Connection tunnel connecting upper and lower existing tunnels and construction method thereof

Abstract

To provide a connection tunnel extending vertically to connect two existing tunnels positioned at upper and lower positions to each other, capable of effectively absorbing relative displacement of an existing tunnel upon frost heave settlement and lower existing tunnels, and a construction method of the connection tunnel.SOLUTION: A connection tunnel 50 extending vertically to connect existing tunnels 10, 20 being two existing subsurface structure positioned at upper and lower positions to each other, is formed by connecting a plurality of segment rings 60A, wherein one of the segments rings is made of a flexible segment 70A.SELECTED DRAWING: Figure 10

Description

[Technical Field]

[0001] The present invention relates to a joint tunnel that connects existing tunnels above and below, and a construction method thereof. [Background technology]

[0002] When excavating between two existing underground structures that are vertically or horizontally related to each other to connect them, protective work is sometimes carried out around the area by ground improvement using methods such as freezing, chemical injection, and high-pressure spraying, and then an unimproved area inside the improved ground area is excavated, and the two existing underground structures are then joined after excavation. For example, when two existing underground structures one above the other are both shield tunnels or other tunnels, a segment tunnel using steel segments may be used for the connecting pipe connecting the two, due to the ease of processing and welding. When the above-mentioned freezing method is applied to protective work, the ground frost heaves during the construction of the frozen soil, and when the frozen soil thaws, the ground subsides, which is known as frost heave settlement. Therefore, when two existing tunnels that are in a vertical relationship as described above are connected by a connecting tunnel extending in the vertical direction, there is a concern that when the frozen soil thaws, the frost heave settlement of the connecting tunnel will damage the joint between the connecting tunnel and the existing tunnel.

[0003] Furthermore, if the ground on which the existing tunnel is located is one that has a significant impact on seismic resistance considerations, there is a concern that the joint between the connecting tunnel and the existing tunnel may be damaged due to relative displacement during an earthquake between the existing tunnels located above and below and the connecting tunnel that connects them. For the above reasons, a connecting tunnel that extends vertically and connects two existing tunnels that are located above and below each other is desired, and a connecting tunnel and its construction method that can effectively absorb relative displacement with the existing tunnel during frost heave settlement and earthquakes.

[0004] Here, Patent Document 1 proposes a tunnel structure for branching and merging sections. In this tunnel structure, tunnels constructed adjacent to each other in the horizontal direction underground are connected by a ceiling slab located above and a floor slab located below, and the tunnel branching and merging sections are constructed adjacent to each other in the approximately vertical direction, and the floor slab of the tunnel constructed above and the ceiling slab of the tunnel constructed below are connected by a tensile resistance member. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-144485 Summary of the Invention [Problem to be solved by the invention]

[0006] According to the tunnel structure of the branching and merging section described in Patent Document 1, it is possible to expand the empty space inside the tunnel by reducing the size of the segment structures of the ceiling slab and floor slab that make up the tunnels of the branching and merging section that are adjacent to each other above and below. However, this does not disclose the technology related to the above-mentioned problem, namely, the joining tunnel that extends vertically and joins two existing tunnels that are in a vertical relationship.

[0007] The present invention relates to a connecting tunnel that extends vertically to connect two existing tunnels that are in an upper and lower position, and aims to provide a connecting tunnel and a construction method therefor that can effectively absorb relative displacement with the existing tunnels during frost heave settlement and earthquakes. [Means for solving the problem]

[0008] In order to achieve the above object, one aspect of the joining tunnel that connects upper and lower existing tunnels according to the present invention is as follows: A connecting tunnel that connects two existing tunnels, which are existing underground structures in an up-and-down relationship, by extending up and down. It is formed by joining a plurality of segment rings, Any of the segment rings may be a flexible segment ring formed by a flexible segment.

[0009] According to this aspect, the joining tunnel, which extends vertically and joins two existing tunnels that are in an upper and lower relationship, is formed by joining a plurality of segment rings, and one of the segment rings is a flexible segment ring formed from a flexible segment, so that the flexible segment ring can effectively absorb relative displacement with the existing tunnel during frost heave settlement or earthquakes, thereby effectively preventing damage at the joint between the existing tunnel and the joining tunnel.

[0010] Here, "any of the segment rings is a flexible segment ring" includes, for example, a configuration in which one of three or more segment rings is a flexible segment ring, a configuration in which two are flexible segment rings, as well as a configuration in which all of three or more segment rings are flexible segment rings.

[0011] Another aspect of the joining tunnel that connects existing tunnels above and below according to the present invention is as follows: The segment rings adjacent to the two existing tunnels above and below are characterized by being flexible segment rings.

[0012] According to this aspect, the segment rings adjacent to the two existing tunnels above and below are flexible segment rings, which effectively prevents damage at the joints between the two existing tunnels and the joining tunnel. Here, "segment rings adjacent to existing tunnels" means, for example, a segment ring adjacent to a segment ring directly joined to the existing tunnel.

[0013] In another aspect of the joining tunnel connecting upper and lower existing tunnels according to the present invention, The segment ring is characterized by being formed of a plurality of steel segments.

[0014] According to this aspect, the segment rings that make up the joining tunnel are made of steel segments, which makes it easy to process and weld when joining the joining tunnel to the two existing tunnels above and below, improving workability. In addition, because the flexible segments are steel segments, flexible segment rings with excellent deformation performance can be formed.

[0015] Another aspect of the joining tunnel that connects existing tunnels above and below according to the present invention is as follows: A secondary lining formed of a filler material is provided on the inside of the steel segment.

[0016] According to this aspect, by providing a secondary lining formed of a filler material on the inside of the steel segments, even though the connecting tunnel is formed of steel segments, if both the existing tunnel and the connecting tunnel are used as sewage or rainwater storage pipes, the inner secondary lining prevents corrosion of the steel segments, which are the primary lining, and provides inner surface smoothness, making it possible to minimize resistance to the flow of rainwater, etc. Here, examples of the filler material include concrete, mortar, and resins such as acrylic resin.

[0017] Another aspect of the joining tunnel that connects existing tunnels above and below according to the present invention is as follows: The ground around the joint tunnel is frozen by a freezing method.

[0018] According to this aspect, a freezing method is applied to the ground around the joining tunnel as a protective measure, and since the ground is frozen, even if the joining tunnel sinks due to frost heave, the flexible segment rings equipped in the joining tunnel can absorb the amount of sinking, preventing damage to the joint between the existing tunnel and the joining tunnel.

