JPH04285297A - Method for constructing underground space - Google Patents

Abstract

PURPOSE:To safely and economically construct an underground space within a short period of construction and at a low noise level without requiring the conveyance and back filling of surface soil, disturbing activities above the ground and being obstructed by any existing buried object. CONSTITUTION:Lead guides 12 are buried along an underground space after the excavation of a work zone and a plasticizer is substituted for earth and sand in a plate insert portion. Arched hollow plates 122 are inserted along the lead guides 12 and concrete 38 is packed in the hollow portion of each plate and an underground space 4 is excavated after the concrete becomes stiff.

Description

[Detailed description of the invention]

0001

FIELD OF THE INVENTION This invention relates to a method for creating underground space.

0002

BACKGROUND OF THE INVENTION Underground spaces are mainly created using open cut construction methods and shield construction methods. In the shield construction method, the excavated underground space is supported with temporary frames until the concrete hardens, so there is a risk of collapse, and it cannot be used in shallow underground areas. In addition, with the shield method,
Since the excavated earth and sand is turned into muddy water and discharged, the amount of discharge is extremely large. For this reason, underground space creation in urban areas is mainly carried out using the open cut method. The open-cut construction method involves cutting open the part that forms the underground space, which obstructs above-ground traffic, and also generates a lot of noise during construction. Another problem with the open cut construction method is that the earth and sand must be scraped down to the underground space and backfilled after the underground space has been created. There is a need for a place to store the soil. In addition, with the open cut construction method, gas pipes, water pipes, sewage pipes, etc. become obstacles to construction.
Enforcement becomes difficult if there are elevated railways or small buildings.

0003

[Problems of the Invention] The problems of this invention are: (1) There is no need to remove or backfill topsoil, (2) construction can be carried out without interfering with above-ground activities, (3) the construction period is short, and (4)
To provide a method for creating an underground space that is safe because it is low in noise, (5) existing buried objects such as gas pipes and building foundations do not obstruct construction work, and (6) there is no risk of collapse. It is in.

0004

[Structure of the Invention] This invention comprises a process of excavating a work space, a process of inserting a lead guide from the excavated work space along a target underground space, and a process of forming a plate while determining the direction by the inserted lead guide. A method for creating an underground space includes a step of inserting the plate along the ceiling of the underground space, and a step of excavating the lower part of the plate to create an underground space. The plates are, for example, two plates corresponding to the ceiling and floor of the underground space. More preferably, the plate is an arch-shaped plate that corresponds to the sides and ceiling of the underground space to be formed, so that the ceiling and both walls of the underground space can be formed at once. In this invention, the underground space is excavated after the ceiling is formed with plates, and the main steps up to that point are insertion of lead guides and plates. Therefore, with this invention, an underground space can be created without fear of collapse.

[0005]

[Example] Example 1 Excavation of work area (FIG. 1) The steps of the example will be explained step by step. First, a work section 2 for horizontal hole digging shown in FIG. 1 is excavated. For example, if the cross section of the underground space to be provided is 8 m x 8 m, a work section 2 with a cross section of about 10 m x 10 m, which is slightly larger than the size of the underground space, is excavated. Further, in this case, the work section 2 is excavated at intervals of, for example, 20 m to 40 m. In the figure, 4 is an underground space forming part, and 6 is a sheet pile, which is also provided on the opposite side of the work area 2. The excavation of the work zone 2 and the work after the excavation are performed, for example, at night, and the work zone 2 is covered with a lid (not shown) in the middle so as not to become an obstruction to traffic. In this embodiment, the plates and the like are inserted into the soil using hydraulic pressure, so the support plate 7 is provided so that the plates and the like can be inserted against the earth pressure. Although the support plate 7 using a concrete wall is shown in the figure, a foundation may be poured into the bottom of the work area 2 and this may be used as the support plate. The support plate 7 supports the civil engineering machine used for inserting plates and the like.

Drilling holes for inserting lead guide columns (FIG. 2) After excavating the work section 2, the excavator is lowered and holes 10 for inserting lead guide columns for the plate are drilled at the four corners of the underground space forming part 4 using an auger 8 or the like. excavate. In this case, work area 2
If the interval is as long as 40 m, for example, insert holes 10 for the lead guide pillars are dug from both opposing work sections 2, and the lead guide pillars are docked at the center. If the distance is as short as 20 m, the excavation is continued from one working section 2 to the other working section 2. If the plate is installed only on the ceiling of the underground space, the insertion holes for the lead guide pillars shall be at the top two corners.

