Method for constructing underground structure and underground structure

Abstract

To implement a reinforcing bar erection process that improves workability, safety, and quality using simple equipment.SOLUTION: A construction method of an underground structure is provided, and comprises a first excavation step of excavating the ground and forming a shaft, an earth retaining member installation step of installing an earth retaining member along a wall surface of the shaft, an initial reinforcing bar installing step of suspending and lowering the reinforcing bar into the shaft while fixing the horizontal position using a fixing jig attached to the earth retaining member, and a final excavation step in which the ground is further excavated and the shaft is excavated in the vertical direction, a final reinforcement step of suspending and lowering the reinforcing bar into the shaft excavated in the final excavation step while fixing the horizontal position using the fixing jig, and a concrete placing step of placing concrete inside the shaft including the reinforcing bars.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for constructing an underground structure and to an underground structure. [Background technology]

[0002] A deep foundation is generally constructed by excavating the ground and repeatedly connecting the left and right and top and bottom edges of a liner plate to a specified depth, constructing an earth retaining wall inside the shaft, erecting rebar inside the wall, and then pouring concrete. Generally, the rebar erected inside the earth retaining wall includes main rebars and hoops, and various techniques for erecting these rebars have been proposed.

[0003] For example, Patent Document 1 describes a technology in which support columns with supports set at predetermined intervals are installed vertically along the wall surface inside a cylindrical pile hole to temporarily hold hoop reinforcement, and the hoop reinforcement is bundled using a hanging scaffold that can be moved up and down, concrete is poured using a floating scaffold, and steel bars are connected using a mechanical joint method to form vertical main reinforcement.

[0004] Furthermore, Patent Document 2 describes a technology in which scaffolding for assembling reinforcing bars is formed inside an excavated shaft, hoop bars are divided into multiple stages and temporarily assembled into a ring using an assembly device on the ground around the shaft, the temporarily assembled multiple stages of hoop bars are carried into the shaft one by one, and then main bars are inserted into the shaft and the main bars and hoop bars are connected using the scaffolding.

[0005] Furthermore, Patent Document 3 describes a technology in which a tower-shaped temporary platform is constructed above ground at the entrance of a deep foundation shaft, multiple chain wheel winches that can be operated synchronously are arranged to suspend and support a support ring, the vertical bars that make up the reinforcing bar cage are attached to the support ring one by one, tie bars are welded around the vertical bars, and the part that has secured the rigidity of the reinforcing bar cage is successively lowered into the deep foundation shaft together with the support ring by operating the chain wheel winch. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-038475 [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-313860 [Patent Document 3] Japanese Patent Application Publication No. 09-177070 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the conventional rebar installation techniques described above have several issues. First, the method of assembling rebar inside a vertical shaft, as described in Patent Document 1, allows for high construction accuracy because the concrete cover thickness outside the rebar can be checked while assembling, but requires scaffolding for assembly, making it less workable, and also requires a lot of work inside the shaft, making it less safe. The method of assembling a certain amount of rebar above ground and then joining it inside the shaft, as described in Patent Document 2, improves construction efficiency and safety, but requires large lifting machinery. Furthermore, while the amount of work inside the shaft is reduced, it still exists. The method of continuously feeding rebar while assembling at the shaft entrance, as described in Patent Document 3, further improves construction efficiency and safety, but requires large-scale equipment and makes it difficult to check the concrete cover thickness because the assembly work is done from outside the shaft.

[0008] Therefore, an object of the present invention is to provide a method for constructing an underground structure and an underground structure that can carry out a reinforcing bar installation process using simple equipment with improved workability, safety, and quality. [Means for solving the problem]