[0019] In addition, one aspect of the construction method of the joining tunnel that connects upper and lower existing tunnels according to the present invention is as follows: A construction method for a joining tunnel that connects two existing tunnels, which are existing underground structures that are in an upper and lower relationship, by extending the tunnel vertically and joining them. A freezing process of freezing the ground around the construction area of ​​the joint tunnel by a freezing method; a connecting step of cutting open the existing tunnel above to form a second entrance, excavating the unfrozen ground below the second entrance to reach the existing tunnel below, and cutting open the existing tunnel to form a first entrance, thereby connecting the two existing tunnels via a borehole extending vertically; a forming process of hanging a segment ring down from the existing tunnel above and joining it to the first portal of the existing tunnel below, stacking the segment rings upward while successively hanging and joining them, and joining the uppermost segment ring to the second portal of the existing tunnel above, thereby forming the joined tunnel; In the forming step, any of the segment rings is made into a flexible segment ring formed by a flexible segment.

[0020] According to this aspect, in construction to connect two existing tunnels that are in an upward and downward relationship with a joining tunnel that extends upward and downward, the joining tunnel is formed by sequentially stacking multiple segment rings, and by making one of the rings a flexible segment, the flexible segment rings provided in the constructed joining tunnel can effectively absorb relative displacement with the existing tunnel during frost heave settlement or earthquakes, thereby effectively preventing damage at the joint between the existing tunnel and the joining tunnel. [Effects of the Invention]

[0021] According to the joining tunnel connecting two existing tunnels above and below and the construction method thereof of the present invention, the joining tunnel extends vertically to join two existing tunnels that are in an upper-lower position, and can effectively absorb relative displacement with the existing tunnel during frost heave settlement or earthquakes. [Brief explanation of the drawings]

[0022] [Figure 1A] FIG. 1 is a plan view of an example of two existing tunnels in an upper-lower relationship to which the groundwater drainage structure according to the embodiment, the joint structure between an existing tunnel and a branch tunnel according to the embodiment, the portal reinforcement structure for an existing tunnel according to the embodiment, the joint tunnel connecting existing tunnels above and below according to the embodiment, and the excavated soil removal device for frozen ground according to the embodiment are applied, and also shows the frozen ground in its construction state. [Figure 1B] 1B is a view taken along the arrow BB in FIG. 1A, and is a vertical cross-sectional view of two existing tunnels in an upper and lower relationship, cut along the axial direction of the lower existing tunnel. [Figure 1C] 1B is a view taken along the CC arrow in FIG. 1A, and is a longitudinal cross-sectional view of two existing tunnels in an upper and lower relationship, cut along a direction perpendicular to the axial direction of the lower existing tunnel. [Figure 2A] FIG. 1 is a side view of an example of a tunnel entrance reinforcement structure for an existing tunnel according to an embodiment. [Figure 2B] 2B is a view taken along the arrow BB in FIG. 2A, and is a front view of an example of a tunnel portal reinforcement structure for an existing tunnel. [Figure 3] FIG. 2 is a vertical cross-sectional view of an example of a groundwater drainage structure according to an embodiment. [Figure 4A] This is a process diagram of an example of a construction method for a joint tunnel that connects existing tunnels above and below the tunnel, according to an embodiment. [Figure 4B] FIG. 4B is a view taken along the arrow BB in FIG. 4A. [Figure 5A] 4A, followed by a process diagram of an example of a construction method for a joint tunnel that connects upper and lower existing tunnels according to an embodiment. FIG. [Figure 5B] FIG. 5B is a view taken along the arrow BB in FIG. 5A. [Figure 6A] 5B is a process diagram of an example of a construction method for a joint tunnel that connects upper and lower existing tunnels according to an embodiment, following FIG. 5A. [Figure 6B] FIG. 6B is a view taken along the arrow BB in FIG. 6A. [Figure 7A]FIG. 1 is a perspective view of an example of a joint structure between an existing tunnel and a branch tunnel according to an embodiment, viewed from above. [Figure 7B] FIG. 1 is a perspective view of an example of a joint structure between an existing tunnel and a branch tunnel according to an embodiment, viewed from below. [Figure 8] FIG. 10 is a plan view showing a state in which a steel plate is blocking the gap between the portal of the existing tunnel below and the segment ring in the joint structure between an existing tunnel and a branch tunnel according to an embodiment. [Figure 9A] 6B is a process diagram of an example of a construction method for a joint tunnel that connects upper and lower existing tunnels according to an embodiment, following FIG. 6A. [Figure 9B] FIG. 9B is a view taken along the arrow BB in FIG. 9A. [Figure 10] This is an enlarged vertical cross-sectional view of a joining tunnel that connects upper and lower existing tunnels in the embodiment. [Figure 11] 1 is a perspective view showing an example of an excavated soil discharge device for frozen ground according to an embodiment. FIG. [Figure 12] This figure shows a vacuum truck, which constitutes an example of an excavated soil discharge device for frozen ground in an embodiment, positioned on the ground next to a manhole and discharging slurried excavated soil through a vacuum pipe. DETAILED DESCRIPTION OF THE INVENTION

[0023] The following describes, with reference to the accompanying drawings, a groundwater drainage structure and a groundwater drainage method according to embodiments, a joint structure and a joint method for connecting an existing tunnel and a branch tunnel according to embodiments, a portal reinforcement structure and a portal reinforcement method for an existing tunnel according to embodiments, a joint tunnel connecting upper and lower existing tunnels according to embodiments and a construction method thereof, and an excavated soil removal device and a soil removal method for frozen ground according to embodiments. Note that in this specification and the drawings, substantially identical components may be designated by the same reference numerals to avoid redundant description.

[0024] [Groundwater drainage structure and groundwater drainage method according to embodiments, joint structure and joint method for existing tunnel and branch tunnel, existing tunnel portal reinforcement structure and portal reinforcement method, joint tunnel connecting upper and lower existing tunnels and construction method thereof, and soil removal device and soil removal method for excavated soil from frozen ground] Referring to Figures 1 to 12, we will explain examples of a groundwater drainage structure and a groundwater drainage method according to the embodiment, a joining structure and joining method for an existing tunnel and a branch tunnel, a tunnel entrance reinforcement structure and tunnel entrance reinforcement method for an existing tunnel, a joining tunnel connecting existing tunnels above and below and a construction method therefor, and an excavated soil removal device and soil removal method for frozen ground.