Embedding the lead guide pillar (FIG. 3) A lead guide pillar 12 (made of steel, for example) for a plate is embedded in the insertion hole 10 of the lead guide pillar that has been excavated. Since it is difficult to bring the 20 m long lead guide pillar 12 into the work section 2, the lead guide pillar 12 is divided into several pieces and is completed by joining them together. Reference numeral 14 in the figure is the tip of the lead guide column, which has a sharpened tip to facilitate embedding, and 16 is a supplementary portion of the lead guide column. When the lead guide column 12 is docked at the center, the tip 14 is also shaped accordingly.

Improving the soil quality of the plate insertion part (FIG. 4) The soil quality of the plate insertion part between the lead guide columns 12 and 12 is improved to give it plasticity. For this purpose, first, a plurality of holes 18 are dug at intervals in the insertion portion of the plate, and earth and sand are removed. Next, a plasticizer 20 is injected into the hole 18 from which the earth and sand have been removed. The material of the plasticizer 20 may be, for example, clay, thermoplastic synthetic resin, etc., as long as it deforms when the plate is inserted and allows the plate to be inserted. A preferred material for the plasticizer 20 is clay kneaded with oil, and the plasticizer 20 is injected in an area slightly larger than the cross-sectional area of the plate. The injection interval of plasticizer 20 is changed depending on the soil quality of the area to be applied.
For example, if the soil is originally clayey and highly plastic, the injection of the plasticizer 20 may be omitted. On the other hand, in hard soil, the earth and sand in the plate insertion portion between the lead guide columns 12, 12 is completely replaced with plasticizer. If the entire area is replaced at once, the hole 18 will collapse, so after replacing one part, the hole 18 is dug in another part and replaced there. For normal soil conditions, the soil is replaced with plasticizer 20 at intervals, and the injection interval of plasticizer 20 is determined depending on the plasticity of the soil. The injected plasticizer 20 is soft in the depths of the hole 18, and becomes harder as it approaches the entrance.

Inserting the plate (FIGS. 5 to 7) Next, the steel plate 22 is oriented along the lead guide column 12 and inserted, connecting each other at intervals of, for example, several meters. This is possible because the earth and sand between the lead guide columns 12, 12 is replaced with the plasticizer 20. plate 2
2, the plasticizer 20 is deformed and a part of it escapes around the plate 22, and most of it enters the inside of the plate 22 through the hole at the tip of the plate 22. Some of the earth and sand and plasticizer 20 that have entered the inside of the plate 22 flow out from the holes 24 of the plate 22 to the surrounding area in areas where the earth pressure is low, and harden the surrounding area. Conversely, in areas with high earth pressure or areas with large friction with the surrounding ground, surrounding earth and sand and plasticizer 20 will flow through the holes 24 to the plate 2.
Enter the hollow part 2. This process equalizes the surrounding earth pressure. If the plasticizer 20 is made by kneading clay with oil,
When the plate 22 is inserted, oil oozes out due to the pressure applied to the plasticizer 20 and the deformation of the plasticizer 20, and the oil spreads along the surface of the plate 22, reducing friction with the ground above and below. In addition, if the plasticizer 20 is injected into the entire surface of the insertion part of the plate 22 and the injection area of the plasticizer 20 is made slightly wider than the area of the plate 22, the plate 22 is not directly connected to the ground above and below, but the plasticizer 20 The friction between the upper and lower ground and the plate 22 is reduced. These make insertion easy. Since it is difficult to bring a plate 22 with a length of 20 m into the work area 2, the plate 22 is divided into a tip part 26 and a joining part 28, and the plate 22 is made into one piece by joining them. Inserting the plate 22 becomes more difficult as it goes deeper into the hole 18, but since the plasticizer 20 also becomes softer as it goes deeper into the hole 18, it becomes easier to insert it.

