CN210888966U - Assembled pre-supporting trolley device for tunnel in-situ extension construction - Google Patents

Abstract

The utility model discloses pin-connected panel of tunnel normal position extension construction protects platform truck device in advance relates to underground tunnel construction equipment technical field in the construction, especially relates to the supporting device who is suitable for in the tunnel normal position extension construction. The utility model discloses a: a trolley structure, a support structure and a guard plate structure; the supporting structures are arranged on the left side, the right side and the top of the trolley, and the guard plate structures are propped against the two side walls and the vault of the tunnel through the supporting structures. The technical scheme of the utility model old tunnel among the prior art has been solved because of lining cutting fracture, dislocation, corrosion phenomenon are serious, and the country rock that causes when the enlargement is unstable to and expand and dig existing lining cutting in front of the face and have not hard up risk during blasting construction, unable support personnel safety, and enlarge again after strengthening the existing tunnel and increase engineering cost scheduling problem.

Description

Assembled pre-supporting trolley device for tunnel in-situ extension construction

Technical Field

The utility model discloses pin-connected panel of tunnel normal position extension construction protects platform truck device in advance relates to underground tunnel construction equipment technical field in the construction, especially relates to the supporting device who is suitable for in the tunnel normal position extension construction.

Background

With the rapid development of economic construction in China, the traffic construction in China also rapidly increases, the requirement on the traffic volume of the tunnel is higher and higher, and in some areas with developed economy, because the construction time of the existing tunnel is long, the tunnel line cannot be changed, the existing tunnel needs to be subjected to in-situ expansion construction to meet the increasing traffic volume. The tunnel in-situ extension is different from a newly-built tunnel, the stratum is disturbed by construction in the early excavation process of the existing tunnel, the soil body is in a loose state, the integral stability of the stratum is reduced, and a relatively stable balance system is formed between the stratum and the original lining structure of the tunnel through coordinated deformation. The existing lining structure is removed and replaced and the stratum is disturbed again in the expanding excavation modification construction, so that the balance is broken, and a larger safety risk is met. At present, the related construction cases of tunnel in-situ extension in China are few, and a matched construction method and construction equipment formed aiming at tunnel in-situ extension are lacked.

Most of in-situ extension construction is to build old tunnels which are long in the past, the existing lining cracks, dislocation and corrosion phenomena are serious, the in-situ extension can cause great influence on the existing tunnels, the safety of construction operators can not be guaranteed during extension, and the method of re-extension after reinforcing the existing tunnels on the whole line is adopted, so that the engineering investment is greatly increased. Therefore, in actual construction, a construction device which can effectively support the existing lining in front of the enlarged excavation surface and has relatively low manufacturing cost is urgently needed.

Aiming at the problems in the prior art, a novel spliced pre-supporting trolley device for tunnel in-situ extension construction is researched and designed, so that the problem in the prior art is very necessary to be solved.

Disclosure of Invention

According to the old tunnel that prior art provided because of lining cutting fracture, dislocation, corrosion phenomenon are serious, the country rock that causes when the enlargement is unstable to and the existing lining cutting that expands in front of digging the face during blasting operation has not hard up risk, can't guarantee personnel's safety, and to the existing tunnel after strengthening the extension again increase engineering cost etc. technical problem, and provide a tunnel normal position extension construction usefulness pin-connected panel supporting platform truck device. The utility model discloses but mainly adopt on-the-spot concatenation, mobilizable, the preliminary supporting platform truck device that has safeguard measure to play and to digging the existing lining cutting formation of face front effectively strutting, cost is the effect of relative cheap again expanding.