[0025] First, referring to Figures 1A to 1C, an example of existing tunnels in an upper-lower relationship that are joined by a joining tunnel according to an embodiment will be described. Here, Figure 1A is a plan view of an example of two existing tunnels in an upper-lower relationship, showing the state in which frozen ground has been constructed. Also, Figure 1B is a view taken along arrow BB in Figure 1A, showing a vertical cross-section of the two existing tunnels in an upper-lower relationship cut along the axial direction of the existing tunnel below. Figure 1C is a view taken along arrow CC in Figure 1A, showing a vertical cross-section of the two existing tunnels in an upper-lower relationship cut along a direction perpendicular to the axial direction of the existing tunnel below.

[0026] The two existing tunnels (examples of existing underground structures) shown in the figure, which are in an upper-lower relationship, are tunnels that serve utility sewers, water supply and sewerage systems, rainwater storage pipes, etc., and the following describes an example of construction in which a first existing tunnel 10 located below and a second existing tunnel 20 located above are joined by a joining tunnel at the location where they intersect in a plan view.

[0027] In both the first existing tunnel 10 and the second existing tunnel 20, the area joined by the joining tunnel is a joint of steel segment rings, and the other areas are a joint of concrete segment rings, and the joining tunnel will be connected to both portals created by cutting open parts of both steel segment rings.

[0028] When constructing a joint tunnel, as shown in Figures 1A to 1C, multiple freezing pipes P are inserted into the ground G around the construction area, for example from the first existing tunnel 10, and a freezing method is applied as a protective work to freeze the surrounding ground G, thereby constructing frozen ground GA around the construction area.

[0029] For example, as shown in Figure 1C, the construction area where the connecting tunnels will be constructed will be excavated by manual excavation, so the excavation area will be unfrozen ground GB, and frozen ground GA will be created around it. Note that even though it is called unfrozen ground GB, part of the unfrozen ground GB will freeze due to the effects of the frozen ground GA. Note that Figure 1C also shows a casing 42 (drainage pipe) that forms a groundwater drainage structure 40 (see Figure 3), which will be described below.

[0030] When applying the freezing method, the freezing expansion pressure acts on the unfrozen ground GB inside the created frozen ground GA. This freezing expansion pressure is calculated using the Takashi cylinder theory formula (Δσ=E / (1+ν)×ξ / 2×(1-r0 2 / r1 2 ), where Δσ is the frost swelling pressure, E is the deformation coefficient of unfrozen ground, ν is the Poisson's ratio of unfrozen ground, ξ is the frost swelling rate in the direction of the lining, r0 is the radius of the cooling surface, and r1 is the radius of the frozen surface. This calculation using the cylindrical theory formula uses the deformation coefficient and frost swelling rate of the ground, and both elements are proportional to the frost swelling pressure. In order to accurately determine physical property values, boring tests are carried out on site, and in-situ tests (underground horizontal loading tests, standard penetration tests, permeability tests, salinity tests, etc.) and laboratory tests (fine particle content tests, frozen uniaxial compression tests, frost heave settlement tests, etc.) are carried out to reflect the local conditions.

[0031] However, even if the physical properties and freezing expansion coefficients are set by conducting the various in-situ and laboratory tests described above, the actual freezing expansion coefficient may be higher. One reason for this is that even if a boring test is conducted, there is uncertainty in the set values. Another reason is that when the construction area is located in a city center or other area and it is difficult to conduct a boring test directly above the construction area, conducting a boring test at a location far from the construction area may not accurately reflect the ground properties of the construction area.

[0032] In this way, if it is discovered during the process of frozen soil construction that the actual freezing expansion rate is higher than the initially set freezing expansion rate for some reason, and that the freezing expansion pressure on the unfrozen ground is high, it will be necessary to quickly construct a reinforcement structure in the construction area of ​​the portal 18 (see Figure 2A) of the first existing tunnel 10, which leads to this unfrozen ground. However, the inside of the first existing tunnel 10 is generally a tangle of piping, wiring, hoses, etc. used during freezing construction, and it is not easy to construct a temporary reinforcement structure around the portal 18 inside the first existing tunnel 10.

[0033] Therefore, the portal reinforcement structure shown in Figures 2A and 2B is applied inside the first existing tunnel 10. Here, Figure 2A is a side view of an example of a portal reinforcement structure for an existing tunnel according to an embodiment, and Figure 2B is a front view of the example of a portal reinforcement structure for an existing tunnel as viewed from the arrow BB in Figure 2A.

[0034] As described above, the area of ​​the first existing tunnel 10 where the joining tunnel 50 (see Figure 9A) is constructed is a joined body in which multiple steel segment rings 12 are joined in the axial direction, and each steel segment ring 12 is formed by assembling multiple first steel segments 11 in the circumferential direction.

[0035] At the top of the first existing tunnel 10 is a mirror cutting area 13 where the first tunnel entrance 18 will be formed, and the area t1 slightly wider than the mirror cutting area 13 is the designed area A, and the area t2 on its periphery is area B. Area A is the area that will be cut directly, and area B is the area that will bear the additional load that has been distributed as a result of the cutting in area A, in addition to the load (earth overburden load, etc.) that area A had previously supported, and each first steel segment ring 12 is designed based on these loads during construction.

[0036] The first steel segment 11 includes a main girder 15, a skin plate 16, a longitudinal rib 17a, and a joint plate 17b.

[0037] The tunnel entrance reinforcement structure 30 comprises a pair of girders 35 arranged on a plurality of sleepers 32 spanning in a direction perpendicular to the axial direction of the first existing tunnel 10, and a plurality of pillars 36 arranged to the side of the mirror cutting area 13 where the first tunnel entrance 18 is constructed and extending in the vertical direction, and the lower end of each pillar 36 is fixed to the girders 35 by bolting or the like.

[0038] In the first existing tunnel 10, the second bracket 33 is welded to the lower main girder 15 in advance (welded before the freezing method is applied), and a portion of the girder material 35 is joined to the second bracket 33 by bolting, welding, clamping, etc.

[0039] Meanwhile, the first brackets 31 are welded in advance to the main girders 15 at positions corresponding to the top of each pillar 36 on the side of the mirror cutting area 13 where the first tunnel entrance 18 is constructed, similar to the second brackets 33, and the top of each pillar 36 is joined to the corresponding first bracket 31 by bolting, welding, clamping, etc. It is also possible to weld a pair of girders 35 directly to the lower main girders 15 without using sleepers 32.