[0009] [1] A method for constructing an underground structure, comprising: an initial excavation step of excavating the ground to form a vertical shaft; an earth retaining member installation step of installing earth retaining members along the wall surface of the vertical shaft; an initial reinforcing bar installation step of lowering reinforcing bars into the interior of the vertical shaft while fixing the horizontal position using a fixing jig attached to the earth retaining members; a final excavation step of further excavating the ground to excavate the vertical shaft in the vertical direction; a final reinforcing bar installation step of lowering reinforcing bars into the interior of the vertical shaft excavated in the final excavation step while fixing the horizontal position using the fixing jig; and a concrete pouring step of pouring concrete into the interior of the vertical shaft containing the reinforcing bars. [2] A method for constructing an underground structure as described in [1], further comprising at least one intermediate excavation step between the first excavation step and the final excavation step, in which the ground is excavated and the shaft is dug vertically, and an intermediate reinforcing bar erection step, in which the reinforcing bar is lowered into the shaft dug in the intermediate excavation step while fixing its horizontal position using the fixing jig. [3] A method for constructing an underground structure described in [1] or [2], wherein the retaining member has a corrugated cross section in the height direction. [4] A method for constructing an underground structure described in any one of [1] to [3], further comprising a grouting step of applying grout between the wall surface of the shaft and the retaining member by either spraying or filling, or both. [5] A method for constructing an underground structure described in any one of [1] to [4], wherein the reinforcing bars are arranged vertically and include at least a first main reinforcing bar and a second main reinforcing bar, each of which is composed of multiple reinforcing bars connected longitudinally, and when the tips of the first main reinforcing bar and the second main reinforcing bar reach the bottom of the excavated shaft in the final reinforcing bar installation process, the connection position of the first main reinforcing bar and the connection position of the second main reinforcing bar are located at different heights. [6] A method for constructing an underground structure described in [5], which includes a step of connecting a new reinforcing bar to the upper end of an already installed reinforcing bar among the plurality of reinforcing bars, and then suspending the already installed reinforcing bar and the new reinforcing bar together. [7] A method for constructing an underground structure described in any one of [1] to [6], wherein the reinforcing bars include main reinforcing bars arranged vertically, and further includes a fixing step of engaging the protruding portion of the main reinforcing bars with the fixing jig to fix the height position of the main reinforcing bars after the first reinforcing bar installation step. [8] An underground structure comprising: retaining members installed along the wall of a shaft formed by excavating the ground; fixing jigs attached to the retaining members; reinforcing bars arranged to engage with the fixing jigs; and concrete poured inside the shaft containing the reinforcing bars. [9] An underground structure as described in [8], wherein the reinforcing bars include main reinforcing bars extending vertically, and the fixing jig has a structure capable of engaging the protruding portions of the main reinforcing bars.

[10] An underground structure as described in [9], wherein the fixing jig has a notch that allows the main reinforcing bars to be inserted and removed.

[11] An underground structure as described in [9], wherein the fixing jig has a curved portion that moves toward and away from the main reinforcing bars by rotating in a horizontal plane. [Effects of the Invention]