The utility model discloses a technical means as follows:

an assembled pre-support trolley device for tunnel in-situ extension construction comprises: a trolley structure, a support structure and a guard plate structure; the supporting structures are arranged on the left side, the right side and the top of the trolley, and the guard plate structure is propped against the two side walls and the vault of the tunnel through the supporting structures;

further, the dolly structure includes: the gate-type steel frame, the connecting structure and the travelling wheels; four portal steel frames are connected into a trolley body through a connecting structure, and four walking wheels are respectively installed at four corners of the bottom of the trolley body to form a trolley structure, so that the trolley structure can be synchronously pushed along with the pushing of the expanding excavation surface;

further, the support structure comprises: vertical supports and diagonal braces; at least two vertical supports are uniformly distributed on the top of the portal steel frame and used for supporting the arch steel plates; a plurality of inclined struts are uniformly distributed on two sides of the portal steel frame;

further, backplate structure is for directly bearing construction load's atress component in strutting the platform truck, includes: a left steel plate, an arch steel plate and a right steel plate; the left steel plate and the right steel plate are supported and attached to two side walls of the tunnel through inclined struts arranged on two sides of the portal steel frame; the arch steel plate is supported against the top of the tunnel arch through a vertical support arranged at the top of the portal steel frame.

Furthermore, the portal steel frame is a portal structure made of 20# I-steel serving as steel beams and steel columns, and beam-column joints of the portal steel frame are of a variable-slope haunched structure and are formed by welding steel plates.

Further, the connection structure includes: the top longitudinal beam, the bottom longitudinal beam and the cross braces; the two bottom longitudinal beams are respectively connected with the bottoms of two sides of the uniformly distributed multi-pin portal steel frame; the two top longitudinal beams are respectively connected with the tops of two sides of the uniformly distributed multi-pin portal steel frame; a cross brace is arranged between the steel columns of two adjacent portal rigid frames, so that the rigidity of the whole structure is improved; a plurality of portal steel frames are connected into a trolley structure through top longitudinal beams, bottom longitudinal beams and cross braces.

Further, the vertical support comprises: the support rod, the vibrating wire type pressure box and the jack are arranged on the support rod; the jack is fixed on the top longitudinal beam, the upper part of the jack is provided with a vibrating wire type pressure box, and the upper part of the vibrating wire type pressure box is provided with a support rod; the support rod is propped against the lower part of the arch steel plate; the supporting rods are jacked by the jacks to apply prestress on the arch steel plates, so that the arch steel plates generate radial stress on the existing tunnel arch and the overlying rock mass thereof, and deformation caused by stress redistribution due to tunnel expanding excavation is reduced; the surrounding rock stress value of the existing tunnel arch is measured through the vibrating string type pressure cell, monitoring is carried out along with tunnel expanding excavation, the surrounding rock stress change value of the existing lining arch in front of the expanding excavation face is monitored, and monitoring and early warning of tunnel collapse disasters are achieved. At least two vertical supports are mounted on each top longitudinal beam.

Furthermore, the inclined struts are of telescopic structures and are welded and fixed on steel columns on two sides of the portal rigid frame, and at least two inclined struts are installed on the outer side of each steel column.

Furthermore, the left steel plate and the right steel plate are respectively connected with the arch steel plate through hinges, so that the supporting trolley has certain coordinated deformation capacity as a whole, and the assembling flexibility of each ring of steel plates is ensured. If tunnel collapse accident happens, the tunnel collapse emergency rescue device is convenient to disassemble and carry out emergency rescue. The assembled steel plate should be prefabricated in a factory, and the detailed design of specific material selection, geometric dimension, assembling mode and the like should be determined by detailed calculation according to boundary conditions (transverse span, longitudinal truss spacing, construction load and the like).

Further, a blasting buffer layer is arranged at the front part of the trolley structure; the blasting buffer layer is made of elastic rubber products with certain hardness and serves as the blasting buffer layer of the face blasting surface, and damage to the trolley structure caused by impact of flying stones generated by blasting, expanding and digging on the supporting trolley is avoided.

Furthermore, the rear portion of the trolley structure is provided with a crawling ladder and an operating platform, and therefore operation personnel can conveniently conduct rescue construction on tunnel collapse accidents.

Furthermore, a plurality of openable observation windows are arranged on the arch steel plate in a quincunx manner, when local instability occurs in the arch of the existing tunnel, the observation windows can be opened, and the rock mass in the instability area is reinforced by applying an advance anchor rod or grouting reinforcement mode.

