Regulating reservoir and construction method of regulating reservoir

Abstract

To provide a regulating reservoir that can be properly constructed without relying on a large-sized crane.SOLUTION: A regulating reservoir 1, which temporarily stores rainwater and regulates the outflow volume of the rainwater, has an U-shaped cross-section and comprises a straight water tank section 2 extending in the left-right direction (first direction), and a pair of side wall parts 3 blocking both ends of the straight water tank section 2 in the left-right direction. The straight water tank section 2 includes a pair of retaining wall parts 10 arranged at intervals in the front-back direction (second direction), and a bottom slab part 20 that connects the lower portions of the pair of retaining wall parts 10, wherein the retaining wall part 10 has a structure in which multiple retaining wall blocks 11 that form an L-shape by protruding a bottom surface part 13 from the lower portion of an upright surface part 12 are arranged and connected in the left-right direction, the side wall part 3 has a structure in which multiple plate-like side wall blocks 30 are arranged and connected in the vertical direction, and the side wall part 3 is installed between the pair of retaining wall parts 10 and is connected to a pair of upright surface parts 12 and a pair of bottom surface parts 13 located at the left-right direction ends of the pair of retaining wall parts 10.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a regulating pond that temporarily stores rainwater and adjusts the amount of rainwater that runs off, and a method for constructing the regulating pond. [Background technology]

[0002] A balancing pond for temporarily storing rainwater and releasing it at the appropriate time is known (Patent Document 1). The balancing pond is constructed so that the planned area of ​​the balancing pond is surrounded by corner peripheral edge blocks and straight peripheral edge blocks. The straight peripheral edge blocks are L-shaped with a vertical wall erected on one side of the base plate. The corner peripheral edge blocks are shaped with vertical walls erected on two consecutive sides of the base plate. If the balancing pond is constructed using only straight peripheral edge blocks, there is a problem that the base plates of the two straight peripheral edge blocks arranged at the corner parts of the balancing pond will interfere with each other. Therefore, the straight peripheral edge blocks are arranged in a row on the straight part of the balancing pond, and corner peripheral edge blocks are arranged at the corner parts of the balancing pond, thereby preventing the above-mentioned interference between the base plates. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 7-197510 Summary of the Invention [Problem to be solved by the invention]

[0004] In the above-mentioned technology, the corner outer perimeter blocks are larger and heavier than the straight outer perimeter blocks. Therefore, in places where it is not possible to introduce a large hoist, such as places adjacent to rivers or on steep slopes, it is sometimes difficult to transport the corner outer perimeter blocks to the intended installation location, making it difficult to build the balancing pond.

[0005] In consideration of the above circumstances, the present invention provides a balancing reservoir and a method for constructing a balancing reservoir that can be properly constructed without relying on large cranes. [Means for solving the problem]

[0006] The present invention is a balancing pond that temporarily stores rainwater and adjusts the amount of rainwater runoff, and comprises a linear water tank section having a U-shaped cross-section and extending in a first direction, and a pair of side wall sections that block both ends of the linear water tank section in the first direction, and the linear water tank section has a pair of retaining wall sections that are spaced apart in a second direction perpendicular to the first direction, and a bottom slab section that connects the lower parts of the pair of retaining wall sections, and the retaining wall sections have a structure in which a plurality of L-shaped retaining wall blocks are arranged and connected in the first direction, with their bottom surfaces protruding from the lower parts of their upright surfaces, and the side wall sections have a structure in which a plurality of flat side wall blocks are arranged and connected in the vertical direction, and are installed between the pair of retaining wall sections, and are connected to the pair of upright surface sections and the pair of bottom surfaces located at the ends of the pair of retaining wall sections in the first direction.

[0007] In this case, the side wall block is divided into a pair of first side plates arranged on both sides in the second direction, and at least one second side plate arranged between the pair of first side plates, and the side wall portion has a pair of first wall portions connected to the pair of retaining wall portions in a structure in which the pair of first side plates are arranged and connected in the vertical direction, and at least one second wall portion connected to the pair of first wall portions in a structure in which the pair of second side plates are arranged and connected in the vertical direction.

