CN116623702A - Construction method of underwater bearing platform - Google Patents

Abstract

The invention relates to a construction method for an underwater cap, which is suitable for an underwater environment with a flow-plastic silt layer, comprising the following steps: setting an anti-silting protection groove on the periphery of a preset foundation pit position of the cap, and the anti-silting The bottom of the protection groove is higher than the elevation of the bottom of the platform; excavate the foundation pit of the platform, and regularly clean the silt in the anti-silting protection channel; set steel casings and steel hanging boxes in sequence in the foundation pit of the platform, so that The steel hanging box has a reinforced concrete bottom plate; concrete is poured in the steel hanging box to obtain a cap. By setting the anti-silting protection groove around the foundation pit, it is possible to prevent the river bed from silting after the excavation of the cap foundation pit is in place. By regularly cleaning the silt in the anti-silting protection groove, the depth of the foundation pit can meet the requirements of the steel hanging box. construction space.

Description

Construction method of underwater bearing platform

This application is a divisional application of the patent application "Construction method and cofferdam structure of underwater caps". The application date of the original application is 2020-07-28, the application number is 2020107404919, and the publication number is CN 111894025A.

technical field

The invention relates to the technical field of bridge construction, in particular to a construction method of an underwater cap.

Background technique

Deep-water caps are the infrastructure and bottom support of construction projects such as water highways and bridges, and play a very important role in the stability of buildings. During the construction of deep-water caps, it is usually necessary to excavate the foundation pit of the cap into the water area with a flow-plastic silt layer. to the elevation requirement. Therefore, it is usually necessary to excavate a deeper foundation pit, resulting in a deep foundation pit that needs to be dredged to build a cap, which increases the construction workload and reduces the construction efficiency. In addition, in the process of building caps, first use concrete to build the bottom cover plate, that is, make a concrete seal layer for the bottom of the steel hanging box in water, which is used to seal the bottom of the steel hanging box. However, due to the deep position of the deep water cap, the flow plasticity of the silt in the deep water is strong, in order to meet the requirements of the construction of the cap, a thicker back-sealing concrete is required, resulting in a deeper dredging thickness for the foundation pit for making the cap , the thickness of the steel hanging box lifting system is relatively high.

Contents of the invention

In order to overcome the above technical problems, especially in waters with flow plastic silt layer, the thickness of the foundation pit that needs to be dredged is relatively deep and the bottom plate of the steel hanging box is relatively thick, the following technical solutions are proposed:

A construction method for an underwater platform provided in an embodiment of the present application includes:

Set up anti-silting protection grooves at preset positions on the periphery of the foundation pit of the cap;

Excavating a cap foundation pit at a preset position, and setting a steel casing in the cap foundation pit;

Assemble a steel hanging box with a reinforced concrete bottom plate on the steel casing, and pour concrete into the steel hanging box to obtain a platform.

Optionally, setting the anti-silting protection groove at the preset position on the periphery of the cap foundation pit includes:

The anti-silting protection groove is excavated at a preset position on the periphery of the construction platform of the foundation pit of the cap.

Optionally, the anti-silting groove has a width of 2-3 meters.

Optionally, the bottom of the anti-silting tank is 0.5-1.5 meters higher than the bottom level of the platform.

Optionally, said assembling a steel suspension box with a reinforced concrete back cover bottom plate on said steel casing includes:

The reinforced concrete back cover bottom plate is sleeved on the steel casing;

A steel hanging box side plate is arranged on the outer edge of the bottom cover plate.

Optionally, after the reinforced concrete bottom cover is sleeved on the steel casing, it includes:

A supporting structure for supporting the reinforced concrete back cover bottom plate is arranged at the top of the steel casing.

Optionally, said sheathing said reinforced concrete bottom cover plate on said steel casing includes:

The reinforced concrete bottom cover is sheathed on the steel casing, and the steel casing and the reinforced concrete bottom cover are connected by a shear key.

The embodiment of the present application also provides a cofferdam structure for an underwater cap, including: a steel hanging box that is sleeved on a steel casing and has a reinforced concrete bottom plate, and is arranged at a preset position on the periphery of the foundation pit of the cap The anti-silting protection groove at the place, and the side plate surrounding the outer edge of the reinforced concrete back cover bottom plate.