[0010] According to the above configuration, the rebar installation process is carried out by sequentially lowering the rebars in parallel with the process of excavating the shaft in stages. This eliminates the need for scaffolding inside the shaft for installing the rebars, improving workability and safety. Furthermore, because the horizontal position of the rebars is fixed by a fixing jig attached to the earth retaining member, the concrete cover thickness outside the rebars can be reliably set without work inside the shaft. Because the rebars only need to be lowered in stages, machines such as cranes used for lowering them can be made smaller. Thus, according to the present invention, a rebar installation process can be carried out using simple equipment with improved workability, safety, and quality. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a diagram showing the steps of a method for constructing a deep foundation, which is an underground structure according to one embodiment of the present invention. [Figure 2]1 is a diagram showing the steps of a method for constructing a deep foundation, which is an underground structure according to one embodiment of the present invention. [Figure 3] 1 is a diagram showing the steps of a method for constructing a deep foundation, which is an underground structure according to one embodiment of the present invention. [Figure 4] 1 is a diagram showing the steps of a method for constructing a deep foundation, which is an underground structure according to one embodiment of the present invention. [Figure 5] 1 is a diagram showing the steps of a method for constructing a deep foundation, which is an underground structure according to one embodiment of the present invention. [Figure 6] 1 is a diagram showing the steps of a method for constructing a deep foundation, which is an underground structure according to one embodiment of the present invention. [Figure 7] 1 is a diagram showing the steps of a method for constructing a deep foundation, which is an underground structure according to one embodiment of the present invention. [Figure 8] 1 is a diagram showing the steps of a method for constructing a deep foundation, which is an underground structure according to one embodiment of the present invention. [Figure 9] 1 is a diagram showing the steps of a method for constructing a deep foundation, which is an underground structure according to one embodiment of the present invention. [Figure 10] FIG. 10 is a diagram showing a modified example of a construction method for an underground structure according to one embodiment of the present invention. [Figure 11] FIG. 10 is a diagram showing a modified example of a construction method for an underground structure according to one embodiment of the present invention. [Figure 12] FIG. 10 is a diagram showing a modified example of a construction method for an underground structure according to one embodiment of the present invention. [Figure 13] 10A and 10B are diagrams illustrating an example of a method for fixing the height position of a reinforcing bar in one embodiment of the present invention. [Figure 14] FIG. 14 is a diagram showing a first example of a fixture that can be used in the example shown in FIG. 13. [Figure 15] FIG. 14 is a diagram showing a second example of a fixture that can be used in the example shown in FIG. 13. [Figure 16] FIG. 14 is a diagram showing a third example of a fixture that can be used in the example shown in FIG. 13. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functional configurations are designated by the same reference numerals, and redundant description will be omitted.

[0013] (Construction method for underground structures) A method for constructing a caisson foundation, which is an underground structure according to one embodiment of the present invention, will be described below with reference to FIGS. 1 to 9. First, FIG. 1 illustrates the first stage (first excavation step) of the excavation process in which the ground is excavated to form a vertical shaft 1, the process of installing a liner plate 2, which is an earth-retaining member, along the wall of the vertical shaft 1, and the process of applying grout 3 between the wall of the vertical shaft 1 and the liner plate 2, followed by the process of installing main reinforcing bars 4A inside the vertical shaft 1 (first reinforcing bar installation step). The main reinforcing bars 4A are vertically arranged along the inner periphery of the vertical shaft 1. In this installation step, the horizontal position of the main reinforcing bars 4A is fixed using a fixing jig 5 attached to the liner plate 2, and the main reinforcing bars 4A are lowered into the vertical shaft 1 from the entrance 1A side of the vertical shaft 1. Although omitted for simplicity, the horizontal position of the main reinforcing bars 4A located at the back of the figure is also fixed using a fixing jig attached to the liner plate 2 at the back.

[0014] In this embodiment, the main reinforcing bars 4A include main reinforcing bars 41A (first main reinforcing bars) whose tips reach the excavated bottom surface 1B of the shaft 1 at the time shown in Fig. 1, and main reinforcing bars 42A (second main reinforcing bars) whose height positions are fixed with their tips not reaching the excavated bottom surface 1B at this time. Note that the advantages of making the heights of the main reinforcing bars 4A different from each other during the erection process, and an example of the structure of the fixing jig 5 for fixing the height position of the main reinforcing bars 42A with their tips not reaching the excavated bottom surface 1B will be described later.

[0015] Next, the second stage (intermediate excavation stage) of the excavation process is performed, in which the ground is excavated and a vertical shaft 1 is dug vertically, as shown in FIG. 2 . Then, as shown in FIG. 3 , sprayed grout 31 is applied to the wall of the excavated shaft 1. As shown in FIG. 4 , a liner plate 2 is installed along the wall of the excavated shaft 1. Furthermore, as shown in FIG. 5 , a filling grout 32 is applied between the sprayed grout 31 and the liner plate 2. Such a method of applying the liner plate 2 and grout 3 is also described in JP 2020-105806 A, and adopting such an application method enables economical design and improved workability. However, the application method of the liner plate and grout according to the embodiment of the present invention is not limited to this example. For example, the sprayed grout 31 may be omitted and only the filling grout 32 may be applied. Alternatively, the grout 3 may be omitted and concrete may be poured through holes in the liner plate 2 to fill the gap between the wall of the shaft 1 and the liner plate 2. The same applies to excavation other than the second stage, and an appropriate method for applying grout 3 can be selected depending on ground conditions, etc. In a multi-stage excavation process, the grout 3 may be applied all at once after only the liner plate 2 is installed, or the method for applying grout 3 may be different for each stage of excavation.

