Pipe gallery foundation pit enclosure construction method

Abstract

This invention provides a construction method for retaining wall of a utility tunnel foundation pit, comprising the following steps: Step S1, determining the boundary line of the foundation pit excavation and identifying the drilling positions of the piles; Step S2, installing a guide frame along the boundary line to ensure the neatness of the sheet piles after installation and prevent displacement during installation; Step S3, driving the sheet piles, placing them opposite each other on both sides of the foundation pit, with the upper ends of the sheet piles protruding above the ground surface, and then excavating the foundation pit between the two rows of sheet piles; Step S4, installing the retaining support structure, which includes brackets, walers, and inner support beams. First, brackets and walers are installed on both sides of the foundation pit, and then tension arms capable of applying prestress to the pit walls are installed outside the inner support beams. This invention can apply prestress to the pit walls, which actively resists the lateral pressure from the pit walls, thereby effectively controlling the pit displacement and rationally controlling the lateral pressure released from the pit walls.

Description

Technical Field

[0001] This invention belongs to the field of underground engineering technology, and specifically relates to a construction method for the retaining structure of a pipe gallery foundation pit. Background Technology

[0002] With the rapid development of urban infrastructure in my country, underground utility tunnels are being used in various enterprise sites. This is an important measure for the rational development and utilization of underground space, which not only saves above-ground space but also greatly facilitates the maintenance and repair of facilities such as electricity, communication, gas, water supply and drainage.

[0003] All underground utility tunnels require the excavation of a foundation pit during construction. Currently, during the excavation of existing utility tunnel foundation pits, the lateral stress of the foundation pits is extremely strong because they are longer and narrower than those of building foundation pits. The commonly used method of protection is to construct steel walers and steel supports on steel sheet piles inserted on opposite sides of the foundation pit, and then attach the two ends of the steel supports to the steel walers on both sides.

[0004] As the excavation depth of the foundation pit increases, the soil around the pit will shift, and the lateral stress of the foundation pit will gradually increase. However, with the existing construction methods, the steel support structure can only passively bear the lateral pressure from the pit wall, which is difficult to effectively control the foundation pit displacement. This can easily lead to an increase in the displacement amplitude of the foundation pit, which poses a significant threat to the retaining structure of the foundation pit. It can easily cause the load of the retaining support system to exceed the limit, resulting in the risk of unstable support.

[0005] Therefore, there is a need to provide an improved technical solution that addresses the shortcomings of the existing technology. Summary of the Invention

[0006] The purpose of this invention is to overcome the problem that the existing construction methods for pipe gallery foundation pit protection cannot effectively control the foundation pit displacement, which easily leads to an increase in the displacement amplitude and poses a significant threat to the foundation pit protection system.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] A method for constructing retaining walls for a utility tunnel foundation pit includes the following steps: Step S1, determining the boundary line of the foundation pit excavation and identifying the drilling locations for the piles; Step S2, installing a guide frame along the boundary line to ensure the neatness of the sheet piles after installation and prevent displacement during installation; Step S3, driving sheet piles, placing them opposite each other on both sides of the foundation pit, with the upper ends of the sheet piles protruding above the ground surface, and then excavating the foundation pit between the two rows of sheet piles; Step S4, installing the retaining support structure, which includes brackets, walers, and internal bracing beams. The brackets and walers are first installed on both sides of the foundation pit. Step S5: Before the purlins are installed in the foundation pit along with the inner support beam, the purlins are adjusted so that both ends of the purlins contract to store energy. The purlins are then hoisted into the foundation pit along with the inner support beam. The purlins are then adjusted again to release energy, so that both ends of the purlins extend synchronously towards the corresponding foundation pit walls. Thus, the purlins in the released energy state apply prestress to the foundation pit walls. Step S6: The inner support beams are connected to the purlins to complete the uppermost layer of the foundation pit enclosure.