[0008] The present invention is a method for constructing a balancing pond that temporarily stores rainwater and adjusts the amount of rainwater runoff, and includes a retaining wall construction process for providing a pair of retaining wall sections, each of which is made by arranging and connecting a plurality of L-shaped retaining wall blocks, each of which has a bottom portion protruding from the bottom of an upright surface portion, in a first direction, with a gap between them in a second direction perpendicular to the first direction; a sidewall construction process for erecting a sidewall section, each of which is made by arranging and connecting a plurality of flat sidewall blocks in the vertical direction, between the pair of retaining wall sections; a base slab construction process for pouring concrete between the pair of retaining wall sections to construct a base slab section connecting the lower parts of the pair of retaining wall sections; and a sidewall connecting process for connecting the sidewall sections to a pair of upright surface sections and a pair of bottom sections located at the ends of the pair of retaining wall sections in the first direction.

[0009] In this case, the side wall block is divided into a pair of first side plates arranged on both sides in the second direction and at least one second side plate arranged between the pair of first side plates, and in the side wall construction process, a pair of first wall portions are constructed by arranging and connecting a plurality of the pair of first side plates in the vertical direction, at least one second wall portion is constructed by arranging and connecting a plurality of the second side plates in the vertical direction, and the side wall portion is constructed by connecting the second wall portion to the pair of first wall portions, and in the side wall connecting process, the pair of first wall portions of the side wall portion are connected to a pair of the retaining wall portions. Effect of the Invention

[0010] According to the present invention, a balancing pond can be properly constructed without relying on a large hoist. [Brief description of the drawings]

[0011] [Figure 1] FIG. 2 is a perspective view showing a balancing pond according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a plan view showing a regulating pond according to one embodiment of the present invention. [Diagram 3] 1 is a flowchart showing a method for constructing a reservoir according to an embodiment of the present invention. [Figure 4] 1 is a cross-sectional view (side view) illustrating a retaining wall construction process of a reservoir construction method according to one embodiment of the present invention. [Diagram 5] 1 is a perspective view illustrating a side wall construction process of a reservoir construction method according to one embodiment of the present invention. FIG. [Figure 6] A front view showing the rear retaining wall portion of a regulating pond in one embodiment of the present invention. [Figure 7] 1 is a cross-sectional view (side view) illustrating a side wall construction process of a reservoir construction method according to one embodiment of the present invention. [Figure 8] 1 is a cross-sectional view (side view) illustrating a side wall construction process and a bottom slab construction process of a reservoir construction method according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. Note that Fr, Rr, L, R, U, and Lo shown in the drawings indicate front, rear, left, right, top, and bottom. The left-right direction (first direction), the front-back direction (second direction), and the up-down direction are perpendicular to each other. Terms indicating directions and positions are used in this specification, but these terms are used for convenience of explanation and do not limit the technical scope of the present invention.

[0013] A balancing reservoir 1 and a method for constructing a balancing reservoir according to one embodiment will be described with reference to Figures 1 and 2. Figure 1 is a perspective view showing the balancing reservoir 1. Figure 2 is a plan view showing the balancing reservoir 1.

[0014] [Adjusting pond] The regulating pond 1 is a facility that temporarily stores rainwater and adjusts the amount of rainwater that flows out into a river to prevent flood damage. As shown in FIG. 1, the regulating pond 1 includes a linear water tank section 2 and a pair of side walls 3. The linear water tank section 2 has a substantially U-shaped cross section and extends in the left-right direction (first direction). The pair of side walls 3 close both left-right ends of the linear water tank section 2. The area surrounded by the linear water tank section 2 and the pair of side walls 3 is a storage space S for storing rainwater. An inflow opening 2A is provided on the upper left rear side of the linear water tank section 2 to allow rainwater to flow into the storage space S, and an outflow basin section 2B is provided on the right front side of the linear water tank section 2 to allow rainwater to flow out of the storage space S (see FIG. 2). The pair of side walls 3 are symmetrical with respect to the storage space S, so this specification will mainly describe one side wall 3.

[0015] <Straight tank section> As shown in Figures 1 and 2, the linear water tank section 2 has a pair of retaining wall sections 10 and a bottom slab section 20. The pair of retaining wall sections 10 are provided with a gap in the front-to-rear direction (second direction). In other words, the pair of retaining wall sections 10 face each other in the front-to-rear direction with the storage space S in between. Note that since the pair of retaining wall sections 10 have a structure that is symmetrical in the front-to-rear direction with the storage space S in between, this specification will mainly describe one of the retaining wall sections 10.