[0040] Here, the sleepers 32 and the beams 35 are formed from shaped steel materials such as H-shaped steel, and the supports 36 are formed from a flexible temporary support material.

[0041] A plurality of pillars 36 are erected at intervals on each beam 35, and brace materials 38 are attached to straddle each pillar 36, thereby maintaining a stable erect position of each pillar 36.

[0042] In this way, the wellhead reinforcement structure 30 is formed by the first bracket 31, the second bracket 33, the beam 35, the support pillars 36, etc.

[0043] Below the mirror cutting area 13 of the first existing tunnel 10, a segment ring support D on which the lowest steel segment ring 60A (see Figure 6A) of the joining tunnel 50 is placed, temporary scaffolding F, etc. are installed.

[0044] Strain gauges (not shown) are installed on the first bracket 31 and other parts, and measurement results from the strain gauges are obtained as needed. In addition, the displacement of the crown, shoulder, etc. of the first existing tunnel 10 is measured as needed, and if it is determined based on these measurement results that unexpected freezing expansion pressure is acting, the portal reinforcement method according to the embodiment involves installing multiple pillars 36 on the already installed first bracket 31, second bracket 33, and girder 35, and reinforcing each pillar 36 with brace material 38 to quickly form the portal reinforcement structure 30.

[0045] According to the illustrated portal reinforcement structure 30, on the side of the mirror cutting area 13 where the first portal 18 of the first existing tunnel 10 will be constructed, the upper part of a pillar 36 extending in the vertical direction is joined to a first bracket 31 which has been pre-joined to the main girder 15 before the freezing method is applied, and the lower end of the pillar 36 is joined to a girder 35 extending in the axial direction of the first existing tunnel 10.This means that the lower ends of multiple pillars 36 can be joined to a common girder 35 and their upper parts can be joined to multiple first brackets 31 which are joined to the main girder 15 around the first portal 18.Even if the interior of the first existing tunnel 10 is complicated by protective work equipment, etc., the area around the first portal 18 can be quickly reinforced with multiple pillars 36.

[0046] In addition, the top and bottom of the pillar 36 can be joined to the main girder 15 of the first steel segment 11 via the first bracket 31, the second bracket 33 and the girder material 35, and the area around the first tunnel entrance 18 can be reinforced with the pillar 36 in a stable upright position.

[0047] Furthermore, because the supports 36 are extendable temporary support materials, the temporary support materials 36, which have excellent handling and installation properties, can be quickly erected even in the complicated conditions inside the first existing tunnel 10, and the area around the first tunnel entrance 18 can be reinforced with the relatively rigid temporary support materials 36. Furthermore, because the supports 36 are temporary support materials, they can be dismantled and removed smoothly.

[0048] When applying the freezing method as a protective work as in the illustrated example, it is common for the pressure of the groundwater in the unfrozen ground GB inside the frozen ground GA to increase, and it is advisable to install a drain pipe from the first existing tunnel 10 to the unfrozen ground GB in order to drain the high-pressure groundwater, for example, into the first existing tunnel 10 below. The drain pipe installed in the unfrozen ground GB not only reduces the water pressure of the groundwater in the unfrozen ground GB, but also makes it possible to check the blockage status of the frozen ground GA constructed around the unfrozen ground GB (to check whether pressure has returned).

[0049] However, when using a normal drainage pipe, there is a risk that the drainage pipe will freeze due to the influence of the frozen ground GA that has been created.Furthermore, after the creation of the frozen ground GA is completed, the unfrozen ground GB will also freeze, and the unfrozen ground GB that was originally supposed to be excavated will freeze, which will significantly reduce the ease of work during excavation, which is undesirable.

[0050] Therefore, as shown in Fig. 3, when draining groundwater from the unfrozen ground GB into the inside of the first existing tunnel 10, a groundwater drainage structure 40 is applied, which can effectively drain the groundwater without freezing it. Note that the groundwater drainage structure 40 may also be applied to the second existing tunnel 20 located above, to drain the groundwater into the inside of the second existing tunnel 20. Here, Fig. 3 is a vertical cross-sectional view of an example of a groundwater drainage structure according to an embodiment.

[0051] An opening 16a facing the unfrozen ground GB is provided in the skin plate 16 (an example of a wall surface) of the first existing tunnel 10. This opening 16a may be a pre-formed opening, or may be formed by rotating the casing 42 (described below) using a boring bit or the like at the tip of the casing.

[0052] The guide pipe 41 having a hollow 41a is fixed to the inside of the skin plate 16 by welding or the like so that the hollow 41a communicates with the opening 16a.

[0053] The guide pipe 41 has a branch pipe 41c at its midpoint, and a casing 42 is inserted into the hollow 41a via the end opening 41b, thereby forming a first flow path 43 between the guide pipe 41 and the casing 42.

[0054] The casing 42 is inserted to a predetermined position in the unfrozen ground GB by being rotationally pressed into the unfrozen ground GB via the guide pipe 41. Even if the unfrozen ground GB is hard, by applying the casing 42, it is possible to install a double pipe 44 that extends to a predetermined position in the unfrozen ground GB.

[0055] Through the gap between the casing 42 and the opening 16a of the skin plate 16, groundwater flows from the unfrozen ground GB into the first flow path 43 in the X1 direction.

[0056] A double pipe 44 (an example of an inserted pipe) is inserted inside the casing 42. The double pipe 44 includes an outer pipe 45 and an inner pipe 46, a second flow path 44a is provided between the outer pipe 45 and the inner pipe 46, and a third flow path 44b is provided inside the inner pipe 46.

[0057] A filler 49 is filled between the casing 42 and the outer pipe 45. The filler 49 may be bentonite, a mixture of cement and bentonite, or the like.

[0058] Filling the gap between the casing 42 and the double pipe 44 with filler 49 improves thermal conductivity compared to when there is an air gap between the two, allowing the heat of the fluid flowing inside the double pipe 44 to be effectively transferred to the groundwater in the first flow path 43. Heat can also be effectively transferred from the double pipe 44 to the unfrozen ground GB via the casing 42, effectively preventing the groundwater in the unfrozen ground GB from freezing and becoming unable to be drained. Furthermore, the filler 49 allows the casing 42 and the double pipe 44 to be installed with their axes aligned to a certain extent.

