Elliptical tunnel surrounding rock pressure calculation method based on surrounding rock attribute partition

Abstract

The invention provides an elliptical tunnel surrounding rock pressure calculation method based on surrounding rock attribute zoning, and the method comprises the steps: dividing surrounding rock disturbance caused by tunnel construction into three attribute regions, namely a loose circle, a collapse arch and a stable arch; therefore, the engineering load generated by the loose ring and the engineering load generated by the caving arch can be superposed to form the surrounding rock pressure. The method can more accurately predict the distribution characteristics of the surrounding rock pressure according to the tunnel surrounding rock failure characteristics and the surrounding rock attribute partition. Furthermore, in consideration of possible partial bearing of collapse arches of different grades of surrounding rocks, the surrounding rock pressure is given to different weight grades through an adjusting coefficient delta, and the flexibility and accuracy of a calculation algorithm are improved.

Description

Technical Field

[0001] This application belongs to the technical field of surrounding rock pressure, and specifically, relates to a calculation method for the surrounding rock pressure of an elliptical tunnel based on surrounding rock attribute zoning. Background Technique

[0002] With the rapid economic development and the acceleration of the urbanization process, tunnel engineering has been more and more widely applied in the fields of transportation, water conservancy, energy, etc. By the end of 2022, remarkable achievements have been made in the construction of traffic tunnels in China, and the total mileage has reached 55,892 kilometers. The safety of tunnel engineering is directly related to people's lives and property safety and the economic benefits of the project. Therefore, the accurate calculation and control of the surrounding rock pressure of the tunnel are particularly important.

[0003] The calculation of the surrounding rock pressure is a prerequisite for the structural design and internal force analysis of highway tunnels. The nature, magnitude, direction, and occurrence and development laws of the surrounding rock pressure have important impacts on tunnel design and construction. The determination of the surrounding rock pressure usually involves various methods, including theoretical calculation, numerical simulation, and on-site monitoring, etc. For example, the main theoretical calculation methods are the Fenner formula and the Kastner formula based on the Mohr-Coulomb yield criterion, but these methods often difficult to accurately reflect the surrounding rock pressure distribution in actual projects. In actual projects, the calculation of the surrounding rock pressure also needs to consider various factors such as geological conditions, design loads, and construction techniques. For example, factors such as the classification of the surrounding rock, the integrity coefficient of the rock mass, and the influence of groundwater will all affect the calculation of the surrounding rock pressure. In addition, the burial depth, span, and construction method of the tunnel are also important factors affecting the surrounding rock pressure.

[0004] In the current lining structure design, the surrounding rock pressure is indeed an important load item for structural safety control. Due to the scouring and corrosion effects caused by the seepage and flow of groundwater, the lining foundation may sink, the side walls may crack, or the invert and the integral track bed may sink and crack. In addition, the surrounding rock slip and dislocation may also cause the lining to deform and crack. If the over-excavation is not filled solidly or not filled completely, it may cause the surrounding rock to collapse, resulting in the lining failure.

[0005] The influence of the surrounding rock pressure on the tunnel lining is multi-faceted. In the case of voids behind the lining, the lining structure bears unevenly and stress concentration occurs, resulting in the secondary lining being extremely prone to cracking, which in turn causes a series of hazards such as water seepage, freezing damage, and steel bar corrosion. Seriously, it may induce the relaxation or instability and shedding of the surrounding rock, and even sudden collapse accidents may occur, posing a serious threat to the maintenance cycle and service life of the tunnel, and posing a serious threat to the traffic quality and the lives and safety of personnel. In tunnel design, the calculation of the surrounding rock pressure is a key factor to ensure structural safety and economy.