The utility model discloses the application method of pin-connected panel preliminary bracing platform truck device of tunnel normal position extension construction as follows:

1. the system comprises factory prefabricated door-type steel frames and factory prefabricated assembled steel plates, wherein steel frames are connected through bottom longitudinal beams, top longitudinal beams and cross braces to form a supporting trolley main body structure, outer wing edge plates of the door-type steel frames are welded with diagonal braces to hang and assemble a left steel plate and an arch steel plate, and the tops of the door-type steel frames are fixedly assembled with vertical supporting assembled arch steel plates to form an annular closed space together;

2. waste tires for engineering machinery are hung at the front end of the supporting trolley and serve as a blasting buffer layer of an adjacent blasting surface, and the tail end of the supporting trolley is provided with a ladder stand and an operation platform, so that operation personnel can conveniently carry out emergency construction on tunnel collapse accidents;

3. before blasting and expanding excavation, the trolley is conveyed to a position 1m away from the blasting range in the existing tunnel to carry out supporting protection of the existing lining;

4. adjusting two vertical supports symmetrically arranged at the tops of the portal steel frames along the center line of the tunnel, jacking a support rod by a hydraulic jack to apply prestress on an arch steel plate, enabling the arch steel plate to generate radial stress on the existing tunnel arch and an overlying rock mass, reading an initial stress value through a vibrating string type pressure box, and monitoring the surrounding rock stress change value of the existing lining arch in front of an expanded excavation surface along with the expansion excavation of the tunnel in real time;

5. the existing tunnel is blasted and expanded, and the supporting trolley is pushed forward along with the expansion excavation face.

Compared with the prior art, the utility model has the advantages of it is following:

1. the utility model provides an assembled pre-support trolley device for tunnel in-situ extension construction, which can lead the trolley structure to be synchronously pushed along with the pushing of the extended excavation surface through the walking wheels arranged at the lower part of the bottom longitudinal beam;

2. the utility model provides a tunnel normal position extension construction's pin-connected panel pre-support platform truck device, left side steel sheet, right side steel sheet are connected with the hunch portion steel sheet through the hinge respectively, make the whole platform truck that supports have certain coordinated deformability, have ensured the flexibility that every ring of steel sheet was assembled, if take place the tunnel collapse accident, also be convenient for dismantle and speedily carry out rescue work;

3. the utility model provides a tunnel normal position extension construction's pin-connected panel pre-support platform truck device, plum blossom shape arranges a plurality of openable observation windows on the hunch portion steel sheet, when the existing tunnel hunch portion has occurred local unstability, can open the observation window, consolidates the rock mass in unstability district through applying the mode of leading stock or slip casting reinforcement;

4. the utility model provides a splicing type pre-support trolley device for tunnel in-situ extension construction, the front part of a trolley structure is provided with a blasting buffer layer as a blasting buffer layer of an explosive surface, and the trolley structure is damaged as the impact of flying stones generated by blasting and expanding excavation on a support trolley is avoided;

5. the utility model provides a splicing type pre-support trolley device for tunnel in-situ extension construction, the rear part of the trolley structure is provided with a crawling ladder and an operation platform, which is convenient for operators to carry out emergency construction on tunnel collapse accidents;

6. the utility model provides a tunnel normal position enlargement construction's pin-connected panel pre-support platform truck device, through jack-up bracing piece to arch steel sheet application prestressing force, make arch steel sheet to existing tunnel arch and its overlying rock mass produce radial stress, reduce because of tunnel expand dig cause stress redistribution lead to the deformation, the effectual probability that reduces the emergence landslide accident;

7. the utility model provides a tunnel normal position enlargement construction's pin-connected panel pre-support platform truck device measures the country rock stress value of existing tunnel hunch portion through the string vibrating type pressure cell, monitors along with the tunnel expands digs, expands the country rock stress variation value of digging the face front existing lining cutting hunch portion, realizes the monitoring early warning to the tunnel disaster that collapses;

8. the utility model provides a tunnel normal position enlargement construction's pin-connected panel pre-support platform truck device, through the cooperation of platform truck mechanism and backplate structure, the supporting effect is showing, has effectually ensured construction operation personnel's safety, has improved the construction progress, reduction of erection time;

9. the utility model provides a pin-connected panel of tunnel normal position enlargement construction protects platform truck device in advance, supporting existing tunnel normal position enlargement construction method has avoided adopting the method of rebuilding after consolidating to the existing tunnel of full line, has saved engineering cost greatly.