FIG. 6 shows the additional part 28 of the plate 22.
FIG. 7 shows the hard steel tip 26. The tip portion 26 and the additional portion 28 are formed by supporting two steel plates with a girder 31 and forming a hollow portion 32. A hole 24 is provided in the outer plate 29 of the two steel plates, so that the dirt and plasticizer 20 that entered through the opening at the tip escape to the surrounding ground, or conversely, the plasticizer 20 and dirt from the surrounding area are trapped in the hollow part 32. Try to get into it. Furthermore, when pouring concrete, the concrete poured into the hollow portion 32 enters the external ground through the hole 24 to help the plate 22 bond to the ground. More hole 2
The surface of the outer panel 29 is partially covered with the concrete escaped from the outer panel 4 to the outside, and corrosion of the outer panel 29 is suppressed. Outer panel 29
Since there is a risk of corrosion due to the surrounding ground, the inner plate 30 is made thicker than the outer plate 29 so that the concrete poured into the inner plate 30 and the hollow part 32 can form a structure for the underground space. Since the inner surface of the inner plate 30 is used as the ceiling surface and floor surface of the underground space, no hole 24 is provided in the inner plate 30. Hollow part 3
Holes 27 are provided in the girder 31 so that the concrete poured into the girders 2 and 2 are bonded to each other. The tip 26 of the plate 22 is made of hard steel, the tip 33 of the inner plate 30 and the tip 34 of the outer plate 29 are edge-shaped, and the tip 35 of the girder 31 is also blade-shaped to facilitate insertion. . Here the two tips 33
, 34 so that their edges face each other (see the left end of Figure 7).
, the plasticizer 20 and earth and sand are introduced into the hollow part 32 so that the plate 22 can be easily inserted.

Discharging earth and sand from the hollow part of the plate (FIG. 8) After the insertion of the plate 22 is completed, the hollow part 3 of the plate 22
Completely scrape out the remaining soil and plasticizer 20. For example, scrape it out with a brush and wash it with water to completely remove any remaining dirt or plasticizer.

Pouring Concrete (FIG. 9) After this, concrete 38 is poured into the hollow portions 32 of the lead guide columns 12 and plates 22. As a result, the lead guide column 12
The plate 22 becomes the skeleton of the steel concrete structure. The poured concrete 38 is poured into the hole 27 made in the girder 31.
It is joined to the concrete in the adjacent hollow part 32. In addition, a portion of the concrete 38 seeps into the ground through the holes 24, suppressing corrosion of the outer panel 29, and also serves to connect the plate 22 to the ground.

Forming the Support Pillar (FIG. 10) When concrete 38 is poured, the strength of the plate 22 increases and it becomes able to support the earth pressure. two plates 22
, 22 was locally excavated, and the pillars 40 and side walls (
A temporary wall) is constructed to prevent collapse even if the entire lower part of the plate 22 is excavated. The number of pillars 40 and the thickness of the side walls may be determined depending on the soil quality.

Completion of underground space (FIG. 11) After this, the underground space between the plates 22 is excavated to complete the underground space 42. The side wall is shown as 44. The provided underground space 42 can also be used as a work area when one or more underground spaces are added laterally, or when an underground space is added at right angles to the underground space 42.

The process of extending the underground space 42 is shown in FIG. When the underground space 42 between the two central work sections 2, 2 is completed, an underground space between the two work sections 2 and 2 is created. Of course, a large number of work sections 2 may be provided at the same time, and all the work sections may be executed at the same time.

In order to widen the width of the underground space, once one underground space 42 is completed, an adjacent underground space 42 is created which also serves as the lead guide column 12, and the earth and sand between these spaces are removed to create a continuous, integrated underground space. It may be a space (Figure 13).

[0017] Characteristics and modifications of the embodiment will be shown. First, excavation in work zone 2 may be carried out at intervals of 40 m to 20 m, for example.
After excavation, work should be done only at night, and if the area in between is covered, it will not cause an obstruction to traffic. Since no open cuts are made in areas other than the work zone 2, there is no problem even if there are buildings, elevated railway sections, etc. above the underground space 42. Gas pipes, water pipes, sewage pipes, etc. are usually buried shallower than 2 to 3 meters underground, and the underground space 42 is usually provided deeper than this, so the gas pipes etc. do not interfere with the construction work. Furthermore, if there are foundations of other buildings or elevated railroad tracks, the underground space 42 may be provided deeper than these foundations. That is, if the work section 2 can be provided, implementation is easy. The construction method of the embodiment is suitable for construction in shallow underground areas in large cities, but it can also be used in construction at great depths by making the work section 2 deeper. Further, when the intervals between the work sections 2 are short, the work is carried out only in one work section 2 and executed toward the other work section 2. If the distance between the work sections 2 is large, the work will be carried out from both work sections 2, 2, and docked at the center. Note that this construction method may be used in combination with conventional construction methods such as open cut and shield construction for part of the construction area.