[0016] (Retaining wall) The retaining wall portion 10 is structured by arranging and connecting a plurality of retaining wall blocks 11 in the left-right direction (first direction). The retaining wall blocks 11 are reinforced concrete members (precast concrete) manufactured in a factory different from the construction site of the reservoir 1. The retaining wall blocks 11 are formed in a block shape that is short in the left-right direction and elongated in the up-down direction (see FIG. 1). The retaining wall blocks 11 are formed in a substantially L-shape with the bottom surface portion 13 protruding from the lower part of the standing surface portion 12 (see FIG. 1). Although it is not clear from the drawing, the retaining wall block 11 is separated into an L-shaped portion formed by integrally molding the substantially lower half of the standing surface portion 12 and the bottom surface portion 13, and a flat plate portion forming the substantially upper half of the standing surface portion 12. The retaining wall block 11 is formed by connecting these two portions. The above-mentioned retaining wall portion 10 is also formed in a substantially L-shape when viewed from the side (first direction). The pair of retaining wall portions 10 are arranged with their bottom surfaces 13 facing each other, so that the linear water tank portion 2 is formed into a substantially U-shaped groove shape.

[0017] (bottom plate) The bottom slab portion 20 connects the lower parts of the pair of retaining wall portions 10 and forms the bottom surface of the storage space S. As will be described in detail later, the bottom slab portion 20 is constructed at the construction site of the balancing reservoir 1 by arranging reinforcement and pouring concrete.

[0018] <Side wall> As shown in FIG. 1, the side wall portion 3 has a structure in which a plurality of side wall blocks 30 each having a substantially flat plate shape are arranged and connected in the vertical direction. The side wall block 30 is formed in a flat plate shape that is elongated in the front-rear direction and has a substantially rectangular shape. The side wall portion 3 is formed in a substantially rectangular flat plate shape by integrating a plurality of side wall blocks 30. The side wall portion 3 is installed between a pair of retaining wall portions 10 and is connected to a pair of upright surface portions 12 and a pair of bottom surface portions 13 located at the left-right ends of the pair of retaining wall portions 10 (see FIG. 2). The regulating pond 1 has a simple structure in which the opening surface (left-right end surface) of the linear water tank portion 2 having a substantially U-shaped groove shape is blocked by the side wall portion 3 having a substantially flat plate shape.

[0019] Although details will be described later, each side wall block 30 is divided into a pair of first side plates 31 arranged on both sides in the front-rear direction, and one second side plate 32 arranged between the pair of first side plates 31. The first side plate 31 and the second side plate 32 (side wall block 30) are reinforced concrete members (precast concrete) manufactured in a factory different from the construction site of the balancing pond 1. A plurality of first side plates 31 are arranged in the vertical direction and connected to each other to form a first wall portion 33. A plurality of second side plates 32 are arranged in the vertical direction and connected to each other to form a second wall portion 34. The side wall portion 3 has a structure in which a pair of first wall portions 33 and one second wall portion 34 are arranged in the front-rear direction (second direction) and connected to each other.

[0020] [Regulation pond construction method] A method for constructing a balancing pond 1 will be described with reference to Figs. 1 to 8. Fig. 3 is a flow chart showing the balancing pond construction method. Fig. 4 is a cross-sectional view (side view) explaining the retaining wall construction step S1 of the balancing pond construction method. Fig. 5 is a perspective view explaining the side wall construction step S2 of the balancing pond construction method. Fig. 6 is a front view showing the retaining wall portion 10 at the rear of the balancing pond 1. Fig. 7 is a cross-sectional view (side view) explaining the side wall construction step S2 of the balancing pond construction method. Fig. 8 is a cross-sectional view (side view) explaining the side wall construction step S2 and the base slab construction step S3 of the balancing pond construction method.

[0021] As shown in Fig. 3, the adjusting pond construction method includes a retaining wall construction step S1, a side wall construction step S2, a base slab construction step S3, and a side wall connection step S4. The adjusting pond 1 is constructed in a place where a large crane (hoist) cannot be introduced, such as a place adjacent to a river or on a steep slope. Therefore, a small crane (not shown) is used in the adjusting pond construction method.

