Construction method of super-large section loess tunnel

Abstract

This invention discloses a construction method for extra-large cross-section loess tunnels, belonging to the field of tunnel construction. The method includes a two-stage, five-step method with an inverted arch and a three-stage, seven-step method with a single core soil. The two-stage, five-step method with an inverted arch is applicable under the following conditions: elastic modulus greater than 60 MPa, moisture content less than 18%, and density greater than 1.55 and less than 1.70 g / cm³. 3 The three-step, seven-stage method for single-core soil is applicable under the following conditions: elastic modulus greater than 70 MPa, moisture content less than 15%, and density greater than 1.60 and less than 1.76 g / cm³. 3 This invention improves upon existing single-sidewall pilot tunnel methods and three-step seven-step methods, forming a two-step five-step method with inverted arch and a single-core soil three-step seven-step method suitable for extra-large cross-section loess tunnels. Furthermore, the two-step five-step method with inverted arch is suitable as a transitional method between the double-sidewall pilot tunnel method and the single-core soil three-step seven-step method, enabling the improved construction method for extra-large cross-section loess tunnels to combine safety and construction progress.

Description

Technical Field

[0001] This invention relates to the field of construction technology for extra-large cross-section tunnels, and in particular to a construction method for extra-large cross-section loess tunnels. Background Technology

[0002] With the rapid development of loess tunnel construction, more and more three-lane and four-lane tunnels have emerged. However, increasing the number of lanes inevitably increases the tunnel width, leading to a larger excavation area and the creation of extra-large cross-section loess tunnels. Furthermore, due to the characteristics of loess, such as well-developed vertical fissures and low shear strength, tunnel excavation in loess strata often triggers sudden soil collapse and excessive deformation of the support structure. This is especially true for tunnels with an excavation cross-sectional area greater than 100 m². 2 The tunnel has a high risk of construction safety.

[0003] Currently, the double-sidewall pilot tunnel method is commonly used in China for the excavation of extra-large cross-section loess tunnels. The double-sidewall pilot tunnel method divides the tunnel cross-section into several parts, excavating them in stages. A cross-section constructed using the double-sidewall pilot tunnel method is shown in the attached figure. Figure 1 As shown in Table 1, the construction sequence is as follows.

[0004] Table 1 Construction sequence of traditional double-sided wall pilot tunnel method

[0005]

[0006]

[0007] It is evident that the existing double-sided wall pilot tunnel method requires the continuous erection and dismantling of temporary steel frames during construction. Furthermore, the connection process of the support structure of each small section is highly demanding, with numerous and complex procedures that are prone to mutual interference. In particular, the limited working space hinders mechanized construction, resulting in slow construction progress and difficulty in controlling the actual deformation of each procedure.

[0008] Currently, another method for tunnel construction is the traditional single-sidewall pilot tunnel method. This method divides the tunnel cross-section into four parts and excavates each part separately. The construction cross-section is shown in the attached figure. Figure 2 As shown in Table 2, the construction sequence is as follows.

[0009] Table 2 Construction Sequence of Traditional Single-Side Wall Pilot Tunnel Method

[0010]

[0011] Although the traditional single-side wall pilot tunnel method has relatively simple construction steps, it is not suitable for the excavation of loess tunnels with extra-large cross sections, as the surrounding rock is unstable and safety cannot be guaranteed.

[0012] Furthermore, the three-stage, seven-step method, due to its advantages of rapid construction and low cost, is also widely used in the excavation of loess tunnels. The existing three-stage, seven-step method uses an arc-shaped pilot tunnel excavation with core soil retention as its basic model, divided into upper, middle, and lower stages and seven excavation faces. The excavation and support of each part are carried out in parallel and staggered manner along the longitudinal direction of the tunnel. The cross-section of the three-stage, seven-step method is shown below. Figure 3 As shown in Table 3, the construction sequence is as follows.

[0013] Table 3. Construction Sequence of the Three-Step, Seven-Step Method

[0014]

[0015] It is evident that the existing three-stage seven-step method divides the tunnel cross-section into several sections for excavation and then applies initial support after excavation. However, due to the large number of excavation steps, the initial support closes into a ring relatively late, failing to form a complete load-bearing structure in time, which has a certain impact on the safety of construction. This is especially true for the excavation of loess tunnels with extra-large cross-sections, where the surrounding rock is unstable and safety cannot be guaranteed.

[0016] Therefore, it is evident that the existing traditional single-sidewall pilot tunnel method, double-sidewall pilot tunnel method, and three-step seven-stage method all have inconveniences and defects, and urgently need further improvement. The question is how to create a new construction method for extra-large cross-section loess tunnels, allowing for the selection of different construction methods based on varying loess conditions, ensuring both construction safety and reliability, thereby overcoming the problems existing in current traditional tunnel construction methods. Summary of the Invention

[0017] The technical problem to be solved by the present invention is to provide a construction method for loess tunnels with extra-large cross sections, which allows for the selection of different construction methods according to different loess conditions, meets the construction requirements of loess tunnels with extra-large cross sections, ensures both construction safety and construction safety, and overcomes the shortcomings of existing tunnel construction methods.

[0018] To address the aforementioned technical problems, this invention provides a construction method for extra-large cross-section loess tunnels, including a two-stage five-step method with an inverted arch and a single-core soil three-stage seven-step method. The two-stage five-step method with an inverted arch serves as a transitional method from the double-sidewall pilot tunnel method to the single-core soil three-stage seven-step method. The applicable conditions for the two-stage five-step method with an inverted arch are: elastic modulus greater than 60 MPa, moisture content less than 18%, and density greater than 1.55 g / cm³. 3 And less than 1.70 g / cm 3 The applicable conditions for the single-core soil three-step seven-stage method are: elastic modulus greater than 70 MPa, moisture content less than 15%, and density greater than 1.60 g / cm³. 3 And less than 1.76 g / cm 3 .

[0019] Further improvements include the following steps in the two-step, five-step method with an inverted arch:

[0020] S1. Divide the soil in the tunnel excavation section into upper and lower steps. First, excavate the left area of ​​the upper step. At the same time, provide initial support for the left side wall section after the excavation of the left area of ​​the upper step, and provide temporary support for the right side section. The bottom width of the left area of ​​the upper step is less than 50% of the width of the entire tunnel excavation section.

[0021] S2. Excavate the right area of ​​the upper step, leaving core soil. The bottom of the core soil is less than 40% of the width of the entire tunnel excavation section. At the same time, provide initial support for the right side wall section after the right area is excavated.

[0022] S3. Excavate the core soil;

[0023] S4. Excavate the left side area of ​​the lower step and provide initial support for the left side wall section after excavation of the left side area of ​​the lower step; wherein, the top width of the left side area of ​​the lower step is less than 20% of the width of the entire tunnel excavation section;

[0024] S5. Excavate the right side area of ​​the lower step and provide initial support for the right side wall section after excavation of the right side area of ​​the lower step; wherein, the top width of the right side area of ​​the lower step is less than 20% of the width of the entire tunnel excavation section.

[0025] S6. Remove the central partition wall formed by temporary support, excavate the central area of ​​the lower step, and carry out the initial support of the invert arch. Then, complete the secondary lining of the full-section invert arch, the filling of the full-section invert arch, and the secondary lining of the full-section arch wall in sequence to complete the construction of the extra-large section loess tunnel.

[0026] In a further improvement, the bottom width of the left region of the upper step in the two-step, five-step method with an inverted arch accounts for 40-45% of the total tunnel excavation cross-section width, the bottom of the core soil accounts for 34-40% of the total tunnel excavation cross-section width, the height of the core soil accounts for 22-30% of the total tunnel excavation cross-section width, and the top width of the left and right regions of the lower step each accounts for 10-15% of the total tunnel excavation cross-section width.

[0027] Further improvements were made: in step S1, the excavation of the pilot tunnel in the left area of ​​the upper step was 10-15m ahead of the excavation of the pilot tunnel in the right area of ​​the upper step in step S2; in step S2, the excavation of the pilot tunnel in the right area of ​​the upper step was 2-4m ahead of the core soil excavation in step S3; in step S3, the core soil excavation was 5-8m ahead of the excavation of the pilot tunnel in the left area of ​​the lower step in step S4; in step S4, the excavation of the pilot tunnel in the left area of ​​the lower step was 2-4m ahead of the excavation of the pilot tunnel in the right area of ​​the lower step in step S5; in step S5, the excavation of the pilot tunnel in the right area of ​​the lower step was 2-4m ahead of the excavation of the pilot tunnel in the center area of ​​the lower step in step S6; and the distance between each excavation surface remained unchanged during the construction process, with an excavation advance of 1m in each step.