[0016] Next, as shown in FIGS. 6 and 7, a process of erecting main rebars (intermediate rebar erection process) is performed inside the excavated shaft 1. First, as shown in FIG. 6, new main rebars 4B (main rebars 41B, 42B) are connected to the top of main rebar 4A (main rebars 41A, 42A) using couplers 61, 62, and the connected main rebars 4A, 4B are lowered as shown in FIG. 7. Here again, the tip of main rebar 41A reaches the excavated bottom surface 1B of the shaft 1, while the tip of main rebar 42A is fixed at a height position without reaching the excavated bottom surface 1B. Note that, although couplers are used to connect the rebars used in the main rebar in this embodiment, in other embodiments, the rebars may be connected using mechanical joints other than those called couplers, lap joints, gas pressure welding joints, or welded joints, and the connection method is not particularly limited.

[0017] In the example shown in FIG. 6 above, new main reinforcing bars 4B are connected to the upper ends of the already-installed main reinforcing bars 4A using couplers 61, 62, etc., and then the main reinforcing bars 4A, 4B are lowered together. As another example, the main reinforcing bars 4A may be lowered first, and then new main reinforcing bars 4B may be connected to the main reinforcing bars 4A using couplers 61, 62, etc. In this case, the couplers 61, 62 may be attached to the upper ends of the main reinforcing bars 4A on the ground beforehand, and then the main reinforcing bars 4A may be lowered, or the couplers 61, 62 may be attached to the upper ends of the main reinforcing bars 4A when connecting the main reinforcing bars 4B inside the shaft 1. As another example, new main reinforcing bars 4B may be connected to the lower ends of the main reinforcing bars 4A using couplers 61, 62, etc. In this case, instead of lowering the main reinforcing bar 4A, couplers 61 and 62 may be attached to the lower end of the main reinforcing bar 4A in advance, and then the main reinforcing bar 4B may be lowered and connected to the lower end of the main reinforcing bar 4A via couplers 61 and 62.

[0018] The intermediate excavation process and intermediate rebar installation process are repeated in the same manner, further excavating the ground to excavate the shaft 1 vertically, while the main rebars are installed inside the excavated shaft 1. Figure 8 shows the state in which the shaft 1 has reached the specified design depth in the final stage of the excavation process (last excavation process), and the process of installing the main rebars (final rebar installation process) inside the excavated shaft 1 has been completed. Note that Figures 8 and 9 are different examples from those up to Figure 7, so the positions of the couplers of each rebar do not necessarily match. As shown, in the final rebar installation process, the tips of all of the main rebars 41 and 42 reach the excavated bottom surface 1B of the shaft 1.

[0019] As described above, in this embodiment, each of the main reinforcing bars 41 and 42 is formed by connecting multiple reinforcing bars in the longitudinal direction with couplers 61 and 62. As shown in the examples of FIGS. 6 and 7 , during excavation, the height positions of the couplers 61 and 62 are aligned between the main reinforcing bars 41A and 41B constituting the main reinforcing bar 41 (first main reinforcing bar) and the main reinforcing bars 42A and 42B constituting the main reinforcing bar 42 (second main reinforcing bar), but the height positions of the tips of the main reinforcing bars are made different from each other. Therefore, when the tips of the main reinforcing bars 41 and 42 reach the bottom surface 1B of the excavation, as shown in FIG. 8 , the connection position of the main reinforcing bar 41 by the coupler 61 and the connection position of the main reinforcing bar 42 by the coupler 62 are located at different heights. This prevents the heights of the connection positions from being aligned for all of the main reinforcing bars, which can be a structural weakness, and improves the strength of the entire caisson foundation, including the main reinforcing bars. In the illustrated example, two types of main reinforcing bars are placed with their connecting positions located at different heights, but three or more types of main reinforcing bars may be placed with their connecting positions located at different heights.