[0022] <Retaining wall construction process> As shown in FIG. 4, in the retaining wall construction process S1, a pair of retaining wall sections 10 are provided with an interval therebetween in the front-rear direction (second direction). Specifically, a plurality of retaining wall blocks 11 are carried into the planned installation positions of the pair of retaining wall sections 10 using a crane. The plurality of retaining wall blocks 11 are arranged side by side in the left-right direction and connected via a plurality of retaining wall tendons 14 (see FIG. 2). The retaining wall tendons 14 are, for example, PC (Prestressed Concrete) steel materials. A portion (a plurality) of the plurality of retaining wall blocks 11 are provided with a plurality of retaining wall fasteners 15 that hold the tension (tensile stress) applied to the retaining wall tendons 14 (see FIG. 2). The plurality of retaining wall blocks 11 are integrated to construct the retaining wall section 10. The pair of retaining wall sections 10 are provided in an orientation in which the bottom surface portions 13 of the respective retaining wall blocks 11 face each other across the planned installation position of the base slab portion 20 (see FIGS. 1 and 2). Incidentally, a small gap (for example, about 5 mm) is formed between adjacent retaining wall blocks 11, and a water-stopping material (not shown) is attached to the gap.

[0023] Considering that the retaining wall block 11 will be transported by a small crane, the rebar for connection to the bottom slab portion 20 does not protrude from the tip surface of the bottom surface portion 13 of the retaining wall block 11. A threaded portion (not shown) for screwing in the bottom slab connection rebar 16 is embedded in the bottom surface portion 13 of the retaining wall block 11. As shown in Fig. 4, the bottom slab connection rebar 16 screwed into the threaded portion of the bottom surface portion 13 is connected to the bottom slab rebar 21 arranged to construct the bottom slab portion 20 via, for example, a torque-fixed joint 22.

[0024] <Side wall construction process> Next, in the side wall construction process S2, each side wall portion 3 is erected between the pair of retaining wall portions 10 at the left and right ends of the pair of retaining wall portions 10. As shown in FIG. 5, each side wall portion 3 includes a pair of first wall portions 33 and a second wall portion 34. The pair of first wall portions 33 is constructed by arranging and connecting a plurality of pairs of first side plates 31 in the vertical direction, and the second wall portion 34 is constructed by arranging and connecting a plurality of second side plates 32 in the vertical direction. The first wall portion 33 and the second wall portion 34 are assembled in a work site different from the construction site of the reservoir 1 before being transported by a crane. The plurality of first side plates 31 constituting the first wall portion 33 are arranged (stacked) in the vertical direction, and the reinforcing bars 35 embedded in the plurality of first side plates 31 in a substantially vertical position are connected, for example, via mortar-filled joints 36 (see FIG. 7). Similarly, the multiple second side panels 32 constituting the second wall portion 34 are arranged side by side (stacked) in the vertical direction, and their reinforcing bars 35 are connected, for example, via mortar-filled joints 36 (see Figure 7).

[0025] Next, the pair of first wall portions 33 and second wall portions 34 are carried by a crane to the intended installation positions of the side wall portions 3. As shown in Fig. 5, the pair of first wall portions 33 are disposed at positions facing the left and right end faces of the pair of retaining wall portions 10, and the second wall portion 34 is disposed with a gap between the pair of first wall portions 33.

[0026] Next, the second wall portion 34 is connected to the pair of first wall portions 33. As shown in FIG. 7, a plurality of side wall reinforcing bars 40 protrude from the inner end surface of the pair of first wall portions 33 facing the second wall portion 34 toward the second wall portion 34. A plurality of side wall reinforcing bars 40 protrude from the front and rear end surfaces of the second wall portion 34 toward the pair of first wall portions 33. The side wall reinforcing bars 40 of the adjacent first wall portion 33 and second wall portion 34 are connected to each other via, for example, a joint 41 of a mortar filling method. After that, a plurality of reinforcing bars 42 are arranged vertically between the pair of first wall portions 33 and second wall portions 34, and concrete is poured to construct a pair of connecting wall portions 43 (see FIG. 8). The pair of first side plates 31 and second side plates 32 may be considered to be connected by the connecting wall portion 43 to constitute one side wall block 30. The side wall portion 3 may be constructed by arranging and connecting a plurality of side wall blocks 30 in the vertical direction.