[0006] Therefore, it is very necessary to conduct an in-depth and systematic study on the calculation of surrounding rock pressure in tunnel design. By scientifically analyzing and evaluating the safety of tunnel lining structures, the influence of surrounding rock pressure on tunnel lining structures can be better understood, thereby improving the safety and durability of tunnel engineering. However, there are certain limitations in the traditional tunnel design code for calculating surrounding rock pressure. For the calculation of surrounding rock pressure in the current tunnel design code, for deep-buried tunnels, the method of multiplying the overlying confining pressure unit weight by the load height is usually used; for shallow-buried tunnels, the Prandtl pressure theory is adopted, and basically it is also calculated by multiplying the unit weight by the load height. Although these methods are widely used in engineering practice, they are usually relatively conservative and may not accurately reflect the actual deformation disturbance mechanism and failure mode of the tunnel.

[0007] In view of the problem that the current traditional calculation method of tunnel surrounding rock pressure may be too simplified and unable to accurately reflect the actual deformation disturbance mechanism and failure mode of the tunnel, it is urgent to propose a new calculation method, which should consider the failure characteristics of tunnel surrounding rock and the zoning of surrounding rock properties to achieve precision design, optimized design and safety design. Summary of the Invention

[0008] Based on the above-mentioned defects of the existing technology, the present invention proposes an elliptical tunnel surrounding rock pressure calculation method based on the zoning of surrounding rock properties, which includes the following steps:

[0009] 1) Conduct zoning of the properties of the surrounding rock of the elliptical tunnel; the property zoning consists of at least 3 parts;

[0010] 2) According to the property zoning, determine the composition of the surrounding rock pressure of the tunnel, and then form the weights of the pressures in the surrounding rock pressure and calculate the values of each pressure.

[0011] Further, the property zoning includes a loosening zone that can no longer bear load, a caving arch that can no longer bear load or can bear part of the load, and a stable arch that can bear load.

[0012] Further, if the engineering load generated by the loosening zone is P1 and the engineering load generated by the caving arch is P2, then

[0013] P = P1 + δP2

[0014] In the formula, δ is an adjustment coefficient.

[0015] Further, the loosening zone is a plastic zone, and the engineering load P1 generated by the loosening zone is calculated by the following formula:

[0016]

[0017] In the formula, γ is the unit weight of the overlying surrounding rock, with the unit kN / m 3 ; 2a is the major axis of the elliptical tunnel, with the unit m; is the internal friction angle of the surrounding rock, with the unit of degree (°).

[0018] Furthermore, the caving arch is a deformation disturbance area, and the calculation process of the engineering load P2 generated by the caving arch at least includes the following steps:

[0019] (1) Qualitatively determine the range of the caving arch;

[0020] (2) Determine the swelling coefficient of the overlying surrounding rock of the tunnel: Determine the swelling coefficient k of the surrounding rock according to the surrounding rock grade;

[0021] (3) Quantitatively determine the range of the caving arch: Take the center of the ellipse as the coordinate origin, with the major axis and minor axis of the ellipse located on the x-axis and y-axis respectively, and establish a plane rectangular coordinate system. Then the cross-sectional shape formed by the caving arch conforms to a parabola, the vertex coordinates of the parabola are (0, h), the major semi-axis of the ellipse is a, and the minor semi-axis is b. Then the surrounding rock pressure P2 formed by the caving arch is:

[0022]

[0023] In the formula, x is the x-axis coordinate of the parabolic distribution, with the unit of m.

[0024] Furthermore, the swelling coefficient k is determined according to the following principles: For Class I surrounding rock, rocky, the swelling coefficient k is 1.85; for Class II surrounding rock, rocky, the swelling coefficient k is 1.80; for Class III surrounding rock, rocky, the swelling coefficient k is 1.70; for Class IV surrounding rock, rocky, the swelling coefficient k is 1.60; for Class V surrounding rock, hard clay, the swelling coefficient k is 1.35; for Class V surrounding rock, sandy soil with pebbles, the swelling coefficient k is 1.30; for Class VI surrounding rock, cohesive soil, the swelling coefficient k is 1.25; for Class V surrounding rock, gravel, the swelling coefficient k is 1.30; for Class VI surrounding rock, cohesive soil, the swelling coefficient k is 1.15.