[0009] In the above-described construction method for the retaining wall of the pipe gallery foundation pit, preferably, in step S2, the guide frame includes: guide piles and guide beams. The guide piles are driven at the beginning and end according to a set distance, and the two guide piles are kept in the same straight line. Then, the guide beam is connected to the two guide piles at the same time, and the guide beam is made to contact the ground surface. The two guide piles are connected together through the guide beam.

[0010] Preferably, the ground at the location of the guide beam is leveled manually, and the guide beam extends in the same direction as the boundary line;

[0011] During the installation of sheet piles, the piles are driven along the side of the guide beam facing the foundation pit.

[0012] Preferably, in step S3, when encountering corners and final angles during the driving of sheet piles, irregular angle piles are used to connect sheet piles in different directions.

[0013] The irregular angle pile is made by dividing the steel sheet pile along its length, so as to use the divided parts of the steel sheet pile to adjust the splicing direction of the steel sheet assembly.

[0014] Preferably, in step S4, the tension arm includes a left arm and a right arm, and both the left arm and the right arm include a support plate, a top plate, a force transmission rod, and a meshing plate, wherein the support plate and the top plate are connected by the force transmission rod.

[0015] Both the support plate and the top plate have through holes in their middle portions, and these through holes are adapted to the inner support beam.

[0016] The top plate, on the opposite side of the support plate, is in close contact with the waler.

[0017] Preferably, the meshing plate is disposed on the side of the top plate that is in close contact with the waler;

[0018] The meshing plate has meshing teeth at one end facing the pit wall to adapt to the retaining wall formed by continuously driven steel sheet piles.

[0019] Preferably, the support plate of the left lever arm is located between the support plate of the right lever arm and the top plate, and the support plate of the right lever arm is located between the support plate of the left lever arm and the top plate.

[0020] Preferably, the force transmission rod of the left lever arm passes through the support plate of the right lever arm;

[0021] The force transmission rod of the right lever arm passes through the support plate of the left lever arm.

[0022] Preferably, an energy storage spring is provided between the support plate of the right lever arm and the top plate of the left lever arm, and correspondingly, an energy storage spring is also provided between the support plate of the left lever arm and the top plate of the right lever arm.

[0023] The energy storage spring is fitted onto the corresponding force transmission rod.

[0024] Preferably, the surface of the support plate is provided with stress grooves, and a slot is formed at the bottom of the stress groove, through which the support plate is opened;

[0025] The diameter of the slot is smaller than the opening of the stress groove, so that the bottom of the stress groove forms a stress groove platform.

[0026] A resisting arm is provided between the support plate of the left lever arm and the support plate of the right lever arm. Both ends of the resisting arm pass through the slots of the corresponding support plates and protrude outwards.

[0027] The protruding portion of the resistance arm is provided with a resistance block.

[0028] Beneficial effects: This invention can apply prestress to the pit wall, which can actively resist the lateral pressure from the pit wall, thereby effectively controlling the pit displacement, reasonably controlling the lateral pressure released by the pit wall, avoiding threats to the pit's retaining structure, and ensuring the stability of the pit's retaining support system. Attached Figure Description

[0029] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. Wherein:

[0030] Figure 1 This is a top view of the foundation pit of the present invention;

[0031] Figure 2 This is a diagram illustrating the application of the irregular corner pile structure of the present invention at the corner of a foundation pit;

[0032] Figure 3 This is a diagram illustrating the application of the irregular angle pile structure of the present invention at the closing angle of a continuous sheet pile;

[0033] Figure 4 This is a front view of the enclosure support structure of the present invention;

[0034] Figure 5 This is an overall schematic diagram of the tension arm structure of the present invention;

[0035] Figure 6 for Figure 5 Enlarged view of point A in the middle;

[0036] Figure 7 This is a schematic diagram of the assembly of the shaping arm structure and the tensioning arm of the present invention;

[0037] Figure 8 This is a schematic diagram of the assembly of the resistance arm and the tension arm structure of the present invention.