[0059] Here, if the ground into which the double pipe 44 is pressed is relatively soft, or if the opening 16a is provided in advance in the skin plate 16 so that it can be opened and closed freely, the casing 42 can be eliminated. In a configuration in which the casing 42 is not present, the double pipe 44 may be pressed directly into the unfrozen ground GB via the hollow 41a of the guide pipe 41. In this case, a first flow path 43 is formed between the guide pipe 41 and the double pipe 44.

[0060] A first valve 47 is installed at an end opening 41b of the guide pipe 41 that is closer to the inside of the first existing tunnel 10 than the branch pipe 41c, and the first flow path 43 is blocked by the first valve 47. In addition, a second valve 48 that opens and closes the branch pipe 41c is installed in the branch pipe 41c.

[0061] A supply port 45a through which a fluid such as hot water or water is supplied is provided behind the outer pipe 45. The ground-side tip of the outer pipe 45 is closed, and the ground-side tip of the inner pipe 46 is open. With this configuration, a fluid such as hot water is supplied to the second flow path 44a in the X3 direction via the supply port 45a, and the fluid that flows through the second flow path 44a in the X4 direction flows into the inner pipe 46 in the X5 direction, flows through the third flow path 44b of the inner pipe 46 in the X6 direction, and is drained into the inside of the first existing tunnel 10 through the end opening of the inner pipe 46.

[0062] The second valve 48 is set to be automatically controlled to open when a predetermined pressure is applied (when the groundwater reaches a predetermined pressure), and when the pressure of the groundwater in the first flow path 43 rises due to freezing expansion pressure and reaches a predetermined pressure, the second valve 48 is opened and the groundwater is automatically drained into the first existing tunnel 10. This predetermined pressure value is, for example, a pressure value higher than the design freezing expansion pressure.

[0063] For example, a pressure gauge (not shown) is provided in either the guide pipe 41, the first valve 47, or the second valve 48 to measure the pressure of the groundwater inside the first flow path 43, and the second valve 48 is automatically controlled to open when the measurement data from the pressure gauge reaches or exceeds a predetermined pressure value.

[0064] An example of a groundwater drainage method according to the embodiment includes a preparation process in which a groundwater drainage structure 40 is installed in the first existing tunnel 10 and a fluid such as hot water is allowed to circulate inside the insertion pipe 44, and a drainage process in which, when the pressure of the groundwater inside the first flow path 43 increases due to the freezing expansion pressure during the freezing method, the second valve 48 is opened to drain the groundwater inside the first flow path 43 into the inside of the first existing tunnel 10.

[0065] In this way, the groundwater drainage structure 40 includes a guide pipe 41 having a hollow 41a communicating with an opening 16a facing the unfrozen ground GB provided in the wall surface 16 of the first existing tunnel 10, a branch pipe 41c located midway, and connected to the wall surface 16; and an insertion pipe 44 having a portion inserted into the unfrozen ground GB through the hollow 41a of the guide pipe 41 to form a first flow path 43 through which groundwater flows between the guide pipe 41 and the insertion pipe 44, and through which a fluid flows. The guide pipe 41 is provided with a first valve 47 that blocks the first flow path 43 and a second valve 48 that opens and closes the branch pipe 41c. By opening the second valve 48, groundwater in the first flow path 43 whose pressure has increased due to freezing expansion pressure is drained into the inside of the first existing tunnel 10, thereby preventing the groundwater flowing through the first flow path 43 from freezing by the fluid flowing inside the insertion pipe 44, and effectively draining the groundwater in the first flow path 43 whose pressure has increased due to freezing expansion pressure via the second valve 48.

[0066] This ensures good workability when drilling the unfrozen ground GB. Also, the blockage status of the frozen ground GA that is constructed around the unfrozen ground GB can be constantly checked, and even if part of the unfrozen ground GB is likely to freeze, it can be partially thawed by the insertion pipe 44 through which fluid flows inside.

[0067] Next, referring to Figures 4 to 10, we will explain the joining tunnel that connects the existing tunnels above and below and its construction method according to the embodiment, as well as the joining structure and joining method between the existing tunnel and the branch tunnel according to the embodiment, which are formed in the early stages of this construction.

[0068] Here, Figures 4A and 4B, Figures 5A and 5B, Figures 6A and 6B, and Figures 9A and 9B are, respectively, process diagrams of an example of a construction method for a joint tunnel that connects existing tunnels above and below in accordance with an embodiment.

[0069] 4A and 4B, like the first existing tunnel 10, the area where the joining tunnel 50 is to be constructed in the second existing tunnel 20 above also consists of a joined body of multiple first segment rings 22, and each first segment ring 22 is formed by assembling multiple first segments 21 in the circumferential direction. A suspension beam B is installed above the area where the joining tunnel is to be constructed in the ceiling of the second existing tunnel 20, and a trolley-equipped chain block C is installed on the suspension beam B so that it can move freely.

[0070] Rails RL are installed inside the second existing tunnel 20, and a cart K (see Figure 5A) that transports steel segments moves along the rails RL.An assembly area A is provided to the side of where the second tunnel entrance 28 is constructed, where multiple steel segments are assembled to produce a steel segment ring.

[0071] Worker S cuts a portion of the lower part of the second existing tunnel 20 to construct the second portal 28, and then manually excavates the unfrozen ground GB below it in the direction Y1 using a chipper M or the like through the second portal 28. Note that part of the unfrozen ground GB is frozen by the surrounding frozen ground GA. This manual excavation creates a borehole H extending in the vertical direction below the second portal 28.

[0072] 5A and 5B, when the borehole H is constructed down to the first existing tunnel 10, mirror cutting is performed on the mirror cutting area 13 of the first existing tunnel 10. Inside the second existing tunnel 20, to the side of the assembly area A, there is a bogie K carrying multiple steel segments 60, and each steel segment 60 is transferred to the assembly area A in the Z1 direction, and the multiple steel segments 60 are assembled in the assembly area A to produce a steel segment ring 60A.

[0073] 6A and 6B, the mirror cutting area 13 of the first existing tunnel 10 is mirror cut to construct the first portal 18, thereby connecting the second portal 28 of the second existing tunnel 20 and the first portal 18 of the first existing tunnel 10 via a borehole H. Here, the borehole H is constructed to have a diameter larger than the outer diameter of the connecting tunnel 50.

[0074] The steel segment ring 60A that has already been manufactured in the assembly area A is lifted up by a trolley-equipped chain block C, and the trolley-equipped chain block C is moved along the suspension beam B to a position above the borehole H. The steel segment ring 60A is then lifted downward in the Y2 direction through the borehole H and placed on a segment ring support D inside the first existing tunnel 10.