[0025] Furthermore, the value range of the adjustment coefficient δ is 0 - 1; when it is Class I surrounding rock, δ = 0; when it is Class VI surrounding rock, δ = 1; when the surrounding rock grade is between I - VI, δ is determined by linear interpolation.

[0026] Furthermore, when the value of the adjustment coefficient δ is distributed in multiple layers of surrounding rock within the h height range, it is determined by thickness weighting according to the unit weight of the rock and soil mass and the surrounding rock grade.

[0027] Furthermore, the ordinate h of the vertex coordinates of the parabola is determined by the following formula:

[0028]

[0029] The object of the present invention is to provide a method for calculating the surrounding rock pressure of an oval tunnel based on the zoning of surrounding rock properties. The surrounding rock disturbance caused by tunnel construction is divided into three property regions: a loosening zone, a caving arch, and a stable arch. The load characteristics and bearing capacities of each region are different. Among them, the surrounding rock pressure is composed of the engineering load generated by the loosening zone and the engineering load generated by the caving arch, and the stable arch does not generate surrounding rock pressure. According to the failure characteristics of the tunnel surrounding rock and the zoning of surrounding rock properties, this method can more accurately predict the distribution characteristics of the surrounding rock pressure, provide a more scientific basis for tunnel design, and achieve precision design, optimized design, and safety design. Further, considering that the caving arches of different grades of surrounding rock may partially bear the load, different weight grades are given to their surrounding rock pressures through an adjustment coefficient δ, which improves the flexibility and accuracy of the calculation algorithm. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present application or related technologies, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained according to the provided drawings.

[0031] The structures, proportions, sizes, etc. shown in the drawings of this specification are only used to cooperate with the content disclosed in the specification for those who are familiar with this technology to understand and read, and are not used to limit the limiting conditions under which the present application can be implemented. Therefore, they do not have a substantial technical meaning. Any modification of the structure, change in the proportional relationship, or adjustment of the size, without affecting the effects that the present application can produce and the purposes that can be achieved, should still fall within the scope that can be covered by the technical content disclosed in the present application.

[0032] Among them,

[0033] Figure 1 is a schematic diagram of the lining failure of a certain tunnel.

[0034] Figure 2 is a generalized diagram of the surrounding rock property zoning of the tunnel of the present invention.

[0035] Figure 3 is a schematic diagram of the technical route of the present invention.

[0036] Figure 4 is a schematic diagram of the cross-sectional shape of the caving arch of the present invention.

[0037] Figure 5 is a schematic diagram of the cross-sectional dimensions of a certain highway tunnel of the present invention.

[0038] Figure 6 is a schematic diagram of the layout of the measuring points of the surrounding rock pressure monitoring section of a certain highway tunnel of the present invention.

[0039] Figure 7a It is the curve of the contact pressure - time variation process between the surrounding rock and the primary support at the left - hand side ZK281 + 890 of the upper - heading left - hand arch waist of a certain tunnel exit in Embodiment 1 of the present invention.

[0040] Figure 7b It is the curve of the contact pressure - time variation process between the surrounding rock and the primary support at the left - hand side ZK281 + 890 of the upper - heading right - hand arch waist of a certain tunnel exit in Embodiment 1 of the present invention.

[0041] Figure 7c It is the curve of the contact pressure - time variation process between the surrounding rock and the primary support at the left - hand side ZK281 + 890 of the upper - heading middle - arch top of a certain tunnel exit in Embodiment 1 of the present invention. Detailed implementation manners

[0042] Next, the embodiments in the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0043] The existing calculation of tunnel surrounding - rock pressure is too simple and cannot accurately reflect the actual deformation disturbance mechanism and failure mode of the tunnel. The present invention proposes a method for calculating the surrounding - rock pressure of an elliptical tunnel based on the zoning of surrounding - rock properties. This method can more accurately predict the distribution characteristics of the surrounding - rock pressure, provide a more scientific basis for tunnel design, and help improve the safety and economy of tunnel engineering.