[0028] Further improvements include the following steps in the single-core soil three-step seven-stage method:

[0029] T1. Divide the soil in the tunnel excavation section into three steps: upper, middle and lower. First, excavate the arc-shaped pilot tunnel of the upper step, and at the same time, provide initial support for the arc-shaped section after the excavation of the upper step.

[0030] T2. Excavate the left side area of ​​the middle bench, and simultaneously provide initial support for the left side section of the middle bench after the left side area is excavated; wherein, the bottom width of the left side area of ​​the middle bench is less than 25% of the width of the entire tunnel excavation section;

[0031] T3. Excavate the right side area of ​​the middle step, and simultaneously provide initial support for the right side section of the middle step after the right side area is excavated; wherein, the bottom width of the right side area of ​​the middle step is equal to the bottom width of the left side area of ​​the middle step;

[0032] T4. Excavation of the core soil for the upper and middle steps;

[0033] T5. Excavate the left side area of ​​the lower step, and at the same time provide initial support for the left side section of the lower step after the excavation of the left side area; wherein, the top width of the left side area of ​​the lower step is smaller than the bottom width of the left side area of ​​the middle step.

[0034] T6. Excavate the right side area of ​​the lower step and provide initial support for the right side section of the lower step after excavation; wherein, the top width of the right side area of ​​the lower step is equal to the top width of the left side area of ​​the lower step.

[0035] T7. Excavate the central area of ​​the lower step and provide initial support for the invert arch section of the lower step;

[0036] T8. The construction of the extra-large cross-section loess tunnel is completed by sequentially carrying out the secondary lining of the full-section inverted arch, the filling of the full-section inverted arch, and the secondary lining of the full-section arch wall.

[0037] In a further improvement, the bottom width of the left side region of the middle step in the single-core soil three-step seven-step method accounts for 15-25% of the total tunnel excavation cross-section width, and the bottom width of the left side region of the lower step accounts for 10-15% of the total tunnel excavation cross-section width. The dividing line between the middle step and the lower step is the tunnel arch waist.

[0038] Further improvements include: in step T1, the arc-shaped pilot tunnel excavated on the upper step is 4-6m ahead of the pilot tunnel excavated on the left side of the middle step in step T2; in step T2, the pilot tunnel excavated on the left side of the middle step is 2-4m ahead of the pilot tunnel excavated on the right side in step T3; in step T3, the pilot tunnel excavated on the right side of the middle step is 2-4m ahead of the core soil excavation in step T4; in step T4, the core soil excavation is carried out simultaneously with the pilot tunnel excavated on the left side of the lower step in step T5; in step T5, the pilot tunnel excavated on the left side of the middle step is 2-4m ahead of the pilot tunnel excavated on the right side in step T6; in step T6, the pilot tunnel excavated on the right side of the lower step is 2-4m ahead of the pilot tunnel excavated in the central area of ​​the lower step in step T7; and the distance between each excavation face remains unchanged during construction, with an excavation advance of 1m in each step.

[0039] Further improvements were made, and the initial support parameters for each step are as follows: HW175 steel arch frame is used, with a longitudinal spacing of 50cm, φ8 steel mesh is hung at 20cm×20cm, and C25 concrete is sprayed at 30cm.

[0040] The temporary support parameters are as follows: I20a type steel arch frame is used, with a longitudinal spacing of 50cm, φ6 steel mesh is hung at 20cm×20cm, and C25 concrete is sprayed at 22cm.

[0041] The support parameters for the secondary lining are: main reinforcement φ25, distribution reinforcement φ14, and sprayed C35 concrete 70cm.

[0042] Further improvements include the two-step five-step method with inverted arch and the single-core soil three-step seven-step method, both of which include the step of constructing anchor pipes for the arch waist, upper part of the arch waist and lower part of the arch waist after the initial support of the cross section. The anchor pipes are φ60*5mm, with a length of 400cm and a vertical outward inclination angle of 30-45°.

[0043] Further improvements include an excavation width of less than 20m, an excavation height of less than 14m, and an excavation cross-sectional area of ​​less than 190m² for the extra-large cross-section loess tunnel. 2 .

[0044] With this design, the present invention has at least the following advantages:

[0045] This invention improves upon the traditional single-sidewall pilot tunnel method by employing a two-stage, five-step method with an inverted arch. Through phased excavation of the upper and lower stages, the span of the excavation on the left side of the upper stage is reduced, decreasing arch crown settlement and surrounding rock deformation during left-side excavation. Furthermore, by reserving core soil during the excavation of the right side of the upper stage, surrounding rock deformation is better controlled. Additionally, by reducing the span of the left and right sides of the lower stage, it adapts to the excavation requirements of extra-large cross-section loess tunnels, ensuring construction safety. This two-stage, five-step method with an inverted arch is suitable as an alternative to the double-sidewall pilot tunnel method and as a transitional method between the double-sidewall pilot tunnel method and the single-core soil three-stage, seven-step method, thus enabling the improved construction method for extra-large cross-section loess tunnels to balance safety and construction progress.

[0046] The single-core soil three-stage seven-step method in this invention improves upon the traditional three-stage seven-step method. Based on the principles of "less disturbance, early support, frequent measurement, and tight closure," it simplifies the construction process of the core soil, reduces disturbance to the surrounding rock, and helps to give full play to the self-supporting capacity of the surrounding rock. It also shortens the distance between the invert arch and the tunnel face, allowing the support to close into a ring earlier, which helps to control the deformation of the surrounding rock, improves the stress on the support structure, and enables the support and surrounding rock to enter a good working state. This further shortens the construction period and ensures the construction safety of the extra-large cross-section loess tunnel. Attached Figure Description

[0047] The above is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0048] Figure 1 This is a schematic diagram of the construction cross-section of the existing double-sided wall pilot tunnel method.

[0049] Figure 2 This is a schematic diagram of the construction cross-section of the existing traditional single-sidewall pilot tunnel method.

[0050] Figure 3 This is a schematic diagram of the construction cross-section of the existing three-stage, seven-step method.

[0051] Figure 4 This is a schematic diagram of the lining structure of the two-step, five-step method with an inverted arch according to the present invention.

[0052] Figure 5 This is a diagram showing the excavation step distance and excavation sequence of the two-step, five-step method with inverted arch of the present invention.

[0053] Figure 6 These are schematic diagrams of finite element calculation models for the single-sidewall pilot tunnel method, the double-sidewall pilot tunnel method, and the two-step five-step method with inverted arch. Figure 6 Method a is the single-sidewall pilot tunnel method. Figure 6 b represents the double-side-wall pilot tunnel method. Figure 6 c represents the two-step, five-step method with an upward arch.

[0054] Figure 7 These are settlement convergence curves for the single-sidewall pilot tunnel method, the double-sidewall pilot tunnel method, and the two-step five-stage method with inverted arch. Figure 7 Method a is the single-sidewall pilot tunnel method. Figure 7 b represents the double-side-wall pilot tunnel method. Figure 7 c represents the two-step, five-step method with an upward arch.

[0055] Figure 8 This is a tunnel cross-sectional displacement cloud diagram (m) of the two-step, five-step method with inverted arch according to the present invention.

[0056] Figure 9 It is a radar image (MPa) of the steel arch frame stress in the tunnel section of the single-side wall pilot tunnel method, the double-side wall pilot tunnel method, and the two-step five-step inverted arch method.

[0057] Figure 10 This is a stress cloud diagram (Pa) of the steel arch frame using the single-side-wall pilot tunnel method and the two-step, five-step method with inverted arch. Figure 10 Method a is the single-sidewall pilot tunnel method. Figure 10 b is a two-step, five-step method with an upward arch.

[0058] Figure 11 This is a schematic diagram of the installation points of the steel arch strain gauges for observing the settlement and horizontal convergence of the arch top using the two-step, five-step method with an inverted arch, according to the present invention.

[0059] Figure 12 This is a numerical simulation of the arch crown settlement and horizontal convergence curves using the two-step, five-step method with inverted arches of this invention.

[0060] Figure 13 This is a graph showing the settlement and horizontal convergence curves of the arch crown during on-site monitoring using the two-step, five-step method with inverted arch of this invention.