Optionally, it is characterized in that the anti-silting groove has a width of 2-3 meters.

Optionally, the bottom of the anti-silting tank is 0.5-1.5 meters higher than the bottom level of the platform.

Optionally, a support structure for supporting the reinforced concrete back cover bottom plate provided at the top of the steel casing is also included.

Optionally, one end of the support structure is fixedly connected to the reinforced concrete back cover bottom plate, and the other end is fixedly connected to the top of the steel casing.

Optionally, a shear key connecting the steel casing and the reinforced concrete back cover bottom plate is also included.

Optionally, one end of the shear key is fixedly connected to the reinforced concrete back cover bottom plate, and the other end is fixedly connected to the steel casing.

Compared with the prior art, the present invention has the following beneficial effects:

The construction method and cofferdam structure of the underwater caps provided in the embodiments of the present application include setting anti-silting protection grooves at preset positions around the foundation pit of the caps; excavating the foundation pits of the caps at the preset positions; A steel casing is arranged in the foundation pit; a steel hanging box with a reinforced concrete bottom plate is assembled on the steel casing, and concrete is poured in the steel hanging box to obtain a cap. By setting anti-silting protection grooves around the foundation pit, it is possible to prevent silting of the riverbed after the excavation of the platform foundation pit is in place, so that the depth of the foundation pit can meet the construction space of the steel hanging box. During the ebb and flow of tides, the fluid plastic silt will first flow back to the anti-silting protection tank during the backflow process. By regularly cleaning the sludge in the anti-silting protection tank, the sludge is prevented from flowing back into the foundation pit, thus satisfying the construction requirements in the foundation pit. The space requirement of the casing ensures the construction progress and efficiency of the casing.

In the construction method and cofferdam structure of the underwater cap provided in the embodiment of the present application, the bottom plate of the steel hanging box is a prefabricated reinforced concrete structure, which has good rigidity and stability, and reduces the thickness of the bottom plate of the steel hanging box. Furthermore, the thickness of the lift-off system for the bottom plate of the steel box is reduced, and the reduction of the thickness of the back cover bottom plate further reduces the dredging depth of the flow plastic mud layer in deep water, thereby reducing the excavation depth of the flow plastic mud layer, ensuring When the construction quality is improved, the construction workload is reduced.

In the construction method and cofferdam structure of the lower cap provided in the embodiment of the present application, a support structure is provided between the bottom cover plate and the steel casing. The support structure is a tension rod. The back cover needs to withstand a large water pressure due to the difference of the back cover. By setting up the support structure, part of the water pressure is distributed to the steel casing, thereby ensuring the safety of the back cover structure and making the back cover have better stability.

In the construction method and cofferdam structure of the lower cap provided in the embodiment of the present application, the width of the anti-silting protection groove is 2-3 meters, and the bottom of the anti-silting protection groove is higher than the bottom elevation of the cap. 0.5-1.5 meters. Avoiding the damage of the anti-silting protection groove structure to the structure of the foundation pit of the cap, it can effectively prevent the silt from flowing back into the foundation pit of the cap, and ensure the construction work space of the foundation pit of the cap, so that the steel hanging box used in the process of constructing the cap can be used It was successfully lowered to the elevation position, and the depth of the foundation pit of the cap platform met the space requirements for the steel hanging box.

Additional aspects and advantages of the invention will be set forth in part in the description which follows, and will become apparent from the description, or may be learned by practice of the invention.

Description of drawings

The above and/or additional aspects and advantages of the present invention will become apparent and easy to understand from the following description of the embodiments in conjunction with the accompanying drawings, wherein:

Fig. 1 is the schematic flow sheet of the construction method of underwater cap of the present invention;

Fig. 2 is a structural schematic diagram of the cofferdam structure of the underwater cap of the present invention.

Detailed ways

Embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by referring to the figures are exemplary only for explaining the present invention and should not be construed as limiting the present invention.

Those skilled in the art will understand that unless otherwise stated, the singular forms "a", "an", "said" and "the" used herein may also include plural forms. It should be further understood that the word "comprising" used in the description of the present invention refers to the presence of features, integers, steps, and operations, but does not exclude the existence or addition of one or more other features, integers, steps, and operations.