To sum up, use the technical scheme of the utility model old tunnel among the prior art has been solved because of lining cutting fracture, dislocation, corrosion phenomenon are serious, and the country rock that causes when the enlargement is unstable to and expand and dig the existing lining cutting in front of the face and have not hard up risk during blasting operation, unable support personnel safety, and build again and increase engineering cost scheduling problem after strengthening to current tunnel.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required to be used in the description of the embodiments or the prior art are briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without inventive labor.

FIG. 1 is a schematic structural view of the present invention;

FIG. 2 is a schematic view of the combination of the portal rigid frame and the guard plate structure of the present invention;

FIG. 3 is a schematic view of the structure of the portal rigid frame of the present invention;

FIG. 4 is a schematic view of a vertical support structure;

FIG. 5 is a schematic view of the structure of the arch steel plate;

fig. 6 is a schematic view of the structure of the utility model in use.

In the figure: 1. the walking wheel 2, the bottom longitudinal beam 3, the blasting buffer layer 4, the portal frame 5, the bridging 6, the top longitudinal beam 7, the vertical support 8, the operation platform 9, the cat ladder 10, the diagonal support 11, the right steel plate 12, the left steel plate 13, the arch steel plate 14, the jack 15, the vibrating string type pressure cell 16, the bracing piece 17 and the observation window.

Detailed Description

It should be noted that, in the present invention, the embodiments and features of the embodiments may be combined with each other without conflict. The present invention will be described in detail below with reference to the accompanying drawings in conjunction with embodiments.

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only some embodiments of the present invention, not all embodiments. The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way intended to limit the invention, its application, or uses. Based on the embodiments in the present invention, all other embodiments obtained by a person skilled in the art without creative work belong to the protection scope of the present invention.

It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments in accordance with the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.

Unless specifically stated otherwise, the relative arrangement of the components and steps, the numerical expressions, and numerical values set forth in these embodiments do not limit the scope of the present invention. Meanwhile, it should be understood that the sizes of the respective portions shown in the drawings are not drawn in an actual proportional relationship for the convenience of description. Techniques, methods, and apparatus known to those of ordinary skill in the relevant art may not be discussed in detail but are intended to be part of the specification where appropriate. Any specific values in all examples shown and discussed herein are to be construed as exemplary only and not as limiting. Thus, other examples of the exemplary embodiments may have different values. It should be noted that: like reference numbers and letters refer to like items in the following figures, and thus, once an item is defined in one figure, further discussion thereof is not required in subsequent figures.

In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the orientation words such as "front, back, up, down, left, right", "horizontal, vertical, horizontal" and "top, bottom", etc. are usually based on the orientation or positional relationship shown in the drawings, and are only for convenience of description and simplicity of description, and in the case of not making a contrary explanation, these orientation words do not indicate and imply that the device or element in question must have a specific orientation or be constructed and operated in a specific orientation, and therefore should not be construed as limiting the scope of the present invention: the terms "inner and outer" refer to the inner and outer relative to the profile of the respective component itself.

Spatially relative terms, such as "above … …," "above … …," "above … …," "above," and the like, may be used herein for ease of description to describe one device or feature's spatial relationship to another device or feature as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if a device in the figures is turned over, devices described as "above" or "on" other devices or configurations would then be oriented "below" or "under" the other devices or configurations. Thus, the exemplary term "above … …" can include both an orientation of "above … …" and "below … …". The device may be otherwise variously oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

It should be noted that the terms "first", "second", and the like are used to define the components, and are only used for convenience of distinguishing the corresponding components, and if not stated otherwise, the terms have no special meaning, and therefore, the scope of the present invention should not be construed as being limited.