[0075] When the steel segment ring 60A is placed on the segment ring support D, the height of the segment ring support D is adjusted so that the upper end of the steel segment ring 60A is close to the first tunnel entrance 18 of the first existing tunnel 10.Therefore, with the steel segment ring 60A placed on the segment ring support D, workers can use temporary scaffolding F to join the two.

[0076] Furthermore, during the joining work between the first tunnel entrance 18 and the steel segment ring 60A, the flexible segment ring 70A to be lowered next is produced in the assembly area A of the second existing tunnel 20.

[0077] As will be explained below with reference to Figure 9A etc., the junction tunnel 50 is provided with flexible segment rings 70A near the first portal 18 and the second portal 28, and therefore, after the lowest steel segment ring 60A is joined to the first portal 18, the flexible segment ring 70A is suspended above this steel segment ring 60A and joined to the steel segment ring 60A at the lower level.

[0078] By joining the lowest steel segment ring 60A to the first portal 18 of the first existing tunnel 10, a joint structure 80 between an existing tunnel and a branch tunnel is formed, as shown in Figures 7A and 7B. Figures 7A and 7B are perspective views, seen from above and below, respectively, of an example of a joint structure between an existing tunnel and a branch tunnel according to an embodiment.

[0079] The "branch tunnel" is another name for the junction tunnel 50, and is so named because the junction tunnel 50 is a tunnel that branches off vertically above the first existing tunnel 10.

[0080] Each second steel segment 60 constituting the second steel segment ring 60A forming the junction tunnel 50 comprises a skin plate 61 (an example of a side plate) extending in the vertical direction and joined to the first tunnel entrance 18, a lower main girder 62 welded to the lower end of the skin plate 61, an upper main girder 63 welded to the upper end of the skin plate 61, a joint plate 64 connecting the ends of the upper main girder 63, the lower main girder 62, and the skin plate 61, and a vertical rib 65 connecting the upper main girder 63, the lower main girder 62, and the skin plate 61. In addition, a flange 66 in an upright position is welded to the inner edge of the lower main girder 62.

[0081] The second steel segments 60 are arranged horizontally, and the joint plates 64 are joined together to form segment joints, thereby forming an annular second steel segment ring 60A.

[0082] As shown in Fig. 7B, a first reinforcing rib 67 is welded to connect the flange surface of the vertical rib of the first steel segment 11 and the main girder 15 to the skin plate 61. Furthermore, as shown in Fig. 7A, a second reinforcing rib 68 is welded to both the skin plate 61 and the lower main girder 62.

[0083] The second reinforcing rib 68 is joined to the skin plate 61 and the lower main girder 62, and is also welded to the longitudinal rib 65.

[0084] In this way, the second steel segment ring 60A located at the lowest end of the branch tunnel 50 (junction tunnel) is joined to the first portal 18 of the first existing tunnel 10, and the outer side of the skin plate 61 of the second steel segment 60 is joined to the flange surface of the main girder 15 and vertical rib of the first steel segment 11 via the first reinforcing rib 67, thereby forming a joint structure 80 between the existing tunnel and the branch tunnel.

[0085] As shown in the illustrated example, in the joint structure 80 between an existing tunnel and a branch tunnel, the first entrance 18 created by cutting through the first existing tunnel 10 formed by the first steel segment 11 is joined to the second steel segment ring 60A, the lowest level of the branch tunnel 50 formed by the second steel segment 60 branching off above it.The skin plate 61 of the second steel segment 60 and the main girder 15 of the first steel segment 11 are joined to each other via the first reinforcing rib 67, so that the load transmitted via the skin plate 61 of the second steel segment 60 (such as the weight of the second steel segment 60) can be transmitted to the main girder 15 of the first steel segment 11 via the first reinforcing rib 67, making it possible to stably transmit the load acting on the joint between the first entrance 18 and the skin plate 61 from the branch tunnel 50 to the first existing tunnel 10.

[0086] Furthermore, a structure in which portal concrete is poured around the first portal 18 of the first existing tunnel 10 to support the branch tunnel 50 is also effective, but if a freezing method is applied to the protective work as in the illustrated example, this is not desirable because there is a risk that the concrete will be damaged by frost due to curing in a freezing environment.

[0087] In addition, the second steel segment 60 further has a second reinforcing rib 68 that is joined to the skin plate 61 and the lower main girder 62. As a result, part of the load transmitted to the skin plate 61 is transmitted via the second reinforcing rib 68 provided on the inside of the skin plate 61, via the lower main girder 62, and via the first reinforcing rib 67 to the main girder 15 of the first steel segment 11 of the first existing tunnel 10, and the remaining load is transmitted via the first reinforcing rib 67 provided on the outside of the skin plate 61 to the main girder 15 of the first steel segment 11, making it possible to transmit the load acting on the joint between the first portal 18 and the skin plate 61 from the branch tunnel 50 to the first existing tunnel 10 more stably.

[0088] Furthermore, since the second reinforcing rib 68 is joined to the vertical rib 65 connecting the upper main girder 63, the lower main girder 62 and the skin plate 61, the skin plate 61, to which the load is transmitted, can be reinforced by the vertical rib 65, and the load can be transmitted through both the skin plate 61 and the vertical rib 65, making it possible to transmit the load acting on the joint between the first portal 18 and the skin plate 61 from the branch tunnel 50 to the first existing tunnel 10 more stably.

[0089] As is also clear from FIG. 7A, the width t5 of the lower main girder 62 toward the center of the segment ring is set wider than the width t6 of the upper main girder 63.

[0090] In the illustrated example, the first existing tunnel 10, the second existing tunnel 20, and the branch tunnel 50 (junction tunnel) connecting them are tunnels used for utility sewerage, water supply and sewerage, rainwater storage pipes, etc., so a secondary lining 55 (see Figure 10) made of a filling material such as concrete or mortar will be constructed on the inside of the junction tunnel 50 in order to prevent corrosion of the steel segments and reduce resistance to the fluid.

[0091] The thickness of this secondary lining 55 is set to the same width as the width t5 of the wider lower main girder 62. Because the width t5 toward the center of the segment ring in the lower main girder 62 is wider than the width t6 of the upper main girder 63 and the thickness of the secondary lining 55 is the same as the width t5 of the lower main girder 62, even if the thickness of the secondary lining 55 is wider than the width t6 of the upper main girder 63, the lower main girder 62 can effectively function as a formwork when constructing the secondary lining 55, and the weight of the secondary lining 55 can be transmitted to the main girder 15 of the first steel segment 11 via the lower main girder 62, the first reinforcing rib 67, the second reinforcing rib 68, etc.