[0061] Figure 14 The stress radar diagram (MPa) of the steel arch frame is obtained by numerical simulation and on-site monitoring at section 1 using the two-step five-step method with inverted arch of this invention.

[0062] Figure 15 The stress radar diagram (MPa) of the steel arch frame is obtained by numerical simulation and on-site monitoring at section 2 using the two-step five-step method with inverted arch of this invention.

[0063] Figure 16 The stress radar diagram (MPa) of the steel arch frame is obtained by numerical simulation and on-site monitoring at three sections using the two-step five-step method with inverted arch of this invention.

[0064] Figure 17 This invention relates to the influence of elastic modulus on the stress of steel arch frame in tunnel surrounding rock when using the two-step, five-step method with inverted arch in numerical simulation analysis.

[0065] Figure 18This invention relates to the influence of elastic modulus on the settlement and horizontal convergence of the tunnel surrounding rock arch when using the two-step, five-step method with inverted arch in numerical simulation analysis.

[0066] Figure 19 This invention relates to the influence of density on the stress of steel arch frames in tunnel surrounding rock during numerical simulation analysis using the two-step, five-step method with inverted arch.

[0067] Figure 20 This invention relates to the influence of density on the settlement and horizontal convergence of the tunnel surrounding rock arch when using the two-step, five-step method with inverted arch of this invention for numerical simulation analysis.

[0068] Figure 21 This is a cross-sectional comparison diagram between the traditional three-step seven-stage method and the single-core soil three-step seven-stage method of this application.

[0069] Figure 22 This is a schematic diagram of the lining structure of the single-core soil three-stage seven-step method of the present invention.

[0070] Figure 23 This is a diagram showing the excavation step distance and excavation sequence of the single-core soil three-step seven-step method of the present invention.

[0071] Figure 24 This is a schematic diagram of the finite element calculation model of the traditional three-step seven-step method and the single-core soil three-step seven-step method. Figure 24 A represents the traditional three-step, seven-stage method. Figure 24 b represents the three-stage, seven-step method for single-core soil.

[0072] Figure 25 These are settlement convergence curves for the traditional three-step seven-stage method and the single-core soil three-step seven-stage method. Figure 25 A represents the traditional three-step, seven-stage method. Figure 25 b represents the three-stage, seven-step method for single-core soil.

[0073] Figure 26 This is a tunnel cross-sectional displacement cloud map (m) based on the single-core soil three-step seven-step method of this invention.

[0074] Figure 27 It is a radar image (MPa) of the steel arch stress of the tunnel section in the traditional three-stage seven-step method and the single-core soil three-stage seven-step method.

[0075] Figure 28 This is the stress cloud diagram (Pa) of the steel arch frame of the single-core soil three-step seven-step method of the present invention.

[0076] Figure 29 This is a numerical simulation of the crown settlement and horizontal convergence curves using the single-core soil three-step seven-step method of this invention.

[0077] Figure 30This is a graph showing the crown settlement and horizontal convergence curves obtained by applying the single-core soil three-step seven-step method of this invention for on-site monitoring.

[0078] Figure 31 The stress radar map (MPa) of the steel arch frame was obtained by numerical simulation and field monitoring at section I using the single-core soil three-step seven-step method of this invention.

[0079] Figure 32 The stress radar map (MPa) of the steel arch frame was obtained by numerical simulation and field monitoring at section II using the single-core soil three-step seven-step method of this invention.

[0080] Figure 33 The stress radar map (MPa) of the steel arch frame was obtained by numerical simulation and field monitoring at section III using the single-core soil three-step seven-step method of this invention.

[0081] Figure 34 This invention relates to the influence of elastic modulus on the stress of steel arch frame in tunnel surrounding rock when using the single-core soil three-step seven-step method for numerical simulation analysis.

[0082] Figure 35 This invention relates to the influence of elastic modulus on the settlement and horizontal convergence of the tunnel surrounding rock arch when using the single-core soil three-step seven-step method for numerical simulation analysis.

[0083] Figure 36 This invention relates to the influence of density on the stress of steel arch frames in tunnel surrounding rock when using the single-core soil three-step seven-step method for numerical simulation analysis.

[0084] Figure 37 This invention relates to the influence of density on the settlement of the tunnel surrounding rock arch and horizontal convergence when using the single-core soil three-step seven-step method for numerical simulation analysis. Detailed Implementation

[0085] Example 1

[0086] This embodiment first improves the traditional single-sidewall pilot tunnel method, transforming it into a two-stage, five-step inverted arch method that can be quickly converted from the double-sidewall pilot tunnel method. This two-stage, five-step inverted arch method can meet the excavation conditions of extra-large cross-section loess tunnels, ensuring the construction safety of extra-large cross-section loess tunnels. (See attached...) Figure 2 As shown, in the traditional single-sidewall pilot tunnel method, the central diaphragm is located at half the width of the excavation tunnel, meaning the bottom width of the upper left area accounts for 50% of the total tunnel excavation cross-section width. For extra-large cross-section tunnels, this results in an excessively large excavation cross-section, easily leading to significant settlement of the arch and deformation of the surrounding rock. This application addresses this shortcoming with a two-stage, five-step method with an inverted arch, as illustrated in the following specific embodiment.

[0087] The extra-large cross-section loess tunnel described in this embodiment has a maximum excavation width of 17.89m, a maximum excavation height of 12.05m, and an excavation cross-sectional area of ​​183m². 2 .

[0088] This embodiment of the two-step, five-step method with an inverted arch first improves the position of the central diaphragm, as shown in the attached figure. Figure 4 As shown, this embodiment first shortens the width of the excavation area on the left side of the upper step, such as making the bottom width of the area on the left side of the upper step only 45% of the width of the entire tunnel excavation cross-section. This reduces the span of the excavation area on the left side, reduces the settlement of the arch crown, and avoids deformation of the surrounding rock. Another improvement of this embodiment is that, based on shortening the excavation distance on the left side, the distance of the central partition wall to the right pilot tunnel cross-section is inevitably increased. To prevent large settlement of the right arch crown, a core soil step is reserved during the excavation of the right pilot tunnel to control the deformation of the surrounding rock and ensure the construction safety of the extra-large cross-section loess tunnel.

[0089] See attached document Figure 4 As shown, the more specific implementation steps of the two-step, five-step method with inverted arch in this embodiment include:

[0090] S1. Divide the soil in the tunnel excavation section into upper and lower steps. First, excavate the left area A in the upper step. At the same time, carry out initial support I on the left side wall section after the excavation of the left area A in the upper step, and carry out temporary support II on the right side section. The bottom width of the left area A in the upper step accounts for 40-45% of the width of the entire tunnel excavation section.

[0091] S2. Excavate the right area B in the upper step, leaving core soil B1. The bottom of the core soil B1 accounts for 34-40% of the width of the entire tunnel excavation section, and the height of the core soil B1 accounts for 22-30% of the width of the entire tunnel excavation section. At the same time, perform initial support I on the right side wall section after the excavation of the right area B.

[0092] S3. Excavate the core soil B1;

[0093] S4. Excavate the left side area C1 of the lower bench, and provide initial support I for the left side wall section after the excavation of the left side area C1 of the lower bench; wherein, the top width of the left side area C1 of the lower bench accounts for 10-15% of the width of the entire tunnel excavation section.

[0094] S5. Excavate the right side area C2 of the lower bench, and provide initial support I for the right side wall section after the excavation of the right side area C2 of the lower bench; wherein, the top width of the right side area C2 of the lower bench accounts for 10-15% of the width of the entire tunnel excavation section;

[0095] S6. Remove the central partition wall formed by temporary support II, excavate the central area D of the lower step, and carry out the initial support I for the invert arch. Then, complete the secondary lining III of the full-section invert arch, the filling of the full-section invert arch, and the secondary lining III of the full-section arch wall in sequence to complete the construction of the extra-large section loess tunnel.