As shown in the figure, the utility model provides a pin-connected panel preliminary bracing platform truck device of tunnel normal position extension construction includes: a trolley structure, a support structure and a guard plate structure; the supporting structures are arranged on the left side, the right side and the top of the trolley, and the guard plate structure is propped against the two side walls and the vault of the tunnel through the supporting structures;

the trolley structure includes: the gate-type steel frame 4, the connecting structure and the travelling wheels 1; 1 portal steel frame 4 is arranged at intervals of 2m, four portal steel frames are arranged and connected into a trolley body through a connecting structure, and four walking wheels 1 are respectively arranged at four corners of the bottom of the trolley body to form a trolley structure;

the support structure includes: a vertical support 7 and a diagonal support 10; at least two vertical supports 7 are uniformly arranged at the top of the portal steel frame 4; a plurality of inclined struts 10 are uniformly distributed on two sides of the portal steel frame 4;

the backplate structure includes: a left steel plate 12, an arch steel plate 13, and a right steel plate 11; the left steel plate 12 and the right steel plate 11 are supported and attached to two side walls of the tunnel through inclined struts 10 arranged on two sides of the portal steel frame 4; the arch steel plate 13 is propped against the top of the tunnel arch through a vertical support 7 arranged at the top of the portal steel frame 4.

The portal steel frame 4 is a portal structure made of 20# I-steel serving as steel beams and steel columns, and beam-column joints of the portal steel frame are of a variable-slope haunch type structure and are formed by welding steel plates. The passing clearance below the portal frame 4 is set to be 3m (width) 4.5m (height).

The connection structure includes: a top longitudinal beam 6, a bottom longitudinal beam 2 and a cross brace 5; two bottom longitudinal beams 2 are made of I-steel No. 20 and are respectively connected with the bottoms of two sides of a plurality of uniformly distributed portal steel frames 4; two top longitudinal beams 6 are made of 16# I-steel and are respectively connected with the tops of two sides of a plurality of uniformly distributed portal steel frames 4; a cross brace 5 is arranged between the steel columns of two adjacent portal steel frames 4; the plurality of portal steel frames 4 are connected into a trolley structure by the top longitudinal beam 6, the bottom longitudinal beam 2 and the cross brace 5.

The vertical support 7 includes: a support rod 16, a vibrating wire type pressure box 15 and a jack 14; a jack 14 is fixed on the top longitudinal beam 6, a vibrating wire type pressure box 15 is arranged at the upper part of the jack 14, and a support rod 16 is arranged at the upper part of the vibrating wire type pressure box 15; the support rod 16 is pressed against the lower part of the arch steel plate 13; at least two vertical supports 7 are mounted on each roof rail 6.

The inclined struts 10 are of telescopic structures and are welded and fixed on steel columns on two sides of the portal steel frame 4, and at least two inclined struts 10 are installed on the outer side of each steel column.

The left steel plate 12 and the right steel plate 11 are respectively connected with the arch steel plate 13 through hinges, so that the supporting trolley has certain coordinated deformation capacity as a whole, and the assembling flexibility of each ring of steel plates is ensured. The outer contour dimension of the trolley on which the left steel plate 12, the right steel plate 11 and the arch steel plate 13 are assembled is 4m (width) × 6m (height).

The front part of the trolley structure is provided with a blasting buffer layer 3; the blasting buffer layer 3 is made of rubber products with certain hardness and elasticity, namely waste tires for engineering machinery, and is used as a blasting buffer layer of an adjacent blasting surface, so that the trolley structure is prevented from being damaged due to impact of flying stones generated by blasting, expanding and digging on a supporting trolley.

The rear portion of platform truck structure is provided with cat ladder 9 and operation platform 8, makes things convenient for the operation personnel to carry out the construction of speedily carrying out rescue work to the tunnel collapse accident.

The arch steel plate 13 is arranged with a plurality of openable observation windows 17 with the size of 0.3m by 0.3m in a quincunx manner.

Finally, it should be noted that: the above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; although the present invention has been described in detail with reference to the foregoing embodiments, it should be understood by those skilled in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; such modifications and substitutions do not depart from the spirit and scope of the present invention.