[0044] To make the above - mentioned objects, features, and advantages of the present application more obvious and understandable, the present application will be further described in detail below in conjunction with the accompanying drawings and specific implementation manners.

[0045] The present invention proposes a method for calculating the surrounding - rock pressure of an elliptical tunnel based on the zoning of surrounding - rock properties, which at least includes the following steps:

[0046] (1) Conduct zoning of the properties of the tunnel surrounding rock. Refer to Figure 3 , the failure mode of the tunnel surrounding rock consists of a loosening zone and a caving arch formed by construction excavation, and the stable arch outside the caving arch can be self - stable. Therefore, the property zoning of the surrounding rock consists of three parts: the loosening zone that can no longer bear load, the caving arch that can no longer bear load or can bear part of the load, and the stable arch that can bear load.

[0047] (2) Determine the composition of the tunnel surrounding - rock pressure p. Refer to Figure 3 , the surrounding - rock pressure is generated by superimposing the engineering loads generated by the loosening zone and the caving arch, and the stable arch does not generate surrounding - rock pressure.

[0048] P = P1 + P2 (1)

[0049] Wherein, P1 is the component of surrounding rock pressure caused by the loosening zone, generally evenly distributed, with the unit of kPa; P2 is the component of surrounding rock pressure caused by the collapse arch, generally parabolic in distribution, with the unit of kPa.

[0050] (2-1) Calculation method of surrounding rock pressure of the loosening zone

[0051] The loosening zone is generally a plastic zone formed by construction disturbance, considered to have basically lost its bearing capacity and all act on the arch frame structure. The specific calculation method is as follows:

[0052]

[0053] Wherein, γ is the unit weight of overlying surrounding rock, with the unit of kN / m 3 ; 2a is the major axis of the elliptical tunnel, with the unit of m; is the internal friction angle of the surrounding rock, with the unit of degree (°).

[0054] (2-2) Calculation method of surrounding rock pressure of the collapse arch

[0055] The calculation method of the surrounding rock pressure formed by the collapse arch. The collapse arch is generally a deformation disturbance zone formed by construction disturbance, which has completely or partially lost its bearing capacity. The higher the surrounding rock grade and the worse the properties, the more the bearing capacity is lost; on the contrary, the lower the surrounding rock grade and the better the properties, the less the bearing capacity is lost. This part of the load acts on the secondary lining structure. The specific calculation consists of the following steps:

[0056] (a) Qualitative determination of the collapse arch range. Assume that the upper surrounding rock collapses along a parabola, and the volume of the loose material formed by the collapse of the rock and soil mass expands relative to the volume of the original rock and soil mass, filling the collapse range and the elliptical tunnel, and no longer continues to collapse, thereby determining the collapse arch range.

[0057] (b) Determine the swelling coefficient of the overlying surrounding rock of the tunnel. Determine the swelling coefficient k of the surrounding rock according to the surrounding rock grade. The specific principles are as follows: For class I surrounding rock, rocky, the swelling coefficient k is 1.85; for class II surrounding rock, rocky, the swelling coefficient k is 1.80; for class III surrounding rock, rocky, the swelling coefficient k is 1.70; for class IV surrounding rock, rocky, the swelling coefficient k is 1.60; for class V surrounding rock, hard clay, the swelling coefficient k is 1.35; for class V surrounding rock, sandy soil with pebbles, the swelling coefficient k is 1.30; for class VI surrounding rock, cohesive soil, the swelling coefficient k is 1.25; for class V surrounding rock, gravel, the swelling coefficient k is 1.30; for class VI surrounding rock, cohesive soil, the swelling coefficient k is 1.15.