[0096] During the specific construction process, as shown in the attached document... Figure 5 As shown, in step S1, the excavation of the pilot tunnel in the left area A of the upper step precedes the excavation of the pilot tunnel in the right area B of the upper step in step S2 by 10-15m, preferably 13m; in step S2, the excavation of the pilot tunnel in the right area B of the upper step precedes the excavation of the core soil B1 in step S3 by 2-4m, preferably 3m; in step S3, the excavation of the core soil B1 precedes the excavation of the pilot tunnel in the left area C1 of the lower step in step S4 by 5-8m, preferably 7m; in step S4, the excavation of the pilot tunnel in the left area C1 of the lower step precedes the excavation of the pilot tunnel in the right area C2 of the lower step in step S5 by 2-4m, preferably 3m; in step S5, the excavation of the pilot tunnel in the right area C2 of the lower step precedes the excavation of the pilot tunnel in the central area D of the lower step in step S6 by 2-4m, preferably 3m. The excavation advance in each step is 1m; and the distance between each excavation surface remains unchanged during the construction process.

[0097] Furthermore, before implementing step S1 above, there is also a step of constructing the upper part of the tunnel excavation section using a pre-construction guide pipe E. The pre-construction guide pipe E is a φ60cm×5cm pre-construction guide pipe, 350cm long, with a ring spacing of 40cm and a horizontal upward inclination angle of 5-12°.

[0098] In this embodiment, the support parameters for initial support I are as follows: HW175 steel arch frame with a longitudinal spacing of 50cm, 20cm×20cm φ8 steel mesh, and 30cm of C25 shotcrete. The support parameters for temporary support II are as follows: I20a steel arch frame with a longitudinal spacing of 50cm, 20cm×20cm φ6 steel mesh, and 22cm of C25 shotcrete. The support parameters for secondary lining III are as follows: main reinforcement φ25, distribution reinforcement φ14, and 70cm of C35 shotcrete.

[0099] In addition, to further improve the reliability of the support, step S1 also includes the step of constructing two anchor pipes F on the arch waist and upper part of the left side wall section of the upper step after the initial support I; step S2 also includes the step of constructing two anchor pipes F on the arch waist and upper part of the right side wall section of the upper step after the initial support I; step S4 also includes the step of constructing two anchor pipes F on the lower part of the left side wall section of the lower step after the initial support I; step S5 also includes the step of constructing two anchor pipes F on the lower part of the right side wall section of the lower step after the initial support I. The anchor pipes F are φ60*5mm anchor pipes, with a length of 400cm and a vertical outward inclination angle of 30-45°.

[0100] This embodiment also compares the improved two-step five-step method with inverted arch with the single-side wall guide tunnel method and the double-side wall guide tunnel method through numerical simulation calculation, on-site monitoring and analysis and indoor tests, and finally determines the applicable conditions of the two-step five-step method with inverted arch.

[0101] (I) Analysis of Numerical Simulation Calculation Methods

[0102] Numerical simulations were performed on the single-side-wall pilot tunnel method, the double-side-wall pilot tunnel method, and the two-step five-step inverted arch method to analyze the deformation of the surrounding rock and the stress changes of the steel arch frame during the construction process.

[0103] ① Model Establishment

[0104] Numerical simulations were conducted using finite element method software, with the tracing element method serving as the modeling approach. To eliminate the influence of boundary effects on the calculation results, the calculation models for all three methods were designed with a longitudinal length of 60m. The surrounding rock of the models was Upper Pleistocene aeolian loess, classified as Class V, with the following parameters: elastic modulus 80MPa, Poisson's ratio 0.4, cohesion 35kPa, internal friction angle 29°, and density 1.57g / cm³. 3 According to Saint-Venant's principle, stress redistribution caused by excavation only occurs within a range of 3-5 times the excavation width from the center of the tunnel. Therefore, the model was selected with a width of 138m and a height of 133m (burial depth of 60m). The established model is attached. Figure 6 As shown. Among them, Figure 6 'a' represents the finite element calculation model of the single-sidewall pilot tunnel method. Figure 6 b is the finite element calculation model of the double-sidewall guide tunnel method. Figure 6 c represents the finite element calculation model of the two-step, five-step method with an inverted arch.

[0105] In this model, the surrounding rock is simulated using the Mohr-Coulomb constitutive model. Both the initial support and secondary lining are simulated using solid elements (C3D8R). The steel reinforcement parameters of the secondary lining are converted into concrete based on the equivalent stiffness principle. The steel arch of the initial support is embedded in the shotcrete using built-in commands. Both the steel arch and the anchor pipes are simulated using beam elements. The model uses the equivalent stiffness principle to simulate the reinforcement effect of the pre-excavated small guide pipes and the base jet grouting piles on the surrounding rock. Due to the disturbance caused by tunnel excavation, the stress in the surrounding rock redistributes, resulting in partial stress release. In the numerical simulation, the method of reducing the elastic modulus of the soil in the excavation area is used to simulate the stress release in the surrounding rock. The surrounding rock density and shear strength indices are obtained from indoor tests at the end of the double-sided pilot tunnel, and are also obtained with reference to the "Specifications for Design of Highway Tunnels" (JTG 3370.1-2018) and relevant existing data.

[0106] ②Results Analysis

[0107] Data on arch crown settlement, horizontal convergence, and steel arch stress were extracted from a section 5m from the opening of the single-sidewall pilot tunnel method, double-sidewall pilot tunnel method, and two-step five-stage inverted arch method models. The results of numerical simulation were then analyzed. (This 5m section location is consistent with the 5m interval between sections in the field monitoring and analysis methods.)

[0108] like Figure 7 The curves showing the changes in crown settlement and horizontal convergence values ​​as a function of excavation progress for three construction method models are presented. Figure 7 a represents the curves showing the changes in crown settlement and horizontal convergence values ​​using the single-sidewall pilot tunnel method. Figure 7 b represents the curves showing the changes in crown settlement and horizontal convergence values ​​using the double-sidewall pilot tunnel method. Figure 7 c represents the curves showing the changes in crown settlement and horizontal convergence values ​​for the two-step, five-step method with inverted arches.

[0109] from Figure 7 As can be seen from c, during the two-step, five-stage construction method with inverted arch, the settlement of the arch crown gradually increased and then stabilized, with a maximum settlement of 9.3 cm. The horizontal convergence of the surrounding rock showed a trend of gradually increasing, then decreasing, and then stabilizing, with a maximum value of 4.27 cm and a subsequent convergence of 1.23 cm. The horizontal convergence gradually decreased after the initial support of the lower step and inverted arch excavation closed into a ring, because the surrounding rock pressure caused the lining to deform outwards, leading to a decrease in horizontal convergence. Figure 7 As shown in section a, during the single-sidewall pilot tunnel construction, the crown settlement exhibits a trend of gradually increasing and then stabilizing, with a maximum value of 12.29 cm. The curve shows two points of significant increase: before the excavation of the upper left pilot tunnel and after the excavation of the upper right pilot tunnel. Additionally, a sudden increase in settlement occurs around analysis step 60. This is because the removal of the central diaphragm wall alters the original balance between the surrounding rock and the support, leading to a sudden increase in surrounding rock settlement. The trend of horizontal convergence of the surrounding rock is initially increasing, then decreasing, and then stabilizing, with a maximum value of 6.92 cm and a subsequent convergence to 2.41 cm. Before the excavation of the lower right pilot tunnel, the horizontal convergence of the surrounding rock continuously increases, and the convergence amplitude increases after the excavation of the upper right pilot tunnel. However, the horizontal convergence then decreases and stabilizes. This is because after the excavation of the lower right pilot tunnel, the initial lining at that section closes into a ring, and the surrounding rock pressure causes the lining to exert a reaction force on the surrounding rock, resulting in a decrease in horizontal convergence. A sudden change also occurs in the convergence decline section, which is due to the removal of the central diaphragm wall. Figure 7 As shown in b, during the double-sided wall pilot tunnel construction, the crown settlement and horizontal convergence exhibit similar trends to those of the single-sided wall pilot tunnel method. Crown settlement gradually increases during excavation, reaching a maximum of 7.3 cm. A sudden change occurs during the removal of the central diaphragm. Horizontal convergence gradually increases before the excavation of the upper step of the central wall, reaching a maximum of 5.9 cm. After the excavation of the lower step of the central wall, the initial lining closes into a ring, applying a support reaction force to the surrounding rock, leading to a decrease in horizontal convergence until it stabilizes at a value of 0.51 cm.

[0110] Please refer to the appendix. Figure 8 As shown, the displacement cloud map of the tunnel using the two-step, five-step method with inverted arch is illustrated. Figure 8 It can be seen that the maximum arch settlement of the tunnel after the completion of the two-step, five-step excavation with inverted arch method is 9.74cm.