Those skilled in the art can understand that, unless otherwise defined, all terms (including technical terms and scientific terms) used herein have the same meaning as commonly understood by those of ordinary skill in the art to which this invention belongs. It should also be understood that terms, such as those defined in commonly used dictionaries, should be understood to have meanings consistent with their meaning in the context of the prior art, and unless specifically defined as herein, are not intended to be idealized or overly Formal meaning to explain.

A method for constructing an underwater platform provided in an embodiment of the present application, as shown in FIG. 1 , includes: S10, S20, and S30.

S10: Set up an anti-silting protection groove at a preset position on the periphery of the foundation pit of the cap.

Optionally, an anti-silting protection groove is set at the preset position on the periphery of the cap foundation pit, including:

At the preset position on the periphery of the construction platform of the cap foundation pit, excavate the anti-silting protection groove.

Optionally, the anti-silting groove has a width of 2-3 meters.

Optionally, the bottom of the anti-silting tank is 0.5-1.5 meters higher than the bottom of the platform.

In the embodiment provided in this application, in order to satisfy the requirement that the anti-silting protection groove bear more silt and avoid damage to the structure of the foundation pit of the cap, an anti-silting protection groove is set at a preset position around the foundation pit of the cap . Based on the fluidity of the silt in the water area with flow plastic silt layer, when the tide rises and falls, the silted silt of the river bed will first flow into the anti-silting protection groove. In order to meet the working space of the cap platform construction, by regularly cleaning the silt in the anti-silting protection tank, the space for receiving the sludge in the anti-silting protection tank is guaranteed, and the probability and amount of sludge flowing back into the foundation pit of the cap platform are reduced, making the bearing The depth of the foundation pit meets the construction requirements, and ensures that the steel hanging box can be lowered to the elevation position to meet the space required for lifting the steel hanging box.

Optionally, also in order to satisfy the requirement that the anti-silting protection groove can accept more silt, reduce the number of sludge cleaning, and at the same time, in order to prevent the anti-silting protection groove from causing damage to the foundation pit structure of the platform, the anti-silting protection groove The width of the tank is 2-3 meters, and the bottom of the anti-silting protection tank is 0.5-1.5 meters above the level of the cap bottom. Optionally, the bottom of the anti-silting tank is 1 meter higher than the bottom of the platform. The tank top of the anti-silting tank is higher than the level of the bottom cover plate at the bottom of the cap. In this application, by setting the anti-return silt protection groove on the periphery of the cap platform, it can effectively prevent the silt from flowing back into the foundation pit of the cap, and ensure the construction work space of the cap foundation pit. The steel hanging box used in the process of constructing the cap can It is smoothly lowered into the elevated position.

S20: Excavating a cap foundation pit at a preset position, and setting a steel casing in the cap foundation pit.

After the excavation of the anti-silting protective groove around the foundation pit of the cap, there is no need to clean the thicker mud layer for the foundation pit of the cap, and there is enough room for operation in the foundation pit of the cap after excavation. Therefore, after the cap foundation pit is completed and the anti-silting protection groove is excavated, the steel casing is lowered at the preset position inside the cap foundation pit to facilitate the construction of the cap pile foundation on the basis of the steel casing and the steel hoisting. Box assembly.

S30: Assemble a steel hanging box with a reinforced concrete bottom plate on the steel casing, and pour concrete into the steel hanging box to obtain a cap.

After the construction of the steel casing is completed, the steel hanging box is assembled on the basis of the steel casing, so that concrete can be poured in the steel hanging box capable of waterproofing and watertight to complete the construction of the cap.

Optionally, assemble a steel drop box with a reinforced concrete back cover over a steel casing, comprising:

Set the reinforced concrete back cover bottom plate on the steel casing;

A steel hanging box side plate is arranged on the outer edge of the bottom plate of the back cover.

When assembling the steel hanging box, first assemble the concrete bottom cover plate capable of bearing pressure in the steel hanging box on the steel casing, and fix the side plate of the steel hanging box on the outer edge of the bottom cover plate. Wherein, there is a waterproof and water-tight object between the bottom plate and the side plate to prevent water from penetrating into the steel hanging box. Optionally, the waterproof object includes one or more of rubber pads, sand layers, and concrete layers.