[0027] As described above, in the side wall construction process S2, a pair of first wall portions 33 and second wall portions 34 are arranged in the front-to-back direction (second direction) across a pair of connecting wall portions 43 and connected (integrated) together to construct a single flat side wall portion 3 (see Figures 1 and 8).

[0028] <Bottom plate construction process> Next, as shown in Figures 1 and 8, in the base slab construction process S3, concrete is poured between the pair of retaining wall sections 10 to construct a base slab section 20 that connects the lower parts of the pair of retaining wall sections 10. To be precise, the concrete is poured in an area surrounded by the pair of retaining wall sections 10 and the pair of side wall sections 3. The base slab section 20 is constructed so as to be at approximately the same height as the bottom surface sections 13 of each retaining wall block 11.

[0029] <Side wall connection process> Next, in the side wall connecting process S4, each side wall portion 3 (a pair of first wall portions 33 (first side plates 31)) is connected to a pair of upright surface portions 12 and a pair of bottom surface portions 13 located at the ends of the pair of retaining wall portions 10 in the left-right direction (first direction). Specifically, as shown in FIG. 6, the pair of first wall portions 33 are connected to the pair of upright surface portions 12 and the pair of bottom surface portions 13 located at the ends of the pair of retaining wall portions 10 in the left-right direction via a plurality of side wall tension members 37. The side wall tension members 37 are, for example, PC steel members, and the ends of the side wall tension members 37 are provided with side wall fixing devices 38 that hold tension (tensile stress). The pair of side wall portions 3 are fixed at three sides, namely, both sides (the pair of upright surface portions 12) and the lower portion (the pair of bottom surface portions 13) in the front-rear direction, at both ends of the pair of retaining wall portions 10 in the left-right direction. In addition, similar to the retaining wall block 11 (bottom portion 13), a bottom slab connection rebar 16 may be connected to the lowest side wall block 30 (first side plate 31, second side plate 32) and connected to the bottom slab rebar 21 of the bottom slab portion 20 via a torque-fixing type joint 22 (not shown).

[0030] With the above, all steps of the balancing pond construction method are completed, and the balancing pond 1 is completed (see FIG. 1). Note that the balancing pond construction method described above is merely one example, and the order (procedure) may be changed in various ways without departing from the spirit of the technical idea, for example, by carrying out the base slab construction step S3 prior to the side wall construction step S2, or by carrying out the side wall connection step S4 prior to the base slab construction step S3.

[0031] In the adjusting pond 1 (method of constructing an adjusting pond) according to the present embodiment described above, the side wall portion 3 is constructed by stacking the side wall blocks 30 divided into a plurality of pieces, and is configured to block the end faces in the left-right direction (first direction) of the linear water tank portion 2 formed into an overall U-shape. According to this configuration, the side wall block 30 can be constructed small and lightweight, so that, for example, even a small crane (hoist) can easily transport the side wall block 30 to the intended installation position. This eliminates the need to rely on a large crane (hoist), so that the adjusting pond 1 can be constructed appropriately even in a place where a large hoist cannot be introduced, such as a place adjacent to a river or a steep slope. In addition, since the plurality of side wall blocks 30 are integrated to construct one flat side wall portion 3, interference between the side wall portion 3 and the bottom surface portion 13 of the retaining wall block 11 can be prevented at the corner portion of the adjusting pond 1.

[0032] Furthermore, according to the adjusting pond 1 (adjustment pond construction method) of this embodiment, each side wall block 30 is divided into a pair of a first side plate 31 and a second side plate 32, which makes it easier to transport the side wall block 30 (each side plate 31, 32) to the intended installation position.

[0033] In the adjusting pond 1 according to this embodiment, the side wall block 30 includes one second side plate 32, but the present invention is not limited to this. For example, the side wall block 30 may include two or more second side plates 32 (not shown). In other words, the side wall portion 3 may include two or more second wall portions 34 (not shown). In addition, the side wall block 30 is divided into the first side plate 31 and the second side plate 32, but the present invention is not limited to this. For example, if it can be transported by a small crane, the side wall block 30 may be formed as a single flat plate without being divided in the front-rear direction (not shown).