[0111] According to the Technical Specification for Highway Tunnel Construction (JTGT3660-2020), the measured displacement value should not be greater than the designed reserved deformation amount U0 of the tunnel. The designed reserved deformation amount of this tunnel is 30cm. The calculated maximum displacement of the two-step five-step method with inverted arch is less than U0 / 3, which belongs to the Class III management level, indicating that the surrounding rock is in a stable state.

[0112] Figure 9 The image shows radar plots of steel arch frame stress at different locations along a 5m cross-section for three different construction method models. Figure 9 It can be seen that under all three construction methods, the steel arch frame is under compression, and the stress is roughly symmetrically distributed. During the single-side-wall pilot tunnel method and the two-step, five-step method with inverted arch, the left arch shoulder experiences the greatest stress, while the inverted arch experiences the least. During the single-side-wall pilot tunnel method, the areas of greatest stress on the steel arch frame are mainly at the arch shoulder, with the left arch shoulder experiencing the greatest stress, exceeding that at the arch crown. During the double-side-wall pilot tunnel method, the left and right arch waists experience the greatest stress, while the inverted arch experiences relatively less stress.

[0113] From again Figure 10 The stress cloud diagrams of the steel arch frame shown in the single-side-wall guide tunnel method and the two-step five-step with inverted arch method show that the maximum stress of the steel arch frame of the single-side-wall guide tunnel method is 181.5 MPa, and the maximum stress of the steel arch frame of the two-step five-step with inverted arch method is 164.3 MPa.

[0114] The "Technical Specification for Highway Tunnel Construction" (JTGT3660-2020) stipulates that the measured stress of the initial support should not exceed the allowable value (235 MPa) by more than 0.8. The calculated ratio of the maximum stress of the steel arch frame to the yield strength of the steel obtained by the two-step, five-stage method with an inverted arch is 164.3 / 235 = 0.7, indicating that the surrounding rock obtained by this method is in a stable state. The calculated ratio of the maximum stress of the steel arch frame to the yield strength of the steel obtained by the single-sidewall pilot tunnel method is 181.5 / 235 = 0.77, close to 0.8, indicating that the surrounding rock is in an unstable state. Furthermore, during construction, situations such as increased loess moisture content or fractured surrounding rock may occur, potentially causing the ratio of the maximum stress of the steel arch frame to the yield strength of the steel to exceed the specification requirements, thus compromising construction safety.

[0115] Based on the above analysis, it can be found that the crown settlement and horizontal convergence of the two-step, five-stage method with inverted arch are less than those of the single-side-wall pilot tunnel method, and slightly greater than those of the double-side-wall pilot tunnel method. However, within the range required by the specifications, the stress on the steel arch frame is greater at the two arch shoulders and the crown than in the other two methods, while it is less at other locations than in the single-side-wall and double-side-wall pilot tunnel methods. The ratio of stress to yield strength of the steel meets the specifications. In terms of construction, the two-step, five-stage method with inverted arch is simpler, requiring less frequent erection and dismantling of the steel arch frame, and its construction speed is greater than that of the double-side-wall pilot tunnel method. In contrast, the stress on the steel arch frame of the single-side-wall pilot tunnel method is close to the specifications. Furthermore, the possibility of increased loess moisture content or fractured surrounding rock during construction may cause the ratio of the maximum stress on the steel arch frame to the yield strength of the steel to exceed the specifications, thus compromising construction safety. In summary, compared with the double-side-wall pilot tunnel method, the two-stage five-step method with inverted arch can accelerate the construction progress and reduce the construction cost while controlling the deformation of the surrounding rock and the stress on the steel arch frame. The two-stage five-step method with inverted arch is feasible for excavation in loess tunnels with extra-large cross-sections.

[0116] (II) On-site monitoring and analysis

[0117] To ensure construction safety, a 30m test section was set up when using the two-step, five-stage inverted arch method. Three monitoring sections (section 1, section 2, and section 3) were established within the test section to monitor surrounding rock deformation and internal forces in the support structure. Each section was equipped with arch crown settlement and horizontal convergence observation points, as well as steel arch strain gauges, to monitor the condition of the surrounding rock and lining. The distribution of the monitoring points on site is as follows: Figure 11 As shown.

[0118] Comparison of numerical simulation calculations and field monitoring data

[0119] Cross-sections at 10m, 15m, and 20m in the numerical simulation model were selected to extract the settlement and horizontal convergence of the surrounding rock arch, as well as the stress values ​​of the steel arch frame. Settlement and horizontal convergence monitoring points were set up at the upper right pilot tunnel of these three cross-sections after excavation, and data were collected. Stress values ​​of the steel arch frame were extracted from stable data after excavation.

[0120] Appendix Figure 12 The figure shows the settlement of the tunnel arch and the horizontal convergence curves during tunnel excavation in the numerical simulation model. Figure 13 The diagram shows the settlement and horizontal convergence curves of the arch crown at the on-site monitoring section after the excavation of the upper right pilot tunnel.

[0121] Depend on Figure 12 It can be seen that the maximum settlement of the arch crown in the three sections of the numerical simulation model is 9.72 cm, the maximum horizontal convergence value is 3.77 cm, and it then stabilizes at 2.38 cm. The results after the excavation of the pilot tunnel in the upper right section of the three sections in the numerical simulation model are compared with the field monitoring data. Figure 13The comparison reveals that both methods exhibit similar trends in crown settlement and horizontal convergence. The numerical simulation of crown settlement ultimately converged to 9.72 cm, less than the maximum value of 11.14 cm observed in the field. The numerical simulation of horizontal convergence ultimately stabilized at 2.38 cm, also less than the maximum value of 3.54 cm observed in the field. This indicates that the numerical simulation can accurately reflect the actual field conditions.

[0122] Appendix Figure 14 The stress radar diagrams of the steel arch frame, obtained by numerical simulation and field monitoring at section 1 (10m) using the two-step five-step method with inverted arch, are shown. Figure 15 The stress radar diagrams of the steel arch frame, obtained by numerical simulation and field monitoring at section 2 (15m), are shown using the two-step five-step method with inverted arch.

[0123] Figure 16 The stress radar diagrams of the steel arch frame, obtained by numerical simulation and field monitoring at section 3 (20m), are shown using the two-step five-step method with inverted arch.

[0124] from Figure 14 , 15 As shown in section 16, the trends of numerical simulation and field monitoring results are quite consistent in each section. The steel arch frame is under pressure at various locations, and the pressure is roughly symmetrically distributed along the tunnel axis. Among them, the areas with greater stress on the steel arch frame are the arch shoulders and arch crown, while the middle of the two inverted arches is under the least stress. The data results of numerical simulation and field monitoring are quite consistent, which shows that the numerical simulation results can accurately describe the field conditions.

[0125] (III) Indoor Testing

[0126] To explore the applicability of the two-step, five-stage arch method with inverted arch, soil samples were collected every 2 meters along a 30m test section for laboratory testing. According to the "Specifications for Geotechnical Testing of Highways" (JTG3430-2020), the moisture content at the sampling points was obtained. Simultaneously, numerical simulations were used to analyze the effects of cohesion, internal friction angle, density, and elastic modulus on the stress of the steel arch frame in the tunnel surrounding rock, arch crown settlement, and horizontal convergence. The results are shown in the appendix. Figures 17 to 20 .

[0127] The results show that the elastic modulus and density have significant effects on the stress of the steel arch frame in the tunnel surrounding rock, the settlement of the arch crown, and the horizontal convergence, while the cohesion and internal friction angle have relatively small effects. Therefore, based on indoor experiments and numerical simulations, the applicable range of the two-step five-step method with inverted arch is further determined to be: elastic modulus greater than 60 MPa, moisture content less than 18%, and density greater than 1.55 g / cm³. 3 And less than 1.70 g / cm 3 .