[0058] (c) Quantitatively determine the collapse arch range. See Figure 4The rock and soil mass in the caving range expands in volume after fragmentation and gradually fills the elliptical tunnel and the caving range, and then stops caving. Eventually, a failure feature with a cross-sectional shape conforming to a parabolic shape is formed.

[0059] Given that the vertex coordinates of the quadratic parabola are (0, h), the major semi-axis of the ellipse is a, the minor semi-axis is b, the center of the ellipse is at the origin of coordinates, the major axis and minor axis of the ellipse are located on the x-axis and y-axis respectively, find the equation of the parabola and the area S of the shaded part. The area of the shaded part of this cross-section:

[0060]

[0061] The total volume per unit length in the caving arch range is the volume of the shaded part plus the volume of the elliptical tunnel:

[0062]

[0063] The volume of the surrounding rock after swelling per unit length in the caving arch range is the volume of the shaded part:

[0064]

[0065] Then h is calculated from V1 = V2:

[0066]

[0067] (d) The surrounding rock pressure formed by the caving arch is obtained by multiplying the unit weight by h determined by the vertex coordinates of the parabola, and the surrounding rock pressure with a parabolic distribution is obtained as follows:

[0068]

[0069] In the formula, x is the x-axis coordinate of the parabolic distribution, with the unit of m. Substituting the x coordinate can obtain the surrounding rock pressure at different positions on the top of the elliptical tunnel.

[0070] (2-3) Precise calculation method for tunnel surrounding rock pressure

[0071] The surrounding rock pressure is composed of the engineering load P1 generated by the loose circle and the engineering load P2 generated by the caving arch. The stable arch does not generate surrounding rock pressure. Considering the influence of the surrounding rock grade on the load generated by the caving arch, the precise calculation method for the surrounding rock pressure is as follows:

[0072] P = P1 + δP2 (8)

[0073] In the formula, δ is the adjustment coefficient, taking 0 for grade I surrounding rock, 1 for grade VI surrounding rock, and the remaining grades of surrounding rock are determined by linear interpolation, which is a dimensionless quantity. If there are multiple layers of surrounding rock distributed within the height h, the unit weight of the rock and soil mass and the surrounding rock grade are determined by thickness weighting.

[0074] Example 1

[0075] A certain highway tunnel is a long extra-long separated tunnel with six lanes in both directions. The starting and ending point stake numbers of the right lane are K281+530 to K289+090, with a length of 7560 m. The starting and ending point stake numbers of the left tunnel are ZK281+506 to ZK289+103, with a length of 7597 m. The tunnel has a flat shape, with a width of 17.15 m, a height of 11.68 m, and a maximum buried depth of 731 m, belonging to a large-span extra-long deep-buried tunnel. The main strata of the tunnel from the surface to the inside are mainly Quaternary slope residual deposits, Devonian limestone, dolomitic limestone, dolomite, Lower Ordovician sand shale, Yanshanian granophyre, and Hercynian diabase and gabbro. The cross-section of the tunnel is as Figure 5 shown.

[0076] Calculation method of surrounding rock pressure in current highway tunnel specifications

[0077] According to Article 6.2.2 of Load in "Highway Tunnel Design Code" (Volume 1, Civil Engineering, JTG 3370.1-2018), for a large-section deep-buried tunnel, the surrounding rock pressure can be calculated by the following formula:

[0078] q = γh (9)

[0079] h = 0.45×2 s-1 ω (10)

[0080] ω = 1 + i(B - 5) (11)

[0081] In the formula, q is the surrounding rock pressure, with the unit of kPa; γ is the unit weight of the surrounding rock, with the unit of kN / m 3 ; h is the calculated height of the surrounding rock pressure, determined by formula (10); s is the surrounding rock grade, taking an integer from 1 to 5; ω is the width influence coefficient, determined by formula (11); B is the tunnel width, with the unit of m; i is the rate of increase or decrease of the surrounding rock pressure when the tunnel width increases or decreases by 1 m, generally determined by referring to Table 6.2.2-1 in the code.