[0092] In addition, a flange 66 that follows the linear shape of the segment ring in the lower main girder 62 is erected upward at the center of the segment ring, so that the secondary lining 55 can be constructed while preventing the filled material from spilling downward from the inside of the lower main girder 62.

[0093] FIG. 8 is a plan view showing a state in which a steel plate is blocking the gap between the portal of the existing tunnel below and the segment ring in the joint structure of the existing tunnel and the branch tunnel according to the embodiment.

[0094] As shown in FIG. 8, the gap between the first wellhead 18 and the second segment ring 60A is closed by an endless steel plate 82 that surrounds the second segment ring 60A.

[0095] In this way, by blocking the gap between the first portal 18 and the second segment ring 60A with an endless steel plate 82, the watertightness around the cut-out first portal 18 in the first existing tunnel 10 is improved, and a joint structure 80 with excellent watertightness can be formed.

[0096] Here, the steel plate 82 is provided with a plurality of injection ports 84, through which a waterproofing material such as non-shrinkage mortar is injected around the joint structure 80, thereby further enhancing the waterproofing properties.

[0097] As shown in Figures 9A and 9B, a flexible segment ring 70A is placed on top of the lowest steel segment ring 60A that is joined to the first tunnel entrance 18, multiple (two in the illustrated example) separate steel segment rings 60A are placed on top of the flexible segment ring 70A, another separate flexible segment ring 70A is placed on top of that, and the topmost steel segment ring 60A is placed on top of that, and the segment rings are joined to each other, and the topmost steel segment ring 60A is joined to the second tunnel entrance 28 of the second existing tunnel 20, thereby constructing a joining tunnel 50 that connects the first existing tunnel 10 and the second existing tunnel 20 vertically.

[0098] As described above, the construction method of the joining tunnel connecting the upper and lower existing tunnels according to the embodiment includes a freezing process in which the ground around the construction area of ​​the joining tunnel 50 is frozen using a freezing method; a connecting process in which the upper second existing tunnel 20 is cut open to form the second portal 28, the unfrozen ground GB below the second portal 28 is excavated to reach the lower first existing tunnel 10, and the first existing tunnel 10 is cut open to form the first portal 18, thereby connecting the two existing tunnels 10, 20 via the borehole H extending vertically; and a forming process in which a steel segment ring 60A is hung down from the second existing tunnel 20 and joined to the first portal 18 of the lower first existing tunnel 10, and multiple steel segment rings 60A and flexible segment rings 70A are hung down sequentially and joined together while stacking them upward, and the top steel segment ring 60A is joined to the second portal 28 of the second existing tunnel 20, thereby forming the joining tunnel 50.

[0099] As shown in the illustrated example, the joining tunnel 50 is equipped with a flexible segment ring 70A at a position close to the first existing tunnel 10 and the second existing tunnel 20 located above and below (close to the tunnel entrances 18, 28 which are not directly joined), thereby effectively preventing damage to the joint between the joining tunnel 50 and the existing tunnels 10, 20 due to frost heave settlement of the joining tunnel 50 or relative displacement between the two existing tunnels 10, 20 during an earthquake.

[0100] That is, when freezing methods are applied to protective work as in the illustrated example, the ground G freezes during the creation of frozen soil, and when the frozen soil thaws, the ground G sinks, posing a problem of so-called frost heave settlement. Therefore, when existing tunnels 10, 20 that are located above and below each other are connected by a connecting tunnel 50 extending in the vertical direction, there is a concern that when the frozen soil thaws, frost heave settlement of the connecting tunnel 50 will damage the joint between the connecting tunnel 50 and the existing tunnels 10, 20. Furthermore, even if the ground on which the existing tunnels 10, 20 are located is ground that has a significant impact on seismic resistance studies, there is a concern that the relative displacement during an earthquake between the existing tunnels 10, 20 located above and below and the connecting tunnel 50 that connects them will damage the joint between the connecting tunnel 50 and the existing tunnels 10, 20.

[0101] In response to such concerns, the connecting tunnel 50 in the illustrated example is equipped with flexible segment rings 70A located close to the first existing tunnel 10 and the second existing tunnel 20 located above and below. This allows the flexible segment rings 70A to effectively absorb relative displacement between the existing tunnels 10, 20 during frost heave settlement or earthquakes, thereby effectively preventing damage at the joints between the existing tunnels 10, 20 and the connecting tunnel 50.

[0102] Fig. 10 is an enlarged longitudinal cross-sectional view of a joining tunnel connecting upper and lower existing tunnels according to an embodiment. As shown in Fig. 10, each steel segment ring 60A has an injection port 69 through which a water-stopping urethane material R is injected into the interface area with the existing tunnels 10 and 20, and a filler Q (e.g., a low-temperature environment filler) is filled into the back surface of the joining tunnel 50, thereby improving the water-stopping performance of the existing tunnels 10 and 20 and the joining tunnel 50.

[0103] In addition, the flexible segment ring 70A is equipped with an expandable waterproof rubber 75, and a joint material 77 that allows deformation is provided on the inside of it, and a secondary lining 55 made of non-shrink mortar or the like is constructed so as to sandwich the joint material 77 above and below.

[0104] Next, an apparatus and method for discharging excavated soil from frozen ground will be described with reference to Figures 11 and 12. Here, Figure 11 is a perspective view showing an example of an apparatus for discharging excavated soil from frozen ground according to an embodiment, and Figure 12 is a diagram showing a vacuum truck constituting an example of an apparatus for discharging excavated soil from frozen ground according to an embodiment, positioned on the ground beside a manhole and discharging slurried excavated soil through a vacuum pipe.

[0105] As shown in Figure 11, the soil removal device 90 includes a pit 91 for collecting excavated soil T in a frozen state, a pump 94 for circulating circulating water supplied to the inside of the pit 91, a heater 93 disposed inside the pit 91, a vacuum pipe 96 having one end 96a inserted into the inside of the pit 91, and a vacuum truck 95 (see Figure 12) with which the other end of the vacuum pipe 96 is fluidly connected.

[0106] A water-permeable gauge 92 having a large number of water-permeable holes 92 a is installed inside the pit 91 , and a heater 93 is disposed inside the gauge 92 .