[0020] Finally, as shown in Figure 9, concrete 7 is poured into the shaft 1, including the main reinforcing bars 41 and 42, to complete the caisson foundation. While the illustrated example of the caisson foundation is a solid structure in which concrete 7 is poured throughout the entire interior of the shaft 1, in other examples it may be a hollow structure in which concrete is poured with a space inside. A hollow underground structure can also be used as a vertical shaft (as a structure) or a drainage well, in addition to a caisson foundation. The hollow structure may also be used as a caisson foundation after filling the internal space with soil, such as surplus soil from excavation work. Furthermore, there is no particular limit to the number of times the excavation process is repeated. For example, the excavation process may be repeated up to the second stage, or more stages may be repeated.

[0021] In this embodiment, a liner plate 2 with a corrugated cross section in the height direction is used as an earth retaining member. The corrugated cross section of the liner plate 2 provides a shear-stop effect, similar to that of deformed reinforcing bars, between the grout 3 applied on the outside and the concrete 7 poured on the inside. Furthermore, because the liner plates 2 are connected to each other in the height and width directions using bolts or the like, they also function as reinforcement for the concrete 7 in the height direction (substitute for main reinforcing bars) and in the width direction (substitute for hoop reinforcing bars). Therefore, in this embodiment, the reinforcing bars of the deep foundation foundation only include main reinforcing bars 4A, 4B, and 4C arranged vertically. As shown in the example, they do not include hoop reinforcing bars arranged circumferentially around the shaft 1, or the number of hoop reinforcing bars is reduced compared to conventional designs. Such a deep foundation foundation design is also described, for example, in JP 2021-080628 A. In other embodiments of the present invention, hoop reinforcing bars can also be arranged. In such cases, the main reinforcing bars connected to the hoop reinforcing bars are lowered with their tips aligned. In this case, the reinforcement of the coupler part of the main reinforcing bars is considered separately. In addition to what is called a liner plate, various other materials with similar functions can be used as earth retaining members.

[0022] According to the underground structure construction method of this embodiment, the rebar installation process is carried out by sequentially lowering the main rebars 4A, 4B, and 4C from the entrance 1A of the shaft 1 in parallel with the stepwise excavation of the shaft 1. This eliminates the need for scaffolding for installing the rebars inside the shaft 1, improving workability and safety. Furthermore, the horizontal positions of the main rebars 4A, 4B, and 4C are fixed by the fixing jigs 5 attached to the liner plate 2, allowing the concrete cover thickness outside the main rebars 4A, 4B, and 4C to be reliably set without work inside the shaft 1. Furthermore, if hoops are omitted, the main rebars 4A, 4B, and 4C can be lowered one by one (one set), eliminating the need for large machines such as cranes for lowering. Even when hoops are installed, if the number of hoops is reduced compared to conventional methods, the machines used for lowering the main rebars can be made smaller.

[0023] Figures 10 to 12 are diagrams showing modified examples of a construction method for an underground structure according to one embodiment of the present invention. As a method for ultimately positioning the connection positions of the first main reinforcing bars and the second main reinforcing bars at different heights, the examples shown in Figures 1 to 7 have been described in which the connection positions during excavation are aligned at the same height, the tip height positions of the main reinforcing bars are made different, and the tip of the main reinforcing bar with the deeper tip position reaches the bottom of the excavation. However, as will be explained below, other methods are also applicable. Note that Figures 10 to 12 below correspond to the stage shown in Figure 1 in the above example.

[0024] For example, as shown in FIG. 10, the tip height positions of the main reinforcing bars 41A and 42A may be staggered, and the height position of the main reinforcing bar 41A with the deeper tip position may be fixed so that it does not reach the bottom surface 1B of the excavation. Alternatively, as shown in FIG. 11, the tip height positions of the main reinforcing bars 41A and 42A may be aligned at a point in the middle of excavation so that both reach the bottom surface 1B of the excavation. In this case, the connecting positions of the main reinforcing bars 41 and 42 will be at different heights at a point in the middle of excavation. Furthermore, as shown in FIG. 12, the tip height positions of the main reinforcing bars 41A and 42A may be aligned, and the height positions of the main reinforcing bars 41A and 42A may be fixed so that neither tip reaches the bottom surface 1B of the excavation.