[0082] Calculation method of surrounding rock pressure based on surrounding rock property zoning

[0083] According to the proposed calculation method of tunnel surrounding rock pressure considering surrounding rock property zoning, the surrounding rock pressure of this large-section deep-buried tunnel can be calculated by the following formula:

[0084] P = P1 + δP2

[0085]

[0086] In the formula, P1 is the component of the surrounding rock pressure caused by the loosening zone, generally evenly distributed, with the unit of kPa; P2 is the component of the surrounding rock pressure caused by the collapse arch, generally parabolic in distribution, with the unit of kPa. γ is the unit weight of the overlying surrounding rock, with the unit of kN / m 3 ; 2a is the major axis of the elliptical tunnel, with the unit of m; is the internal friction angle of the surrounding rock, with the unit of °. To determine the swelling coefficient of the overlying surrounding rock of the tunnel, the swelling coefficient k of the surrounding rock is determined according to the surrounding rock grade. For Class V surrounding rock, hard clay, the swelling coefficient k is 1.35; for Class V surrounding rock, sandy soil with pebbles, the swelling coefficient k is 1.30. δ is the adjustment coefficient, taking 0 for Class I surrounding rock, 1 for Class VI surrounding rock, and the remaining grades of surrounding rock are determined by linear interpolation, which is a dimensionless quantity. If there are multiple layers of surrounding rock distributed within the height h, the unit weight of the rock and soil mass and the surrounding rock grade are determined by thickness weighting.

[0087] Substitute the calculation parameters of this tunnel, where the unit weight of the surrounding rock γ = 22.0 kN / m 3 ; the surrounding rock grade is V, strongly weathered granite, then k = 1.30; the major axis 2a of the elliptical tunnel is 17.15 m, the minor axis 2b of the elliptical tunnel is 11.68 m, x = a; the internal friction angle of the surrounding rock The adjustment coefficient δ = 0.8; it is calculated that P1 is 220.03 kPa, P2 is 961.74 kPa, and P is 989.42 kPa.

[0088] From the above calculations, it can be seen that the surrounding rock pressure of this tunnel calculated based on the specification method is 389.66 kPa, which is less than the calculation result of the surrounding rock pressure of the tunnel based on the surrounding rock property partition, which is 989.42 kPa.

[0089] To verify the accuracy of the calculation method, at the upper heading ZK281 + 890 on the right side of a certain highway tunnel, earth pressure cells are arranged along the left arch waist B measuring point, the middle crown A measuring point, and the right arch waist C measuring point respectively to monitor the surrounding rock pressure at this section. The measuring point layout is shown in Figure 6 , and the monitoring results are shown in Figure 7. A positive value indicates a compressive stress state.

[0090] According to the surrounding rock pressure monitoring curves of the three parts of the inverted arch of the cross-section of the tunnel after closing the loop in Figure 7, the range of the surrounding rock pressure is 0.96 MPa - 1.10 MPa, which is 960 - 1100 kPa. After comparison, the surrounding rock pressure calculated by the specification method is 389.66 kPa, the surrounding rock pressure calculated by the calculation method considering the surrounding rock property partition is 989.42 kPa, and the surrounding rock pressure monitoring result is 960 - 1100 kPa. The results of the two calculation methods differ by 599.76 kPa. Among them, the result obtained by the calculation method considering the surrounding rock property partition of the tunnel surrounding rock pressure is closer to the monitoring result. The surrounding rock pressure calculation method of the present invention is closer to the true pressure. Therefore, it can be determined that a method for calculating the surrounding rock pressure of an elliptical tunnel based on the surrounding rock property partition provided by the present invention is more accurate and reliable than the traditional specification method.

[0091] In this specification, the various embodiments are described in a progressive, or parallel, or a combination of progressive and parallel manners. Each embodiment focuses on the differences from other embodiments. For the same or similar parts among the embodiments, reference can be made to each other. For the devices disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple. For the relevant parts, reference can be made to the description in the method section.