In this application, prefabricated reinforced concrete slabs are used to form the bottom slab of the steel suspension box. Since steel members are added to the bottom slab, the rigidity of the bottom slab is increased, and it can withstand greater pressure. At the same time, the prefabricated reinforced concrete slab The bottom cover of the steel hanging box is rigid enough, so that the thickness of the bottom cover can be reduced, and the thickness of the lifting system of the bottom cover of the steel box is further reduced. Due to the reduced thickness of the bottom cover of the steel box, the excavation depth of the foundation pit of the cap platform is reduced. It also decreases accordingly, thereby reducing the excavation depth of the fluid plastic silt layer, and improving the stability of the fluid plastic silt after excavation and the stability of the foundation pit. The side plates are arranged on the outer edge of the bottom cover plate, and together the side plates and the bottom cover plate form a steel hanging box with a water-proof structure, which is convenient for pouring concrete in the steel hanging box without water to form a construction cap.

Optionally, after the reinforced concrete bottom cover is set on the steel casing, it includes:

On the top of the steel casing, a supporting structure is set to support the reinforced concrete back cover bottom plate.

Before pouring the cap, the water inside the steel hanging box needs to be drained. During the drainage process, there is a water head difference between the inside and outside of the steel hanging box, which causes the bottom slab of the reinforced concrete slab to bear a large water pressure. However, the bottom sealing of the reinforced concrete slab The thickness of the bottom slab is relatively thin, and under the action of water pressure, the back cover bottom slab of the reinforced concrete slab is not stable enough, making its structure not safe enough. Therefore, a supporting structure is set on the bottom plate of the steel hanging box, and the bottom plate of the steel hanging box is connected to the steel casing, so that the supporting structure can disperse part of the water pressure to the steel casing to ensure the stability and stability of the bottom plate. safety.

Optionally, a reinforced concrete bottom cover is set on the steel casing, including:

A reinforced concrete bottom cover is set on the steel casing, and the steel casing and the reinforced concrete bottom cover are connected through a shear key.

Optionally, in order to ensure that the bottom plate is located at the specified position, the steel casing and the bottom plate are connected by a shear key, one end of the shear key is welded on the steel casing, and the other end is fixedly connected in the bottom plate. The connector of the shear key can stably connect the bottom plate of the back cover to the steel casing. Since the position of the steel casing is fixed, the position of the bottom plate of the bottom cover is guaranteed to be at the specified position.

The embodiment of the present application also provides a cofferdam structure of an underwater cap, as shown in FIG. The anti-silting protection groove 4 at the predetermined position on the periphery of the platform foundation pit 3, and the side plate 22 surrounding the outer edge of the reinforced concrete back cover bottom plate 21. Figure 2 shows a partial structural schematic diagram of the cofferdam structure of the underwater cap, and the structural schematic diagrams of other parts are the same as this part, and the shaded part in the figure is the cap structure.

In waters with flow plastic silt, the foundation pit of the underwater cap is prone to silt backflow, resulting in a deep foundation pit dredging thickness. The thickness of the box lifting system is relatively thick, and the rigidity is small, and the water pressure in the water area with flow plastic silt layer is relatively high, so the bottom plate of the steel hanging box needs to withstand high water pressure. The steel hanging box 2 in the cofferdam structure of the present application adopts the back cover bottom plate 21 of prefabricated reinforced concrete. Since the reinforcement member is added in the back cover bottom plate 21, the rigidity of the back cover bottom plate 21 is increased, the thickness of the back cover bottom plate 21 can be reduced, and the thickness of the back cover bottom plate 21 can be reduced. The thickness of the lifting system of the bottom cover bottom plate 21 of the steel suspension box 2 is reduced, thereby reducing the excavation depth of the fluid plastic mud layer, improving the stability after excavation of the foundation pit of the cap platform of the fluid plastic mud layer, and being able to withstand greater water pressure.

In the embodiment provided by the present application, the cofferdam structure steel hanging box 2 also includes a side plate 22 surrounding the outer edge of the bottom plate 21, and the side plate 22 and the bottom plate 21 together form a steel hanging box 2 with a water-stop structure, which is convenient Concrete is poured in the waterless steel hanging box 2 to form a construction cap.

Optionally, a support structure 23 for supporting the reinforced concrete bottom cover 21 is provided at the top of the steel casing 1 .

Optionally, one end of the support structure 23 is fixedly connected to the reinforced concrete bottom cover 21 , and the other end is fixedly connected to the top of the steel casing 1 .