Next, in this construction method, the underground space 42 is not excavated until the pillars 40 or side walls 44 are constructed. In the process up to that point, the plate 22 is simply inserted into the ground, and there is no risk of it collapsing. Although it is difficult to insert the plate 22 into the soil, the soil is modified with a plasticizer to facilitate insertion. At the time of insertion, the plate 22 is aligned with the lead guide column 12 and inserted precisely.

As a result, in this embodiment, there is no need to excavate excess earth and sand, and it is only necessary to excavate only the earth and sand in the area where the underground space 42 is to be provided. This construction method is economical because the amount of earth and sand to be excavated is small, there are no obstructions due to gas pipes, etc., and the plates 22 and lead guide columns 12 can be mass-produced as units.
And there is less noise. Furthermore, since the plate 22 can be made into a unit, the construction period can be shortened compared to the conventional construction method in which the top plate and floor plate are installed on site.

In the embodiment, the plates 22 are provided above and below the underground space 42, but the role of the plates 22 is to prevent the columns 40 and side walls 44 from collapsing when they are formed, and the plates 22 may be provided only at the top. . Further, since a square or rectangular underground space 42 is desired, the plate 22 is made flat, but a half-moon-shaped plate extending toward the inside of the underground space 42 may be used.

Embodiment 2 (FIGS. 14 to 24) As a second embodiment, an arch-shaped plate is shown. This example is Example 1 except that the plate is arched.
The explanation regarding the first embodiment also applies to the second embodiment as is, except for the points specifically indicated. The advantage of having arch-shaped plates is that there is no need to construct supports (Fig. 10), and the inserted plates can be used as they are for the ceiling and both walls of the underground space. If an arch-shaped plate is inserted into the soil and the hollow part of the plate is solidified with concrete, there is no risk of it collapsing, and an underground space can be created by excavating the earth and sand inside the plate and forming a floor surface. The problem with having an arched plate is that the cross-sectional area of the plate is increased, which increases the resistance to insertion of the plate. Therefore, the plasticizer 20 is applied to the entire surface of the insertion part of the plate, and the earth and sand in the part where the plate enters is completely replaced with the plasticizer 20. The details of the embodiment will be explained below.

The excavation process of the working section 2 was carried out in the same manner as in the first embodiment, and the insertion hole 10 of the lead guide column 12 was inserted into the underground space 4.
Excavate at both ends of the lower end (Figure 14). This is because the arch-shaped plate has a large cross-sectional area, so it tends to move downward rather than horizontally, and this is corrected by the lead guide pillars 12 attached to both lower ends of the arch-shaped plate. Of course, in addition to the two lead guide pillars 12 at both lower ends, additional lead guide pillars 12 may be provided at the bending part of the arched plate (the shoulder part between the ceiling part and the wall part). Next, the lead guide column 12 is inserted as shown in FIG. Lead guide pillar 12
The structure of is shown in FIG.

An injection hole 18 for the plasticizer 20 is drilled on the entire surface of the insertion surface of the arched plate, and the plasticizer 20 is injected (
Figure 17). A particularly preferred plasticizer 20 is clay coated with oil, dioctyl phthalate, etc., which is injected or inserted into the hole 18 in a liquid form or in the form of a cylindrical or other molded body. The part where the earth and sand are replaced by the plasticizer 20 is made slightly wider than the cross-sectional area of the arched plate so that the arched plate can be slid and inserted into the plasticizer 20. Also, if all the earth and sand is replaced with plasticizer at once, the holes 18 will be crushed, so the holes 18 are provided alternately, for example, every other hole.
After injecting the plasticizer 20, the remaining holes 18 are drilled and the plasticizer 20 is injected.