[0092] It should also be noted that in this text, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, such that an article or device including a series of elements not only includes those elements but also includes other elements not expressly listed, or further includes elements inherent to such article or device. Without further limitation, an element defined by the statement "including one..." does not exclude the presence of another identical element in the article or device including the above element.

[0093] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present application. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but rather to the broadest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for calculating the surrounding rock pressure of an elliptical tunnel based on surrounding rock property zoning, characterized in that: It includes the following steps: 1) Conduct attribute zoning on the surrounding rock of the oval tunnel; the attribute zoning consists of at least three parts; 2) According to the attribute zoning, determine the composition of the surrounding rock pressure of the tunnel, then form the weights of the pressures in the surrounding rock pressure and calculate the values of each pressure.

2. The calculation method according to claim 1, characterized in that: The attribute zoning includes a loosening zone that can no longer bear the load, a caving arch that can no longer bear the load or can bear part of the load, and a stable arch that can bear the load.

3. The calculation method according to claim 2, wherein: The engineering load generated by the loosening zone is P1, and the engineering load generated by the caving arch is P2, then P = P1 + δP2 In the formula, δ is an adjustment coefficient.

4. The calculation method according to claim 3, characterized in that: The loosening zone is a plastic zone, and the engineering load P1 generated by the loosening zone is calculated by the following formula: where γ is the unit weight of overlying surrounding rock, in kN / m 3 ; 2a is the major axis of the elliptical tunnel, in m; is the internal friction angle of the surrounding rock, in degrees (°).

5. The calculation method according to claim 4, wherein: The caving arch is a deformation disturbance zone, and the calculation process of the engineering load P2 generated by the caving arch includes at least the following steps: (1) Qualitatively determine the range of the caving arch; (2) Determine the swelling coefficient of the overlying surrounding rock of the tunnel: Determine the swelling coefficient k of the surrounding rock according to the surrounding rock grade; (3) Quantitatively determine the range of the caving arch: Take the center of the ellipse as the coordinate origin, with the major axis and minor axis of the ellipse located on the x-axis and y-axis respectively, and establish a plane rectangular coordinate system. Then the cross-sectional shape finally formed by the caving arch conforms to a parabola, and the vertex coordinates of the parabola are (0, h). The major semi-axis of the ellipse is a, and the minor semi-axis is b. Then the surrounding rock pressure P2 formed by the caving arch is: In the formula, x is the x-axis coordinate of the parabolic distribution, with the unit of m.

6. The calculation method according to claim 5, wherein: The swelling coefficient k is determined according to the following principles: For class I surrounding rock, rocky, the swelling coefficient k is 1.85; for class II surrounding rock, rocky, the swelling coefficient k is 1.80; for class III surrounding rock, rocky, the swelling coefficient k is 1.70; for class IV surrounding rock, rocky, the swelling coefficient k is 1.60; for class V surrounding rock, hard clay, the swelling coefficient k is 1.35; for class V surrounding rock, sandy soil with pebbles, the swelling coefficient k is 1.30; for class VI surrounding rock, cohesive soil, the swelling coefficient k is 1.25; for class V surrounding rock, gravel, the swelling coefficient k is 1.30; for class VI surrounding rock, cohesive soil, the swelling coefficient k is 1.

15.

7. The calculation method according to claim 3, characterized in that: The value range of the adjustment coefficient δ is 0 - 1; when it is class I surrounding rock, δ = 0; when it is class VI surrounding rock, δ = 1; when the surrounding rock grade is between I - VI, δ is determined by linear interpolation.

8. The calculation method according to claim 7, characterized in that: When the value of the adjustment coefficient δ is distributed in multiple layers of surrounding rock within the height range of h, it is determined by thickness weighting according to the unit weight of the rock and soil mass and the surrounding rock grade.

9. The calculation method according to claim 6, characterized in that: The ordinate h of the vertex coordinates of the parabola is determined by the following formula:

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