Since the water inside the steel hanging box needs to be drained before pouring the cap, during the drainage process, there is a water head difference between the inside and outside of the steel hanging box 2, causing the bottom cover 21 of the reinforced concrete slab to bear a relatively large water pressure, but the reinforced concrete slab The thickness of the back cover bottom plate 21 of the reinforced concrete slab is relatively thin, and the back cover bottom plate 21 of the reinforced concrete slab is not stable enough under the action of water pressure, resulting in a structure that is not safe enough. Therefore, the support structure 23 provided on the bottom cover plate 21 of the steel suspension box 2 connects the bottom cover plate 21 of the steel suspension box 2 to the steel casing 1, so that the support structure 23 disperses part of the water pressure to the steel casing 1 , to ensure the stability and safety of the back cover bottom plate. Optionally, the support structure 23 is a tension and compression rod.

In order to ensure the stability and safety of the back cover bottom plate 21 , one end of the support structure 23 is fixedly connected inside the back cover bottom plate 21 , and the other end of the support structure 23 is fixedly connected to the top edge of the steel casing 1 . The fixed connection avoids that when the water pressure on the bottom cover plate 21 is not uniform, the bottom cover plate 21 can still be stably located at a fixed position, avoiding the shaking of the bottom cover plate 21, and ensuring that the water pressure can be transmitted to the steel casing 1 in time. Optionally, a connector for connecting the support structure 23 is pre-embedded in the back cover bottom plate to ensure a stable connection between the support structure 23 and the back cover bottom plate 21 .

Optionally, a shear key 24 connecting the steel casing 1 and the reinforced concrete back cover 21 is also included.

Optionally, one end of the shear key 24 is fixedly connected to the reinforced concrete bottom cover 21 , and the other end is fixedly connected to the steel casing 1 .

In order to ensure that the back cover bottom plate is at the specified position, the steel casing 1 and the back cover bottom plate 21 are connected by a shear key 24, one end of the shear force key 24 is welded on the steel casing 1, and the other end is fixedly connected in the back cover bottom plate 21. The connector of the shear key 24 can stably connect the back cover bottom plate 21 to the steel casing 1. Since the position of the steel casing 1 is fixed, the position of the bottom cover bottom plate 21 is guaranteed to be a specified position. Optionally, a connector for connecting the shear key 24 is pre-embedded in the back cover bottom plate to ensure a stable connection between the shear key 24 and the back cover bottom plate 21 .

In the embodiment provided by this application, in order to reduce the amount of silt flowing back into the foundation pit of the cap and the lifting space of the steel hanging box, as mentioned above, a The anti-silting protection groove 4, optionally, the width of the anti-silting protection groove 4 is 2-3 meters. Optionally, the bottom of the anti-silting tank 4 is 0.5-1.5 meters higher than the bottom of the platform. The tank top of the anti-silting protection tank 4 is higher than the level of the back cover bottom plate 21 at the bottom of the platform. In this application, by setting the anti-return silt protection groove on the periphery of the cap platform, it can effectively prevent the silt from flowing back into the foundation pit of the cap, and ensure the construction work space of the cap foundation pit. The steel hanging box used in the process of constructing the cap can It is smoothly lowered to the elevation position, so that the depth of the foundation pit meets the space requirements required by the steel hanging box.

In summary, the steel hanging box provided by the application includes the following beneficial effects:

The construction method and cofferdam structure of the underwater caps provided in the embodiments of the present application include setting anti-silting protection grooves at preset positions around the foundation pit of the caps; excavating the foundation pits of the caps at the preset positions; A steel casing is arranged in the foundation pit; a steel hanging box with a reinforced concrete bottom plate is assembled on the steel casing, and concrete is poured in the steel hanging box to obtain a cap. By setting anti-silting protection grooves around the foundation pit, it is possible to prevent silting of the riverbed after the excavation of the platform foundation pit is in place, so that the depth of the foundation pit can meet the construction space of the steel hanging box. During the ebb and flow of tides, the fluid plastic silt will first flow back to the anti-silting protection tank during the backflow process. By regularly cleaning the sludge in the anti-silting protection tank, the sludge is prevented from flowing back into the foundation pit, thus satisfying the construction requirements in the foundation pit. The space requirement of the casing ensures the construction progress and efficiency of the casing.