[0025] (Method of fixing rebar) FIG. 13 is a diagram showing an example of a method for fixing the height position of rebars in one embodiment of the present invention. For simplicity, only a pair of rebars located within the radial cross section of the shaft 1 and the fixing jigs for securing these rebars are shown. In the illustrated example, the first stage of the excavation process for the shaft 1 has been completed, as in the example shown in FIG. 1 above, and the main rebars have been installed. In this example, the main rebar 4A is shorter than the example shown in FIG. 1, and the main rebar 4B has already been connected to its upper part via coupler 6A. Furthermore, although no other rebars have yet been connected to the upper end of the main rebar 4B, the coupler 6B has already been attached. Here, the couplers 6A and 6B are cylindrical members with threaded holes (internal threads) that thread onto the threaded portions (external threads) formed at the respective ends of the main rebars 4A and 4B. The couplers 6A and 6B attached to the main rebars 4A and 4B form protrusions with larger outer diameters than the main rebars 4A and 4B. In the illustrated example, the height position of the main reinforcing bars 4A, 4B is fixed by positioning the fixing jig 5 to match the length of the main reinforcing bars 4A, 4B and engaging the protruding couplers 6A, 6B with the fixing jig 5. Therefore, thereafter, the height position of the main reinforcing bars 4A, 4B can be maintained during the second stage of excavation of the shaft 1, and the main reinforcing bars 4A, 4B can be lowered after excavation is completed.

[0026] FIG. 14 is a diagram showing a first example of a fixing jig that can be used in the example shown in FIG. 13. In the illustrated example, the fixing jig 5A is generally in the shape of a plate with one fork. An attachment hole 51 for attaching to the liner plate 2 is formed at the base of the fork, and a notch 52 with parallel opposing sides is formed at the fork. As shown in FIG. 14( a), with a coupler 6A attached to the upper end of a main reinforcing bar 4A, the main reinforcing bar 4A is inserted into the notch 52 of the fixing jig 5A, and the coupler 6A is placed on the top surface of the fixing jig 5A on both sides of the notch 52 to lock the coupler 6A and fix the height position of the main reinforcing bar 4A. To enable the insertion of the main reinforcing bar 4A (and main reinforcing bar 4B) and the locking of the coupler 6A (and coupler 6B), the inner dimension of the notch 52 is larger than the outer diameter of the main reinforcing bar 4A (and main reinforcing bar 4B) and smaller than the outer diameter of the coupler 6A (and coupler 6B).

[0027] As shown in Figure 14(b), the main reinforcing bars 4B are attached to coupler 6A from above to connect the main reinforcing bars 4A and 4B, and then coupler 6B is attached to the upper end of main reinforcing bar 4B. After that, as shown in Figure 14(c), the horizontal positions of the main reinforcing bars 4A and 4B are shifted away from fixing jig 5A, that is, toward the center of the shaft 1 (not shown). This causes coupler 6A to disengage from notch 52, and the fixing jig 5A releases the coupler 6A from its engagement with the fixing jig 5A. In this state, the main reinforcing bars 4A and 4B are lowered, and this time the main reinforcing bar 4B is inserted into notch 52, as shown in Figure 14(d). This allows the vertical position of the main reinforcing bars 4A and 4B after lowering to be fixed by engaging coupler 6B with fixing jig 5A.