[0128] Example 2

[0129] This embodiment improves upon the existing three-stage, seven-step method. Based on the New Austrian Tunneling Method's principles of "minimal disturbance, early support, frequent measurement, and tight closure," and considering the favorable surrounding rock conditions of the Zhonghe Tunnel, it proposes a single-core soil three-stage, seven-step construction method. This method further strengthens the control of surrounding rock deformation, improves initial support stress, enables early support closure, shortens the construction period, and is suitable for the excavation of extra-large cross-section loess tunnels, offering high safety. (Appendix) Figure 21 The construction process steps are compared between the traditional three-step seven-stage method (21a) and the single-core soil three-step seven-stage method (21b) of this application. Figure 21 As can be seen, the single-core soil three-stage seven-step method of this application is an improvement on the traditional three-stage seven-step method. It combines the upper, middle and lower core soils, significantly reducing the construction cross-section, simplifying the construction process, reducing disturbance to the surrounding rock, and shortening the distance between the invert and the working face. This allows the support to close into a ring earlier, which is beneficial for controlling the deformation of the surrounding rock, improving the stress on the support structure, and enabling the support and surrounding rock to enter a good working state. The specific embodiments of the single-core soil three-stage seven-step method of this application are as follows.

[0130] See attached document Figure 22 As shown, the more specific implementation steps of the single-core soil three-step seven-step method in this embodiment include:

[0131] T1. Divide the soil in the tunnel excavation section into three steps: upper, middle and lower. First, excavate the arc-shaped pilot tunnel A of the upper step, and at the same time, carry out initial support I on the arc-shaped section after the excavation of the upper step.

[0132] T2. Excavate the left side area B of the middle bench, and at the same time carry out initial support I on the left side section of the middle bench after the excavation of the left side area B; wherein, the bottom width of the left side area B of the middle bench accounts for 15-25% of the width of the entire tunnel excavation section, and the bottom of the middle bench is the tunnel arch waist.

[0133] T3. Excavate the right side region C of the middle step, and simultaneously provide initial support I for the right side section of the middle step after the excavation of the right side region C; wherein, the bottom width of the right side region C of the middle step is equal to the bottom width of the left side region B of the middle step;

[0134] T4. Excavate the core soil D of the upper and middle steps;

[0135] T5. Excavate the left side area E of the lower step, and at the same time, carry out initial support I on the left side section of the lower step after the excavation of the left side area E; wherein, the top width of the left side area E of the lower step is smaller than the bottom width of the left side area B of the middle step, accounting for 10-15% of the width of the entire tunnel excavation section.

[0136] T6. Excavate the right side region F of the lower step, and provide initial support I for the right side section of the lower step after the excavation of the right side region F; wherein, the top width of the right side region F of the lower step is equal to the top width of the left side region E of the lower step;

[0137] T7. Excavate the central area G of the lower step and provide initial support I for the invert arch section of the lower step;

[0138] T8. The construction of the extra-large cross-section loess tunnel is completed by sequentially carrying out the full-section invert arch secondary lining II, the full-section invert arch filling, and the full-section arch wall secondary lining II.

[0139] During the specific construction process, as shown in the attached document... Figure 23 As shown, in step T1, the arc-shaped pilot tunnel excavated on the upper step precedes the pilot tunnel excavated on the left side of the middle step in step T2 by 4-6m, preferably 5m; in step T2, the pilot tunnel on the left side of the middle step precedes the pilot tunnel on the right side of the middle step in step T3 by 2-4m, preferably 3m; in step T4, the pilot tunnel on the right side of the middle step precedes the core soil excavation in step T4 by 2-4m, preferably 3m; in step T4, the core soil excavation is carried out simultaneously with the pilot tunnel excavated on the left side of the lower step in step T5; in step T5, the pilot tunnel on the left side of the middle step precedes the pilot tunnel on the right side of the middle step in step T6 by 2-4m, preferably 3m; in step T6, the pilot tunnel on the right side of the lower step precedes the pilot tunnel in the center area of ​​the lower step in step T7 by 2-4m, preferably 3m. The excavation advance in each step is 1m, and the distance between each excavation face remains unchanged during construction.

[0140] Furthermore, the support parameters for the initial support and secondary lining in each step of this embodiment are the same as those in Embodiment 1. In each step, anchor pipes are constructed at the arch waist, upper arch waist, and lower arch waist after the initial support of the cross-section, which is also the same as in Embodiment 1.

[0141] This embodiment also compares the improved three-step seven-step method for single-core soil with the traditional three-step seven-step method through numerical simulation calculation, field monitoring and analysis, and indoor tests, and finally determines the applicable conditions of the three-step seven-step method for single-core soil.

[0142] (I) Analysis of Numerical Simulation Calculation Methods

[0143] Numerical simulations were performed on the traditional three-step seven-stage method and the single-core soil three-step seven-stage method to analyze the deformation of the surrounding rock and the stress changes of the steel arch during the construction process.

[0144] ① Model Establishment

[0145] Numerical simulation was conducted using finite element method software, with the tracing element method as the modeling approach. To eliminate the influence of boundary effects on the calculation results, the calculation models for both methods were designed with a longitudinal length of 60m. The surrounding rock of the models was Upper Pleistocene loess, classified as Class V, with the following parameters: elastic modulus 90MPa, Poisson's ratio 0.4, cohesion 40kPa, internal friction angle 32°, and density 1.68g / cm³. 3 According to Saint-Venant's principle, stress redistribution caused by excavation only occurs within a range of 3-5 times the excavation width from the center of the tunnel. Therefore, the model was selected with a width of 138m and a height of 133m (burial depth of 60m). The established model is attached. Figure 26 As shown. Among them, Figure 24 'a' represents the finite element calculation model of the traditional three-step, seven-stage method. Figure 24 b represents the finite element calculation model of the three-step, seven-step method for single-core soil.

[0146] In this model, the surrounding rock is simulated using the Mohr-Coulomb constitutive model. Both the initial support and secondary lining are simulated using solid elements (C3D8R). The steel reinforcement parameters of the secondary lining are converted into concrete based on the equivalent stiffness principle. The steel arch of the initial support is embedded in the shotcrete using built-in commands. Both the steel arch and the anchor pipes are simulated using beam elements. The model uses the equivalent stiffness principle to simulate the reinforcement effect of the pre-excavated small guide pipes and the base jet grouting piles on the surrounding rock. Due to the disturbance caused by tunnel excavation, the stress in the surrounding rock redistributes, resulting in partial stress release. In the numerical simulation, the method of reducing the elastic modulus of the soil in the excavation area is used to simulate the stress release in the surrounding rock. The surrounding rock density and shear strength indices are obtained from indoor tests at the end of the double-sided pilot tunnel, and are also obtained with reference to the "Specifications for Design of Highway Tunnels" (JTG 3370.1-2018) and relevant existing data.

[0147] ②Results Analysis

[0148] Data on crown settlement, horizontal convergence, and steel arch stress were extracted from a section 5m from the opening of both the traditional three-step seven-step method and the single-core soil three-step seven-step method model. The numerical simulation results were then analyzed. (This 5m section location is consistent with the 5m interval between sections in the field monitoring and analysis method.)

[0149] like Figure 25 The curves showing the variation of crown settlement and horizontal convergence values ​​with excavation progress for two construction method models are presented. Figure 25 'a' represents the curves showing the changes in crown settlement and horizontal convergence values ​​using the traditional three-step, seven-step method. Figure 25 b represents the curves showing the changes in crown settlement and horizontal convergence values ​​for a single-core soil using the three-step, seven-stage method.

[0150] from Figure 25As shown in curve b, the crown settlement during the single-core soil three-stage seven-step excavation process in this embodiment exhibits a trend of first increasing and then stabilizing, with a maximum value of 9.52 cm. Before the invert excavation, the crown settlement continuously increases. After the invert excavation, the initial support closes into a ring, forming a complete load-bearing structure, and the crown settlement tends to stabilize. The horizontal convergence trend is first increasing, then decreasing, and then stabilizing, with a maximum value of 2.77 cm and a stable value of 0.9 cm. Before the invert excavation, the horizontal convergence continuously increases. After the invert excavation, the initial support closes into a ring, forming a complete load-bearing structure. Under the pressure of the surrounding rock above the tunnel, the lining deforms outward, causing the horizontal convergence to decrease. Figure 25 Figure 'a' shows the crown settlement and horizontal convergence curves during the traditional three-stage seven-step excavation process. As can be seen from the figure, the crown settlement and horizontal convergence trends of the traditional three-stage seven-step method are consistent with those of the single-core soil three-stage seven-step method. Crown settlement continuously increases before invert excavation and then stabilizes, with a maximum value of 10.41 cm. Horizontal convergence also continuously increases before invert excavation. After the initial lining of the invert is closed into a ring, horizontal convergence decreases and stabilizes, with a stable value of 3.67 cm.

