Ⅱ has no support force, that is, it is a sliding block and has the risk of instability. The model under the condition of opening the silo of the rectangular tunnel boring machine provided in this embodiment has a wedge-shaped body as its lower model, that is, the sliding block Ⅱ, which corresponds to the area where the support force is lost in the middle of the working face, and the width is B , and the height of the wedge-shaped body corresponds to the height of the working face D; The upper model is a quadrangular prism - shaped block Ⅴ, which is superimposed on the sliding block Ⅱ of the lower model, and its height corresponds to the tunnel burial depth C .

[0032] In this embodiment, the safety factor of the block Ⅱ in the area without support force at the tunnel working face is calculated, that is, the ratio of the maximum shear strength of the soil to the actually exerted shear strength, and the stability safety factor The definition formula is: , where is the maximum shear strength of the soil, represents the actually exerted shear strength.

[0033] The instability model of the opened - up part of the tunneling face under the condition of opening the silo in the rectangular tunnel constructed in this embodiment. The width of the tunnel working face L , height D , the burial depth is C , and the width of the opened - up area is B , which can be obtained through construction information or on - site measurement. The relevant formation parameters use effective indexes. The unit weight of the soil is , the effective cohesion is , and the effective internal friction angle is , which can be obtained through geological exploration data.

[0034] Step 2:

[0035] Refer to Attachment Figure 4 and 5 , which are the force - analysis diagrams of the strip - method adopted in this embodiment respectively. Figure 4 represents horizontal strip - division of the wedge - shaped body, is the distance from the slice to the bottom of the working face, is the angle between the bottom sliding surface of the sliding block and the horizontal plane in the tunnel advancing direction ， is the load applied by the overlying soil; Figure 5 represents a micro - element taken after strip - division; It can be seen from the force - analysis diagram of a small slice in the strip - method model constructed in this embodiment that the static equilibrium of an infinitesimal slice with a thickness of dz is considered after horizontal strip - division. The forces acting on the slice are the gravity of the slice dG , the "loading" force applied by the overlying soil layer on the slice V + dV , the "support" force applied by the underlying soil layer V , the shear forces on the vertical sliding surfaces on both sides , the shear force on the bottom inclined sliding surface dT and the normal force dN .

[0036] The shear force dT received by the inclined sliding surface cdhgis the ratio of its maximum shear strength to the safety factor and

[0037] (7)

[0038] The shear force on the two vertical slip surfaces abcd (efgh) on both sides is the ratio of its maximum shear strength to the safety factor and

[0039] (8)

[0040] Among them is the coefficient of lateral earth pressure, and , is the vertical stress

[0041] (9)

[0042] It can be obtained that

[0043] (10)

[0044] The gravity of the slice dG is

[0045] (11)

[0046] Among them is the unit weight of soil, dA represents the side area of the slice,

[0047] (12)

[0048] The equilibrium conditions of the slice parallel and perpendicular to the sliding direction are:

[0049] (13)

[0050] (14)

[0051] Substitute equations (7), (10) to (13) into equation (14), and the equilibrium equation parallel to the sliding direction of the wedge can be obtained:

[0052] (15)

[0053] Among them

[0054] (16)

[0055] (17)

[0056] (18)

[0057] Among them

[0058] (19)

[0059] In the formula P 、 M 、 S 、 For simplified representation, they have no practical meaning

[0060] The equation of formula (15) is a differential equation about the vertical stress V ( z ). Combining with the boundary condition V (0) = 0, we can get

[0061] (20)

[0062] Among them

[0063] (21)

[0064] (22)

[0065] In the formula 、 For simplified representation, they have no practical meaning

[0066] Regarding the differential equation about the vertical stress V ( z ), another boundary condition is ,

[0067] (23)

[0068] The load applied by the overlying soil is calculated by the silo theory

[0069] (24)

[0070] Among them represents the surface load, which is set to 0 in this embodiment; R is equal to the ratio of the volume of the soil column to its circumferential area

[0071] (25)