[0028] FIG. 15 is a diagram showing a second example of a fixing jig that can be used in the example shown in FIG. 13. In the illustrated example, the fixing jig 5B is plate-shaped and has an attachment portion and a curved portion. The attachment portion has an attachment hole 51 for connecting to the liner plate 2, and a C-shaped inner edge 53 is formed on the inside of the curved portion. As shown in FIG. 15(a), with a coupler 6A attached to the upper end of a main reinforcing bar 4A, the inner edge 53 of the fixing jig 5B is aligned with the main reinforcing bar 4A, and the coupler 6A is placed on the upper surface of the fixing jig 5B outside the inner edge 53 to lock the coupler 6A and fix the height position of the main reinforcing bar 4A. To stably lock the coupler 6A, it is preferable that the inner edge 53 has a planar shape with a radius of curvature that is greater than the radius of curvature of the outer periphery of the main reinforcing bar 4A (and main reinforcing bar 4B) and smaller than the radius of curvature of the outer periphery of the coupler 6A (and coupler 6B).

[0029] As shown in Figure 15(b), the main reinforcing bars 4B are attached to the coupler 6A from above to connect the main reinforcing bars 4A and 4B. Then, as shown in Figure 15(c), the fixing jig 5B is rotated in the horizontal plane around the mounting hole 51 to move it away from the main reinforcing bar 4A. This moves the coupler 6A away from the inner edge 53 of the fixing jig 5B, releasing the coupler 6A from its engagement with the fixing jig 5B. In this state, the main reinforcing bars 4A and 4B are lowered as shown in Figure 15(d), and the fixing jig 5B is again rotated in the horizontal plane around the mounting hole 51 to move it closer to the main reinforcing bar 4B, this time aligning the inner edge 53 with the main reinforcing bar 4B. The height positions of the main reinforcing bars 4A and 4B after lowering can be fixed by engaging the coupler 6B with the fixing jig 5B. Such a fixing jig 5B can fix and release the vertical position without moving the horizontal position of the main reinforcing bars 4A, 4B, and can therefore be used, for example, when hoops are placed in addition to the main reinforcing bars 4A, 4B and the main reinforcing bars 4A, 4B are lowered while they are joined to the hoops.

[0030] FIG. 16 shows a third example of a fixing jig that can be used in the example shown in FIG. 13 . In the illustrated example, the fixing jig 5C is an annular member fixed to the shaft entrance 1A (not shown) of the shaft 1 using flat bars or angles. Multiple notches 54 are formed on the inner periphery of the fixing jig 5C. Similar to the notches in the example described above with reference to FIG. 14 , the notches 54 are portions into which main reinforcing bars can be inserted. The inner periphery of the fixing jig 5C is also formed with notches 54 at intervals corresponding to the spacing of the main reinforcing bars in the circumferential direction of the shaft 1. By inserting the main reinforcing bars into each notch 54 and engaging the couplers, the height position of the main reinforcing bars can be fixed, as in the examples shown in FIGS. 14 and 15 above. Furthermore, by removing the main reinforcing bars from the notches 54 and unlocking the couplers, the main reinforcing bars can be further lowered. This type of fixing jig 5C can be used, for example, even when it is difficult to attach the fixing jig to a liner plate.

[0031] In the example described above, the height positions of the main reinforcing bars 4A and 4B are fixed by engaging the couplers 6A and 6B with the fixing jigs 5A, 5B, and 5C, but in other examples, protrusions may be formed by pressing a sleeve-shaped member or welding another member to the middle part of the main reinforcing bars 4A and 4B, and then the fixing jigs 5A, 5B, and 5C may be engaged. The fixing jigs 5A and 5B may be attached to multiple stages, including the top stage of the liner plate 2, using the mounting holes 51, as in the example shown in Figure 13, or they may be attached to just the top stage.

[0032] Note that the embodiment of the present invention is not limited to the above-described example in which the vertical position of the main rebars is fixed by engaging the protruding portions of the main rebars with the fixing jig. Separate clamps or other means may be used to fix the vertical position of the main rebars. Furthermore, fixing jigs capable of fixing the horizontal and vertical positions of the main rebars, such as fixing jigs 5A, 5B, and 5C, may be used in combination with fixing jigs that fix only the horizontal position of the main rebars. A fixing jig for fixing the vertical position of the main rebars may be placed, for example, at only one location on the topmost level of each main rebar. While the above-described example shows multiple fixing jigs placed in the vertical direction, if the horizontal position is fixed with at least one fixing jig, two-point support can be achieved by, for example, abutting the tip of the main rebar against the bottom of the excavation at the end of excavation, thereby stabilizing the horizontal position of the main rebars.