In the construction method and cofferdam structure of the underwater cap provided in the embodiment of the present application, the bottom plate of the steel hanging box is a prefabricated reinforced concrete structure, which has good rigidity and stability, and reduces the thickness of the bottom plate of the steel hanging box. Furthermore, the thickness of the lift-off system for the bottom plate of the steel box is reduced, and the reduction of the thickness of the back cover bottom plate further reduces the dredging depth of the flow plastic mud layer in deep water, thereby reducing the excavation depth of the flow plastic mud layer, ensuring When the construction quality is improved, the construction workload is reduced.

In the construction method and cofferdam structure of the lower cap provided in the embodiment of the present application, a support structure is provided between the bottom cover plate and the steel casing. The support structure is a tension rod. The back cover needs to withstand a large water pressure due to the difference of the back cover. By setting up the support structure, part of the water pressure is distributed to the steel casing, thereby ensuring the safety of the back cover structure and making the back cover have better stability.

In the construction method and cofferdam structure of the lower cap provided in the embodiment of the present application, the width of the anti-silting protection groove is 2-3 meters, and the bottom of the anti-silting protection groove is higher than the bottom elevation of the cap. 0.5-1.5 meters. Avoiding the damage of the anti-silting protection groove structure to the structure of the foundation pit of the cap, it can effectively prevent the silt from flowing back into the foundation pit of the cap, and ensure the construction work space of the foundation pit of the cap, so that the steel hanging box used in the process of constructing the cap can be used It was successfully lowered to the elevation position, and the depth of the foundation pit met the space requirements for the steel hanging box.

The above are only some embodiments of the present invention, and it should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, some improvements and modifications can also be made, and these improvements and modifications should also be regarded as Be the protection scope of the present invention.

Claims (10)

1. A construction method of an underwater cap, which is applicable to an underwater environment with a fluid plastic silt layer, is characterized in that, comprising the following steps: An anti-silting protection groove is set on the periphery of the preset foundation pit position of the cap, and the bottom of the anti-silting protection groove is higher than the elevation of the bottom of the cap; Excavate the foundation pit of the cap, and regularly clean up the silt in the anti-silting protection groove; A steel casing and a steel hanging box are sequentially arranged in the foundation pit of the cap, and the steel hanging box has a reinforced concrete bottom cover; Concrete is poured in the steel hanging box to obtain the platform. 2. The construction method of the underwater cap according to claim 1, characterized in that, the width of the anti-silting protection groove is 2-3 meters. 3. The construction method of underwater caps according to claim 1, characterized in that, the bottom of the anti-silting protection tank is 0.5-1.5 meters higher than the bottom elevation of the caps. 4. The construction method of the underwater cap according to claim 1, characterized in that, the steel casing and the steel hanging box are sequentially arranged in the foundation pit of the cap, comprising: inserting a steel casing into the foundation pit of the cap; The reinforced concrete back cover bottom plate is sleeved on the steel casing; A steel hanging box side plate is arranged on the outer edge of the bottom cover plate. 5. The construction method of the underwater cap according to claim 4, characterized in that, after the steel casing is provided with a reinforced concrete back cover, the method includes: A support structure supporting the reinforced concrete back cover bottom plate is arranged at the top of the steel casing. 6. The construction method of the underwater cap according to claim 5, characterized in that, one end of the support structure is fixedly connected to the reinforced concrete back cover bottom plate, and the other end is fixedly connected to the top of the steel casing. 7. The construction method of the underwater cap according to claim 5, wherein the supporting structure is a tension and compression rod. 8. The construction method of the underwater cap according to claim 4, wherein the step of setting the reinforced concrete bottom sealing plate on the steel casing comprises: The steel casing and the reinforced concrete back cover bottom plate are connected by a shear key. 9. The construction method of the underwater cap according to claim 4, characterized in that, a water-stopping object is set between the bottom cover plate and the side plate of the steel suspension box, and the water-stopping object includes a rubber pad, a sand layer and One or more of the concrete layers. 10. The construction method of the underwater cap according to claim 1, characterized in that, the thickness of the reinforced concrete back cover bottom plate is smaller than the thickness of the plain concrete back cover bottom plate with the same back cover effect.