[0107] In addition, a pump 94 is placed on a stand 91a suspended above the pit 91, and the circulating water is heated by a heater 93 inside the gauge 92, and the circulating water is provided to the excavated soil T by the pump 94. The circulating water also flows out of the gauge 92 and is provided to the excavated soil T, causing the frozen excavated soil T to melt and turn into slurry by the circulating water.

[0108] The excavated soil T thus slurried is discharged into a vacuum truck 95 via a vacuum pipe 96. Here, as shown in Figure 12, if the existing tunnels 10, 20 are connected to the ground via a manhole W, the vacuum truck 95 is positioned to the side of the manhole W on the ground, and the vacuum pipe 96 extending from the vacuum truck 95 is passed through the manhole W, with one end of the vacuum pipe 96 inserted into a pit 91 installed within the existing tunnels 10, 20.

[0109] As shown in the example shown, a pit 91 is installed inside the existing tunnels 10, 20 and used to accumulate frozen excavated soil T. Inside the pit 91, a pump 94 for circulating water and a heater 93 are installed, and one end 96a of a vacuum pipe 96 that passes through a connecting pipe such as a manhole W connecting the existing tunnels 10, 20 to the ground and leads to a vacuum truck 95 is inserted into the pit 91. The circulating water heated by the heater 93 melts the frozen excavated soil T into a slurry, and the slurry is discharged into the vacuum truck 95 via the vacuum pipe 96. The frozen soil is efficiently thawed, and the mud produced by the thawed frozen soil can be efficiently discharged to the ground through the relatively small diameter connecting pipe W, and the excavated soil can be transported by the vacuum truck 95 to an industrial waste disposal contractor, etc.

[0110] Here, the soil removal method of this embodiment includes a melting process in which the frozen excavated soil T is melted and turned into a slurry using circulating water heated inside the pit 91 as described above, and a discharge process in which the slurried excavated soil T is sucked up and discharged by a vacuum truck 95 via a vacuum pipe 96 extending from the vacuum truck 95 waiting on the ground.

[0111] It should be noted that the present invention is not limited to the configurations shown here, and other embodiments may be possible in which other components are combined with the configurations described in the above embodiments. In this regard, the present invention can be modified within the scope of the present invention, and can be appropriately determined depending on the application form. [Explanation of symbols]

[0112] 10: Existing tunnel (first existing tunnel, underground structure) 11: First steel segment 12: First segment ring 13: Mirror cut area 15: Main girder 16: Skin plate (wall) 16a:Aperture 17a:Vertical rib 17b: Joint plate 18: Mine entrance (1st mine entrance) 20: Existing tunnel (second existing tunnel, underground structure) 21: First steel segment 22: First segment ring 28: Mine entrance (2nd mine entrance) 30: Portal reinforcement structure (portal reinforcement structure for existing tunnels) 31: First bracket 32: Sleeper 33: Second bracket 35: Girder material 36: Support pillar (temporary support material) 38: Brace material 40: Groundwater drainage structure 41: Guide tube 41a:Hollow 41b: End opening 41c: Branch pipe 42: Casing 43: First flow path 44: Insert pipe (double pipe) 44a: Second flow path 44b: Third flow path 45:Outer tube 46: Inner tube 47: First valve 48: Second valve 49: Filling material 50: Junction tunnel (junction tunnel connecting existing tunnels above and below, branch tunnel) 55: Secondary lining 60A: Steel segment ring (second steel segment ring) 60: Steel segment (second steel segment) 61: Skin plate (side panel) 62:Lower main girder 63: Upper main girder 64: Joint plate 65:Vertical rib 66: Flange 67: First reinforcing rib 68: Second reinforcing rib 69: Inlet 70A: Flexible segment ring 70: Flexible segment 75:Waterproof rubber 77: Joint material 80: Joint structure (joint structure between existing tunnel and branch tunnel) 82: Steel plate (endless steel plate) 84: Inlet 90: Soil removal device (soil removal device for excavated soil from frozen ground) 91: Pit 91a: stand 92: Gauge 92a: Water hole 93: Heater 94: Pump 95: Vacuum truck 96: Vacuum pipe G: Ground GA: Frozen ground (ground) GB: Unfrozen ground (ground) P: Cryotube H: Borehole D: Segment ring support F: Temporary scaffolding A: Assembly area RL: Rail K: Cart E: Heat insulation material M: Chipper S: Worker B: Hanging beam C: Chain hoist with trolley R: Waterproof urethane material Q: Backfill material T: Frozen excavated soil (excavated soil) W: Manhole (connecting pipe)

Claims

1. A junction tunnel that connects two existing tunnels, which are existing underground structures in an up-down relationship, by extending vertically and joining them. It is formed by joining a plurality of segment rings, A joint tunnel connecting upper and lower existing tunnels, characterized in that any of the segment rings is a flexible segment ring formed from a flexible segment.

2. 2. The joint tunnel connecting upper and lower existing tunnels according to claim 1, wherein the segment rings adjacent to the two upper and lower existing tunnels are the flexible segment rings.

3. 3. A joint tunnel connecting upper and lower existing tunnels according to claim 1 or 2, characterized in that the segment ring is formed by a plurality of steel segments.

4. 4. A joint tunnel connecting upper and lower existing tunnels according to claim 3, characterized in that a secondary lining formed of a filling material is provided on the inside of the steel segments.

5. 3. The junction tunnel connecting the upper and lower existing tunnels according to claim 1 or 2, wherein the ground around the junction tunnel is frozen by a freezing method.

6. A construction method for a joining tunnel that connects two existing tunnels, which are existing underground structures that are in an upper-lower relationship, by extending the tunnel vertically and joining the existing tunnels, A freezing process of freezing the ground around the construction area of ​​the joint tunnel by a freezing method; a connecting step of cutting open the existing tunnel above to form a second entrance, excavating the unfrozen ground below the second entrance to reach the existing tunnel below, and cutting open the existing tunnel to form a first entrance, thereby connecting the two existing tunnels via a borehole extending vertically; a forming process of hanging a segment ring down from the existing tunnel above and joining it to the first portal of the existing tunnel below, stacking a plurality of the segment rings upward while successively hanging them down and joining them, and joining the topmost segment ring to the second portal of the existing tunnel above, thereby forming the joined tunnel; A construction method for a joint tunnel connecting existing tunnels above and below, characterized in that in the forming process, any of the segment rings is made into a flexible segment ring formed from a flexible segment.

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