FIG. 18 shows the extension 128 of the arched plate 122, and FIG. 19 shows the hard steel tip 126. The arched plate 122 has an outer plate 129 and a thick inner plate 1.
30 are connected by girders 31 to form an arch shape, and a hollow part 32 is provided between the outer plate 129 and the inner plate 130. Of course, in addition to this, a beam or the like may be provided to reinforce the shoulder portion of the plate 122 (the bent portion between the ceiling portion and both side wall portions). Furthermore, R portions 132 are provided at both lower ends of the arched plate 122, so that the plate 122 is inserted while the R portions 132 follow the lead guide column 12. Furthermore, holes 24 are provided on the surface of the outer panel 129, so that in areas where the earth pressure is low, the plasticizer 20 that has entered the hollow part 32 escapes to the outside and equalizes the earth pressure, and in areas where the earth pressure is high, the plasticizer 20 from the outside is removed. The agent 20 escapes from the hole 24 into the plate 122 to reduce friction due to earth pressure. When concrete 38 is poured into the hole 24, a portion of it overflows from the hole 24 to the surrounding area to connect the arched plate 122 to the ground, and also covers the surface of the outer panel 129 with the overflowing concrete to suppress corrosion. It also has a role. Holes 27 are provided in the girder 31 to allow the concrete 38 to join the girder 31 unhindered. At the tip 126 of the arch-shaped plate 122, an edge-shaped tip 3 is attached to the outer plate 129, the inner plate 130, and the girder 31.
4, 33, and 35 are provided so that they can be easily inserted into the plasticizer 20. Furthermore, the edges 34 and 33 face each other so that the plasticizer 20 enters the hollow portion 38.

[0025] Then, by hydraulic pressure or the like, the arch-shaped plate 1 is
22 into the plasticizer 20 (FIG. 20). This work is carried out by lowering a civil engineering machine equipped with a hydraulic system into the work area 2. This machine has an insertion hole 10 in the lead guide column 12.
It is preferable to integrate the excavating device for the plasticizer injection hole 18, the hydraulic insertion device for the lead guide column 12, the injection device for the plasticizer 20, etc., so that all the work can be performed by one civil engineering machine. The plasticizer 20 is attached to the plate 122
It is injected over a wider area. Therefore, the edges 33, 34, and 35 at the front ends of the plates are inserted into the plasticizer 20, and the outer plate 129 and the inner plate 130 are also inserted while sliding in the plasticizer 20. As a result, not only the resistance to which the tip of the plate 122 is subjected, but also the resistance due to earth pressure from the ground on the upper, lower, right and left sides is reduced. Since the edges 33 and 34 of the plasticizer 20 face each other (Fig. 19), the plasticizer 20 at the tip of the plate
0 is drawn into plate 122. Therefore, the plasticizer 20 enters the hollow portion 32, and even when the plate 122 is inserted, the thickness of the plasticizer 20 layer does not swell, so that the resistance at the time of insertion can be reduced. Plate 122 is plasticizer 2
0, oil seeps out from the plasticizer 20, the bond between the clay particles of the plasticizer 20 is broken, and the plasticizer 2
0 fluidity increases. As a result, the tip 1 of the plate 122
The resistance at 26 is reduced. This is a type of thixotropy, but in order to increase the fluidity of the plasticizer 20, for example, the plate 122 is inserted while applying vibrations such as ultrasonic waves, and the ultrasonic waves weaken the bonds between clay particles and prevent oil from seeping out. May be promoted. The oil that has seeped out forms an oil film on the surface of the plate 122, reducing frictional resistance due to earth pressure from the ground above, below, and to the left and right. Alternatively, a layer of plasticizer such as dioctyl phthalate may be coated on the outer plate 129, inner plate 130, etc. of the plate 122, and the resistance may be weakened as the plasticizer layer peels off. The plasticizer 20 is soft toward the back of the injection hole (FIG. 17) and hard toward the front. Since the resistance to the plate 122 becomes stronger as it goes deeper, the softening of the plasticizer 20 in the depths of the injection hole 18 facilitates the insertion of the plate 122. The additional portions 128 of the plate 122 are connected, for example, by fitting, bolts and nuts, or welding, to form an integral plate 122.