[0034] Furthermore, in the reservoir construction method according to this embodiment, the retaining wall section 10 is assembled at the planned installation location (the construction site of the reservoir 1), but this is not limited thereto, and the retaining wall section 10 may be assembled in advance at a work site different from the construction site of the reservoir 1 and then transported by a crane. Furthermore, the first wall section 33 and the second wall section 34 are assembled in advance and then transported by a crane, but this is not limited thereto, and the retaining wall section 10 may be assembled at the construction site of the reservoir 1.

[0035] The above embodiment is merely an example of the balancing reservoir and the balancing reservoir construction method of the present invention, and the technical scope of the present invention is not limited to the above embodiment. The present invention may be modified, substituted, or altered in various ways without departing from the spirit of the technical idea, and the claims include all embodiments that may be included within the scope of the technical idea. [Explanation of symbols]

[0036] 1. Regulating pond 2 Straight tank section 3 Side wall 10 Retaining wall 11 Retaining wall blocks 12 Standing surface part 13 Bottom part 20 Bottom plate part 30 Sidewall Block 31 1st side plate 32 Second side plate 33 1st wall section 34 Second wall section S1 Retaining wall construction process S2 side wall construction process S3 bottom plate construction process S4 Side wall connection process

Claims

1. A regulating pond that temporarily stores rainwater and adjusts the amount of rainwater that runs off. A linear water tank portion having a U-shaped cross section and extending in a first direction; a pair of side walls closing both ends of the linear water tank portion in the first direction; The linear water tank portion is A pair of retaining wall portions provided at an interval in a second direction perpendicular to the first direction; A bottom plate portion connecting the lower portions of the pair of retaining wall portions, The retaining wall portion has a structure in which a plurality of L-shaped retaining wall blocks are arranged and connected in the first direction, the L-shaped retaining wall blocks having bottom surfaces protruding from the lower parts of the upright surfaces, The side wall portion is a structure in which multiple flat side wall blocks are arranged and connected in the vertical direction, and is installed between a pair of the retaining wall portions and connected to a pair of the upright surface portions and a pair of the bottom surface portions located at the ends of the pair of the retaining wall portions in the first direction.

2. the side wall block is divided into a pair of first side plates arranged on both sides in the second direction and at least one second side plate arranged between the pair of first side plates, The side wall portion is A pair of first wall portions connected to the pair of retaining wall portions in a structure in which the pair of first side plates are arranged and connected in a vertical direction; The reservoir as described in claim 1, characterized in that it has at least one second wall portion connected to a pair of the first wall portions in a structure in which a plurality of the second side panels are arranged and connected in the vertical direction.

3. A method for constructing a reservoir for temporarily storing rainwater and regulating the amount of rainwater runoff, comprising the steps of: a retaining wall construction process for providing a pair of retaining wall sections, each of which is formed by arranging and connecting a plurality of L-shaped retaining wall blocks in a first direction, the retaining wall blocks having bottom surfaces protruding from the lower portions of the upright surfaces, at intervals in a second direction perpendicular to the first direction; a side wall construction process for erecting a side wall portion between the pair of retaining wall portions, the side wall portion being formed by arranging and connecting a plurality of flat side wall blocks in the vertical direction; A bottom slab construction process of pouring concrete between the pair of retaining wall portions to construct a bottom slab portion that connects the lower parts of the pair of retaining wall portions; A method of constructing a reservoir, comprising a side wall connecting process of connecting the side wall portions to a pair of the upright surface portions and a pair of the bottom surface portions located at the ends of the pair of the retaining wall portions in the first direction.

4. the side wall block is divided into a pair of first side plates arranged on both sides in the second direction and at least one second side plate arranged between the pair of first side plates, In the side wall construction step, a pair of first wall portions are constructed by arranging and connecting a pair of the first side plates in a vertical direction, at least one second wall portion is constructed by arranging and connecting a plurality of the second side plates in a vertical direction, and the side wall portion is constructed by connecting the second wall portion to the pair of the first wall portions, The method for constructing a regulating pond according to claim 3, characterized in that in the side wall connecting step, a pair of the first wall portions of the side wall portion are connected to a pair of the retaining wall portions.

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