[0033] Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention. [Explanation of symbols]

[0034] 1...shaft, 1A...pithead, 1B...bottom of excavation, 2...liner plate, 3...grout, 31...sprayed grout, 32...filling grout, 4A, 4B, 4C, 41, 41A, 41B, 42, 42A, 42B...main rebars, 5, 5A, 5B, 5C...fixing jig, 51...mounting hole, 52, 54...cutout, 53...inner edge, 61, 62, 6A, 6B...coupler, 7...concrete.

Claims

1. a first excavation step of excavating the ground to form a vertical shaft; a first earth retaining member installation step of installing a first earth retaining member along a wall surface of the vertical shaft excavated in the first excavation step; A first reinforcing bar erection process in which reinforcing bars arranged vertically inside the vertical shaft are lowered while fixing the horizontal position using a fixing jig attached to the first retaining member; at least one second excavation step of further excavating the ground to excavate the shaft in a vertical direction; At least one second earth retaining member installation step of installing a second earth retaining member along a wall surface of the shaft excavated in the second excavation step; At least one second reinforcing bar erection step of lowering the reinforcing bar into the shaft excavated in the second excavation step while fixing the horizontal position using the fixing jig; a concrete pouring step of pouring concrete into the shaft including the reinforcing bar; Including, A method for constructing an underground structure, wherein the reinforcing bars include a first reinforcing bar and a second reinforcing bar that are placed near the first earth retaining member or the second earth retaining member and connected in the longitudinal direction.

2. A method for constructing an underground structure as described in claim 1, wherein the second reinforcing bar installation process includes a process of connecting the second reinforcing bar to the upper end of the first reinforcing bar that has already been installed, and then suspending and lowering the first reinforcing bar and the second reinforcing bar together.

3. 3. The method for constructing an underground structure according to claim 1, wherein the first and second retaining members have a wave-shaped cross section in the height direction.

4. A method for constructing an underground structure as described in any one of claims 1 to 3, further comprising a grouting step of applying grout between the wall surface of the shaft and at least the second retaining member by either spraying or filling, or both.

5. The reinforcing bars include at least a first main reinforcing bar and a second main reinforcing bar, and the first main reinforcing bar and the second main reinforcing bar are respectively configured by the first reinforcing bar and the second reinforcing bar connected in a longitudinal direction; A method for constructing an underground structure described in any one of claims 1 to 4, wherein when the tips of the first main reinforcing bar and the second main reinforcing bar reach the bottom of the excavated shaft in the final second reinforcing bar installation process, the connection position of the first main reinforcing bar and the connection position of the second main reinforcing bar are located at different heights.

6. At least the end or middle part of the first reinforcing bar is formed with a protrusion having an outer diameter larger than that of other parts, A method for constructing an underground structure as described in any one of claims 1 to 5, further comprising a fixing step of engaging the protrusion with the fixing jig to fix the height position of the first reinforcing bar after the first reinforcing bar erection step.

7. an earth retaining member installed along the wall surface of a vertical shaft formed by excavating the ground; A fixing jig attached to the retaining member; A reinforcing bar arranged vertically and disposed so as to engage with the fixing jig; Concrete poured inside the shaft including the reinforcing bars; and The reinforcing bars include a first reinforcing bar and a second reinforcing bar connected in a longitudinal direction; At least the first reinforcing bar has a protruding portion having an outer diameter larger than that of other portions, The fixing jig is an underground structure having a structure capable of engaging the protrusion.

8. 8. The underground structure according to claim 7, wherein the structure capable of locking the protrusion has a notch into which the portion of the reinforcing bar other than the protrusion can be inserted and removed.

9. The underground structure according to claim 7 , wherein the structure capable of locking the protrusion has a curved portion that moves toward and away from the reinforcing bar by rotating in a horizontal plane.

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