After inserting the plate 122, the plasticizer 20
Scrape it out and wash it with water to remove the remaining plasticizer 20 (Figure 21
). The concrete 38 is then placed on the arched plate 12.
It is injected into the hollow part 32 of No. 2 to form a steel concrete structure (FIG. 22). As a result, a ceiling and both wall surfaces of the underground space are obtained. Once the concrete 38 has hardened, the arched plate 122 can withstand earth pressure and there is no risk of it collapsing even if the interior is excavated. Therefore, the underground space is completed by excavating the earth and sand inside (FIG. 23) and providing a floor surface 140 of concrete or the like (FIG. 24). After the concrete 38 has hardened, if the surface of the inner plate 130 of the plate 122 is painted, it can be used as the ceiling or both side walls of the underground space 4.

[0027]

[Effects of the Invention] The construction method of the present invention provides the following effects. (1) Since the earth and sand are excavated only in the area that will actually be used as underground space, there is no need to remove or backfill the topsoil. (2) During the construction process, only the work zones will be visible on the ground, and the work zones will be set up at intervals and are large enough to be covered during the day, so construction can be carried out without interfering with ground activities. (3) Since plates, lead guide columns, etc. can be mass-produced as units, the construction period is short. (4) Only the work area is visible above the ground, and there is less earth and sand to be excavated and transported, so there is less noise. (5) If a work zone can be established, existing buried objects such as gas pipes and building foundations will not interfere with construction work. (6) It is safe because there is no risk of collapse. (7) It is economical because the construction period is short, there is less earth and sand to be excavated, and unitization is easy.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of the excavation process of a work section for horizontal hole digging in an embodiment.

FIG. 2 is a perspective view of the process of drilling a hole for inserting a lead guide column for a plate in the embodiment.

FIG. 3 is a perspective view of the plate lead guide pillar embedding process in the example.

FIG. 4 is a perspective view of the soil improvement process of the plate insertion part in the example.

FIG. 5 is a perspective view of the plate insertion process in the example.

FIG. 6 is a perspective view of a plate in an example.

FIG. 7 is a sectional view of the tip of the plate in the example.

FIG. 8 is a perspective view of the step of removing earth and sand from the hollow part of the plate in the example.

FIG. 9 is a perspective view of the process of pouring concrete into the hollow part of the plate in the example.

FIG. 10 is a perspective view of the construction process of the pillar and side wall in the example.

FIG. 11 is a perspective view of the completed underground space in the example.

FIG. 12 is a perspective view of the underground space extension process in the example.

FIG. 13 is a perspective view of the step of expanding the underground space in the lateral direction in the example.

FIG. 14 is a perspective view of the step of drilling a hole for inserting a lead guide column for a plate in the second embodiment.

FIG. 15 is a perspective view of the plate lead guide pillar embedding process in the second embodiment.

FIG. 16 is a perspective view of the process of connecting lead guide columns.

FIG. 17 is a perspective view of the soil improvement process of the plate insertion part in the second embodiment.

FIG. 18 is a perspective view of an arched plate in a second embodiment.

FIG. 19 is a perspective view of the main part of the arched plate tip in the second embodiment.

FIG. 20 is a perspective view of the arch-shaped plate insertion process in the second embodiment.

FIG. 21 is a perspective view of the step of removing earth and sand from the hollow portion of the arch-shaped plate in the second embodiment.

FIG. 22 is a perspective view of the process of pouring concrete into the hollow part of the arched plate in the second embodiment.

FIG. 23 is a perspective view of the process of removing earth and sand from the hollow part of the arch-shaped plate in the second embodiment.

FIG. 24 is a perspective view of a completed underground space in the second embodiment.

[Explanation of symbols]

2 Work section 4 Underground space formation part 6 Sheet pile 7 Support plate 10 Lead guide post insertion hole 12 Lead guide post 20 Plasticizer 22, 122 Plate 24 Hole 26, 126 Plate tip 27 Hole 28, 128 Plate addition part 29 ,129
Outer panel 30, 130 Inner panel 31 Girder 32 Hollow section 33, 34, 35 Edge section 38 Concrete 40 Support column 42 Underground space 44 Side wall 140 Floor surface

Claims (2)

[Claims] Claim 1: A step of excavating a work space, a step of inserting a lead guide from the excavated work space along the underground space to be formed, and a step of forming a plate while determining the direction by the inserted lead guide. A method for creating an underground space that includes a process of inserting the plate along the ceiling and a process of excavating the lower part of the plate to create an underground space. 2. The method for creating an underground space according to claim 1, wherein the plate is an arch-shaped plate, and the plate forms a ceiling portion and both side surfaces of the underground space.