[0151] Please refer to the appendix. Figure 26 As shown, the tunnel displacement cloud map of the single-core soil three-step seven-step method is illustrated. Figure 26 It can be seen that the maximum arch settlement of the tunnel after the completion of the three-stage seven-step excavation method for single-core soil is 9.55cm.

[0152] According to the Technical Specification for Highway Tunnel Construction (JTGT3660-2020), the measured displacement value should not be greater than the designed reserved deformation amount U0 of the tunnel. The designed reserved deformation amount of this tunnel is 30cm. The calculated maximum displacement of the single core soil using the three-step seven-step method is less than U0 / 3, which belongs to the Class III management level, indicating that the surrounding rock is in a stable state.

[0153] Figure 27 The image shows radar plots of the steel arch frame stress at different locations along a 5m cross-section, representing two different construction methods. Figure 27 It can be seen that, under both construction methods, the stress on the steel arch frame in the single-core soil three-step seven-step method of this application is lower than that in the traditional three-step seven-step method, and the steel arch frame is under compression with a symmetrical stress distribution. In the traditional three-step seven-step method, the arch crown experiences the greatest stress, followed by the left and right arch shoulders, and the invert arch experiences the least stress. In the single-core soil three-step seven-step method of this application, the stress on the steel arch frame is similar to that of the traditional three-step seven-step method, with the arch crown experiencing the greatest stress, followed by the left and right arch shoulders, and the invert arch experiencing the least stress.

[0154] Appendix Figure 28 This paper presents a stress cloud diagram of the steel arch frame using the three-step, seven-stage method for single-core soil in this application. Figure 28 It can be seen that the maximum stress of the steel arch frame with a single core soil three-step seven-step method is 164MPa.

[0155] The "Technical Specification for Highway Tunnel Construction" (JTGT3660-2020) stipulates that the measured stress of the initial support shall not exceed the allowable value (235 MPa) by more than 0.8. The ratio of the maximum stress of the steel arch frame to the yield strength of the steel, calculated using the three-step, seven-stage method for single-core soil, is 164 / 235 = 0.7, indicating that the surrounding rock obtained using the three-step, seven-stage method for single-core soil is in a stable state.

[0156] Based on the above analysis, it can be found that the crown settlement and horizontal convergence of the single-core soil three-step seven-step method are both smaller than those of the traditional three-step seven-step method, decreasing by 0.89 cm and 2.77 cm respectively. Furthermore, the surrounding rock is in a stable state during excavation using the single-core soil three-step seven-step method. Except for the arch bottom, where the stress of the steel arch frame is higher than that of the traditional three-step seven-step method, the stress at other locations using the single-core soil three-step seven-step method is lower, with a maximum reduction of 37.77 MPa. The stress-to-yield strength ratio of the steel in the single-core soil three-step seven-step method meets the specification requirements. Adhering to the New Austrian Tunneling Method's principle of "minimal disturbance and tight closure," the single-core soil three-step seven-step method simplifies the construction process, shortens the time for initial support closure, and improves the stress on the initial support, making it more advantageous than the traditional three-step seven-step method in terms of surrounding rock deformation and steel arch frame stress. In summary, the single-core soil three-stage seven-step method can better control the deformation of the surrounding rock and reduce the stress on the steel arch frame compared with the traditional three-stage seven-step method. It is feasible to use the single-core soil three-stage seven-step method for excavation in loess tunnels with extra-large cross sections.

[0157] (II) On-site monitoring and analysis

[0158] To verify the feasibility of the three-step, seven-stage excavation method for single-core soil, a 30m test section using this method was set up in the Zhonghe Tunnel. Three monitoring sections were established at chainages ZK57+186.8, ZK57+191.8, and ZK57+196.8 to monitor surrounding rock deformation and internal forces in the support structure. Each section was equipped with arch crown settlement and horizontal convergence observation points, as well as steel arch strain gauges, to monitor the condition of the surrounding rock and lining. The on-site monitoring results are as follows: Figure 11 As shown.

[0159] Comparison of numerical simulation calculations and field monitoring data

[0160] Cross-sections at 10m, 15m, and 20m in the numerical simulation model were selected to extract the settlement and horizontal convergence of the surrounding rock arch, as well as the stress values ​​of the steel arch frame. Since the horizontal convergence measuring points were arranged after the excavation of the right-side middle bench, the settlement and horizontal convergence data of the arch crown after the excavation of the right side of the middle bench were taken, and the stress values ​​of the steel arch frame were extracted from the stable data after the excavation was completed.

[0161] Figure 29 The figure shows the crown settlement and horizontal convergence curves during the three-step, seven-stage tunnel excavation process in a single-core soil system, based on a numerical simulation model. Figure 30 The data shows the crown settlement and horizontal convergence after the excavation of the middle bench on the right side of the on-site monitoring section. Figure 29 It can be seen that the maximum settlement of the arch crown in the three cross-sections of the numerical simulation was 9 cm, the maximum horizontal convergence was 2.77 cm, and it later stabilized at 1.15 cm. The results after excavation of the middle bench on the right side of the three cross-sections in the numerical simulation were compared with the field monitoring data. Figure 30 A comparison reveals that both methods exhibit similar trends in crown settlement and horizontal convergence. The numerically simulated crown settlement ultimately converged to 9 cm, less than the maximum value of 12 cm observed in the field. The numerically simulated horizontal convergence ultimately stabilized at 1.15 cm, less than the maximum value of 2.33 cm observed in the field. Through comparison... Figure 29 and Figure 30 It can be seen that the settlement and horizontal convergence trends of the arch in the numerical simulation results are consistent with the monitoring data, which shows that the numerical simulation can accurately reflect the on-site situation.

[0162] Regarding construction progress, according to on-site construction statistics, the monthly progress of the single-core soil three-stage seven-step method can reach 80m or more, which is significantly better than the 70m monthly progress of the traditional three-stage seven-step method. This indicates that the single-core soil three-stage seven-step method can not only better control the deformation of the surrounding rock and improve the stress of the steel arch frame, but also accelerate the construction progress and save costs.

[0163] Appendix Figure 31 The stress radar diagrams of the steel arch frame, obtained by numerical simulation and field monitoring at section 1 (10m), are shown using the single-core soil three-step seven-step method. Figure 32 The stress radar diagrams of the steel arch frame, obtained by numerical simulation and field monitoring at section II (15m), are shown using the single-core soil three-step seven-step method. Figure 33 The stress radar plots of the steel arch frame obtained by numerical simulation and field monitoring at section III (20m) using the single-core soil three-step seven-step method are shown. Table 4 below lists the comparison of the average stress values ​​of the steel arch frame obtained by numerical simulation and field monitoring at the three sections.

[0164] Table 4 Comparison of Simulated and Measured Data on Stress in Steel Arch Frames

[0165]

[0166] from Figure 31 , 32As shown in Table 33, the trends of numerical simulation and field monitoring results are largely consistent in each cross-section. The steel arch frame is under pressure at all locations, and the stress distribution is roughly symmetrical along the tunnel axis. Among them, the areas of higher stress in the steel arch frame are the arch crown and the left and right arch shoulders, while the stress at the invert arch is relatively low. Table 4 shows that the data results of numerical simulation and field monitoring are largely consistent, with errors within 20%, indicating that the numerical simulation results can accurately describe the field conditions. This further demonstrates that the single-core soil three-stage seven-step method is superior to the traditional three-stage seven-step method in the construction of extra-large cross-section loess tunnels.

[0167] (III) Indoor Testing

[0168] To explore the applicability of the three-step, seven-stage method for single-core soil, soil samples were collected every 2 meters along a 30m test section for laboratory testing. The moisture content at the sampling points was obtained according to the "Specifications for Geotechnical Testing of Highways" (JTG3430-2020). Simultaneously, numerical simulations were used to analyze the effects of cohesion, internal friction angle, density, and elastic modulus on the stress of the steel arch frame in the tunnel surrounding rock, arch crown settlement, and horizontal convergence. The results are shown in the appendix. Figures 34 to 37 .

[0169] The results show that the elastic modulus and density have significant effects on the stress of the steel arch frame in the tunnel surrounding rock, the settlement at the arch crown, and the horizontal convergence, while the cohesion and internal friction angle have relatively small effects. Therefore, based on indoor experiments and numerical simulations, the applicable range of the three-step seven-stage method for single-core soil is further determined as follows: elastic modulus greater than 70 MPa, moisture content less than 15%, and density greater than 1.60 g / cm³. 3 And less than 1.76 g / cm 3 .