Claims (9)

1. The utility model provides a tunnel normal position extension construction's pin-connected panel pre-support platform truck device which characterized in that, pin-connected panel pre-support platform truck device include: a trolley structure, a support structure and a guard plate structure; the supporting structures are arranged on the left side, the right side and the top of the trolley, and the guard plate structure is propped against the two side walls and the vault of the tunnel through the supporting structures;

the trolley structure comprises: the device comprises a door type steel frame (4), a connecting structure and a traveling wheel (1); a plurality of portal steel frames (4) are connected into a trolley body through a connecting structure, and four corners of the bottom of the trolley body are respectively provided with a traveling wheel (1) to form a trolley structure;

the support structure comprises: a vertical support (7) and an inclined support (10); at least two vertical supports (7) are uniformly distributed on the top of the portal steel frame (4); a plurality of inclined struts (10) are uniformly distributed on two sides of the portal steel frame (4);

the guard plate structure comprises: a left steel plate (12), an arch steel plate (13) and a right steel plate (11); the left steel plate (12) and the right steel plate (11) are supported and attached to two side walls of the tunnel through inclined struts (10) arranged on two sides of the portal steel frame (4); the arch steel plate (13) is propped against the top of the tunnel arch through a vertical support (7) arranged at the top of the portal steel frame (4).

2. The assembled pre-supporting trolley device for tunnel in-situ extension construction according to claim 1, wherein the portal steel frame (4) is a portal structure made of 20# I-steel serving as a steel beam and a steel column, and beam-column joints of the portal steel frame are of a variable-slope haunched structure and are formed by welding steel plates.

3. The assembled pre-supporting trolley device for tunnel in-situ extension construction according to claim 1, wherein the connecting structure comprises: the top longitudinal beam (6), the bottom longitudinal beam (2) and the cross braces (5); two bottom longitudinal beams (2) are respectively connected with the bottoms at two sides of a plurality of uniformly distributed portal steel frames (4); two top longitudinal beams (6) are respectively connected with the tops of two sides of a plurality of uniformly distributed portal steel frames (4); a scissor brace (5) is arranged between the steel columns of two adjacent portal steel frames (4); a plurality of portal steel frames (4) are connected into a trolley structure through top longitudinal beams (6), bottom longitudinal beams (2) and cross braces (5).

4. The assembled pre-supporting trolley device for tunnel in-situ extension construction according to claim 1, wherein the vertical support (7) comprises: a support rod (16), a vibrating wire type pressure box (15) and a jack (14); the jack (14) is fixed on the top longitudinal beam (6), the upper part of the jack (14) is provided with a vibrating wire type pressure box (15), and the upper part of the vibrating wire type pressure box (15) is provided with a support rod (16); the supporting rod (16) is propped against the lower part of the arch steel plate (13); at least two vertical supports (7) are arranged on each top longitudinal beam (6).

5. The assembled pre-support trolley device for tunnel in-situ extension construction according to claim 1, wherein the inclined struts (10) are of telescopic structures and are welded and fixed on steel columns on two sides of the portal steel frame (4), and at least two inclined struts (10) are installed on the outer side of each steel column.

6. The assembled pre-supporting trolley device for tunnel in-situ extension construction according to claim 1, wherein the left steel plate (12) and the right steel plate (11) are respectively connected with the arch steel plate (13) through hinges, so that the supporting trolley has certain coordinated deformation capacity as a whole, and the assembling flexibility of each ring of steel plates is ensured.

7. The assembled pre-supporting trolley device for tunnel in-situ extension construction according to claim 1, wherein a blasting buffer layer (3) is arranged at the front part of the trolley structure; the burst buffer layer (3) is made of an elastic rubber product with certain hardness.

8. The assembled pre-supporting trolley device for tunnel in-situ extension construction according to claim 7, wherein a ladder (9) and an operating platform (8) are arranged at the rear part of the trolley structure.

9. The assembled pre-supporting trolley device for tunnel in-situ extension construction according to claim 4, wherein a plurality of openable observation windows (17) are arranged on the arch steel plate (13) in a quincunx manner.

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