[0072] From the differential equation of the vertical stress V ( z ), another boundary condition , that is , and the equation only contains two unknowns, the dip angle and the safety factor . Solving it can obtain the failure mode and the solution of the safety factor under the limit condition

[0073] The calculated safety factor The optimal value is used to evaluate the formation stability under the condition of normal pressure opening of the rectangular tunnel. When the safety factor When, it indicates that the actual shear strength of the soil mass is less than its maximum shear strength, indicating that there is sufficient safety reserve for the evaluation of the formation stability under the condition of normal pressure opening of the rectangular tunnel; when the safety factor When, it indicates that the soil mass has actually reached the maximum shear strength and exerted all its shear resistance. The soil mass is in an ultimate state and is very likely to undergo instability or even failure.

[0074] In this embodiment, according to the actual engineering conditions, through actual measurement at the construction site, it can be obtained that: the working face height of the tunnel D = 4.2m, the working face width L = 6.9m, the tunnel burial depth C = 4.5m, the width of the opening area B = 2.4m; from the geological exploration data, it can be known that: the unit weight of the soil , the effective internal friction angle of the soil shear resistance , the effective cohesion of the soil is .

[0075] Substituting the above conditions into the formula, at this time, there is only One variable, Is the angle between the sliding surface at the bottom of the sliding block and the horizontal plane of the tunnel advancing direction, The range of can be 30° - 80°; Is the coefficient of lateral pressure, ; R Is the ratio of the volume of the prismatic block in the upper model to its surrounding lateral area, .

[0076] In this embodiment, software programming is used for calculation. Each calculation formula is listed according to the calculation sequence, aiming to obtain the minimum value of the safety factor, that is, to calculate the optimal solution and obtain the corresponding Value.

[0077] According to the data provided in this embodiment, the calculated safety factor . Since the safety factor is greater than 1, it can be known that the excavation face will not undergo instability failure in the project. The safety factor is relatively close to 1, and the shutdown and opening of the silo have caused a certain amount of ground settlement, which fits well with the actual situation.

Claims

1. An evaluation method for the formation stability under the condition of normal pressure opening of a rectangular tunnel, characterized in that including the following steps: (1)Establish a model under the condition of normal pressure opening of the rectangular tunnel boring machine. The model includes a lower model and an upper model. The lower model corresponds to the soil area in front of the middle working face where the tunnel loses the support force after the opening of the chamber, and the upper model corresponds to the overlying soil of the soil area of the lower model. The lower model is a sliding block in the shape of a wedge, and the width of the wedge corresponds to the width of the tunnel opening area. B The height of the wedge corresponds to the height of the working face. D， The included angle between the bottom sliding surface of the sliding block and the horizontal plane in the advancing direction of the tunnel is , 30° - 80°. The upper model is a prismatic block, and the height of the prismatic block corresponds to the tunnel burial depth. C ; (2)The following data is obtained from construction information or on-site actual measurements: the working face height of the tunnel D , the tunnel burial depth C , the width of the opening area B ; The following parameters are obtained from geological exploration data: the unit weight of soil , the effective internal friction angle of soil shear resistance , the effective cohesion of soil ; (3) According to the model established in step (1) and the data provided in step (2), define the stability safety factor according to formula (1). : ， （1） Among them, is the maximum shear strength of the soil mass, is the actually developed shear strength; The stability safety factor calculated according to formula (2) The optimal value of: （2） wherein, （3） （4） （5） The load applied to the overlying soil mass is calculated according to Equation (6) as follows: （6） Among them, is the lateral pressure coefficient, ; R is the ratio of the volume of the quadrangular prism block of the upper model to its surrounding lateral area, ; is the surface load, is 0; (4)The optimal value of the calculated safety factor is used to evaluate the formation stability under the condition of normal pressure opening of the rectangular tunnel When it is, the formation stability under the condition of normal pressure opening of the rectangular tunnel is in a safe state; When it is, the formation stability is in the limit state, or the instability and failure states occur.

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