[0170] As can be seen from the above embodiments, the two-stage five-step method with inverted arch can serve as a transitional construction method from the existing double-sidewall pilot tunnel method to the single-core soil three-stage seven-step method. It can ensure the construction safety of extra-large cross-section loess tunnels, as well as the construction progress and improve construction efficiency.

[0171] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, equivalent changes, or alterations made by those skilled in the art using the disclosed technical content shall fall within the protection scope of the present invention.

Claims

1. A construction method for an extra-large cross-section loess tunnel, characterized in that, The methods include the two-step, five-step method with inverted arch and the three-step, seven-step method with a single core soil layer. The two-step, five-step method with inverted arch serves as a transitional method from the double-sidewall pilot tunnel method to the three-step, seven-step method with a single core soil layer. The applicable conditions for the two-step, five-step method with inverted arch are: elastic modulus greater than 60 MPa, moisture content less than 18%, and density greater than 1.55 g / cm³. 3 And less than 1.70 g / cm 3 The applicable conditions for the single-core soil three-step seven-stage method are: elastic modulus greater than 70 MPa, moisture content less than 15%, and density greater than 1.60 g / cm³. 3 And less than 1.76 g / cm 3 ; The two-step, five-step method with an inverted arch includes the following steps: S1. Divide the soil within the tunnel excavation section into upper and lower steps. First, excavate the left area of ​​the upper step. Simultaneously, provide initial support for the left side wall of the excavated left area of ​​the upper step, and provide temporary support for its right side. The bottom width of the left area of ​​the upper step is less than 50% of the width of the entire tunnel excavation section. S2. Excavate the right area of ​​the upper step, leaving core soil. The bottom of the core soil is less than 40% of the width of the entire tunnel excavation section. At the same time, provide initial support for the right side wall section after the right area is excavated. S3. Excavate the core soil; S4. Excavate the left side area of ​​the lower bench, and provide initial support for the left side wall section after excavation of the left side area of ​​the lower bench; wherein, the top width of the left side area of ​​the lower bench is less than 20% of the width of the entire tunnel excavation section; S5. Excavate the right-side area of ​​the lower bench, and provide initial support for the right-side wall section after excavation of the right-side area of ​​the lower bench; wherein, the top width of the right-side area of ​​the lower bench is less than 20% of the width of the entire tunnel excavation section; S6. Remove the central partition wall formed by the temporary support, excavate the central area of ​​the lower step, and carry out the initial support of the invert arch. Then, complete the secondary lining of the full-section invert arch, the filling of the full-section invert arch, and the secondary lining of the full-section arch wall in sequence to complete the construction of the extra-large section loess tunnel. In the two-step, five-step method with inverted arch, the bottom width of the left area of ​​the upper step accounts for 40-45% of the total tunnel excavation cross-section width, the bottom of the core soil accounts for 34-40% of the total tunnel excavation cross-section width, the height of the core soil accounts for 22-30% of the total tunnel excavation cross-section width, and the top width of the left and right areas of the lower step each accounts for 10-15% of the total tunnel excavation cross-section width.

2. The construction method for extra-large cross-section loess tunnels according to claim 1, characterized in that, In step S1, the excavation of the pilot tunnel in the left area of ​​the upper step is 10-15m ahead of the excavation of the pilot tunnel in the right area of ​​the upper step in step S2. In step S2, the excavation of the pilot tunnel in the right area of ​​the upper step is 2-4m ahead of the excavation of the core soil in step S3. In step S3, the excavation of the core soil is 5-8m ahead of the excavation of the pilot tunnel in the left area of ​​the lower step in step S4. In step S4, the excavation of the pilot tunnel in the left area of ​​the lower step is 2-4m ahead of the excavation of the pilot tunnel in the right area of ​​the lower step in step S5. In step S5, the excavation of the pilot tunnel in the right area of ​​the lower step is 2-4m ahead of the excavation of the pilot tunnel in the center area of ​​the lower step in step S6. During the construction process, the distance between each excavation surface remains unchanged, and the excavation advance in each step is 1m.

3. The construction method for extra-large cross-section loess tunnels according to claim 1 or 2, characterized in that, The single-core soil three-step seven-step method includes the following steps: T1. Divide the soil in the tunnel excavation section into three steps: upper, middle and lower. First, excavate the arc-shaped pilot tunnel of the upper step, and at the same time, provide initial support for the arc-shaped section after the excavation of the upper step. T2. Excavate the left side area of ​​the middle bench, and simultaneously provide initial support for the left side section of the middle bench after the left side area is excavated; wherein, the bottom width of the left side area of ​​the middle bench is less than 25% of the width of the entire tunnel excavation section; T3. Excavate the right side area of ​​the middle step, and simultaneously provide initial support for the right side section of the middle step after the right side area is excavated; wherein, the bottom width of the right side area of ​​the middle step is equal to the bottom width of the left side area of ​​the middle step; T4. Excavation of the core soil for the upper and middle steps; T5. Excavate the left side area of ​​the lower step, and at the same time provide initial support for the left side section of the lower step after the excavation of the left side area; wherein, the top width of the left side area of ​​the lower step is smaller than the bottom width of the left side area of ​​the middle step. T6. Excavate the right side area of ​​the lower step and provide initial support for the right side section of the lower step after excavation; wherein, the top width of the right side area of ​​the lower step is equal to the top width of the left side area of ​​the lower step. T7. Excavate the central area of ​​the lower step and provide initial support for the invert arch section of the lower step; T8. The construction of the extra-large cross-section loess tunnel is completed by sequentially carrying out the secondary lining of the full-section inverted arch, the filling of the full-section inverted arch, and the secondary lining of the full-section arch wall.

4. The construction method for extra-large cross-section loess tunnels according to claim 3, characterized in that, In the single-core soil three-step seven-step method, the bottom width of the left side area of ​​the middle step accounts for 15-25% of the total tunnel excavation cross-section width, and the bottom width of the left side area of ​​the lower step accounts for 10-15% of the total tunnel excavation cross-section width. The dividing line between the middle step and the lower step is the tunnel arch waist.

5. The construction method for extra-large cross-section loess tunnels according to claim 3, characterized in that, In step T1, the arc-shaped pilot tunnel excavated on the upper step is 4-6m ahead of the pilot tunnel excavated on the left side of the middle step in step T2. In step T2, the pilot tunnel excavated on the left side of the middle step is 2-4m ahead of the pilot tunnel excavated on the right side of the middle step in step T3. In step T3, the pilot tunnel excavated on the right side of the middle step is 2-4m ahead of the core soil excavation in step T4. The core soil excavation in step T4 is carried out simultaneously with the pilot tunnel excavated on the left side of the lower step in step T5. In step T5, the pilot tunnel excavated on the left side of the middle step is 2-4m ahead of the pilot tunnel excavated on the right side of the middle step in step T6. In step T6, the pilot tunnel excavated on the right side of the lower step is 2-4m ahead of the pilot tunnel excavated in the center area of ​​the lower step in step T7. The distance between each excavation face remains unchanged during the construction process, and the excavation advance in each step is 1m.

6. The construction method for extra-large cross-section loess tunnels according to claim 3, characterized in that, The initial support parameters for each step are as follows: HW175 steel arch frame is used, with a longitudinal spacing of 50cm, φ8 steel mesh is hung at 20cm×20cm, and C25 concrete is sprayed at 30cm. The temporary support parameters are as follows: I20a type steel arch frame is used, with a longitudinal spacing of 50cm, φ6 steel mesh is hung at 20cm×20cm, and C25 concrete is sprayed at 22cm. The support parameters for the secondary lining are: main reinforcement φ25, distribution reinforcement φ14, and sprayed C35 concrete 70cm.

7. The construction method for extra-large cross-section loess tunnels according to claim 6, characterized in that, Both the two-step five-step method with inverted arch and the single-core soil three-step seven-step method include the step of constructing anchor pipes for the arch waist, upper part of the arch waist and lower part of the arch waist after the initial support of the cross section. The anchor pipes are φ60*5mm anchor pipes with a length of 400cm and a vertical outward inclination angle of 30-45°.

8. The construction method for extra-large cross-section loess tunnels according to claim 1, characterized in that, The excavation width of the extra-large cross-section loess tunnel is less than 20m, the excavation height is less than 14m, and the tunnel excavation cross-sectional area is less than 190m². 2 .

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