[0038] In the diagram: 1. Bracket; 2. Waler; 3. Inner support beam; 4. Guide pile; 5. Guide beam; 6. Irregular angle pile; 7. Left lever arm; 8. Right lever arm; 9. Support plate; 10. Top plate; 11. Force transmission rod; 12. Engaging plate; 13. Through hole; 14. Energy storage spring; 15. Stress groove; 16. Slot; 17. Stress groove platform; 18. Resistance arm; 19. Resistance block; 20. Shaping arm. Detailed Implementation

[0039] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art are within the scope of protection of the present invention.

[0040] In the description of this invention, the terms "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," and "bottom," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and do not require the invention to be constructed and operated in a specific orientation; therefore, they should not be construed as limitations on the invention. The terms "connected" and "linked" used in this invention should be interpreted broadly. For example, they can refer to a fixed connection or a detachable connection; they can refer to a direct connection or an indirect connection through intermediate components. Those skilled in the art can understand the specific meaning of the above terms according to the specific circumstances.

[0041] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other.

[0042] Example 1

[0043] A method for constructing a retaining wall for a utility tunnel foundation pit includes the following steps:

[0044] Step S1: Determine the boundary line of the foundation pit excavation and determine the drilling location of the piles;

[0045] Step S1.1: Use the main axis method to determine the excavation boundary line of the foundation pit. Based on the established construction grid and design drawings, find the relationship between the outline and the main axis, determine the excavation boundary, and then determine the drilling position of the pile outside the excavation boundary line of the foundation pit.

[0046] Step S2, refer to Figure 1 Guide frames are installed along the boundary line to ensure the neatness of the steel sheet piles after installation and to prevent displacement during the installation process.

[0047] Step S2.1, drive guide piles 2. The guide frame includes: guide piles 4 and guide beams 5. Drive guide piles 4 at the beginning and end positions of the set distance, and ensure that the two guide piles 4 are in the same straight line. The distance between the two guide piles 4 is 11.0m.

[0048] Step S2.2, guide beam 5 is installed in a single-layer, single-sided manner. It is usually made of waler material (HW350x350b type steel waler). First, the ground at the position of guide beam 5 is manually leveled. Guide beam 5 is then attached to two guide piles 4 and welded to guide piles 4 to prevent displacement during pile driving.

[0049] Step S3, refer to Figure 1 Steel sheet piles are driven and placed opposite each other on both sides of the foundation pit, with the upper end of the steel sheet piles protruding above the ground surface. Then, the foundation pit is excavated between the two rows of steel sheet piles.

[0050] Step S3.1: The pile driving operation is carried out using a robotic arm vibratory pile driver. A typical pile driving team is equipped with 2 people, including 1 pile driver operator and 1 pile holder.

[0051] Step S3.2: First, the sheet piles on the site are picked up by the vibratory hammer of the pile driver and moved to the pile driving location. The pile support personnel cooperate with the machine operator to accurately place the sheet piles into the pile position. At this time, the sheet piles should be close to the guide beam 5. Then the machine operator turns on the vibratory hammer and drives the sheet piles downward. The pile support personnel observe the verticality of the sheet piles at any time and direct the machine operator to make adjustments. After the first sheet pile is driven to the design elevation, the second pile is lifted and driven. Using this individual driving method, the sheet piles are driven into the soil one by one until the foundation pit support is completed.

[0052] Step S3.3: The first sheet pile driven in serves as a guide pile, acting as a template to guide the subsequent sheet piles. Its driving position and verticality should be ensured to avoid affecting the position and verticality of the subsequent sheet piles.

[0053] Step S3.4, refer to Figure 2 and Figure 3 When encountering corners and final angles during the driving of sheet piles, special-shaped corner piles 6 are used to connect sheet piles in different directions. The special-shaped corner piles 6 are made by dividing the sheet piles along the length direction, so as to use the divided parts of the sheet piles to adjust the splicing direction of the sheet piles.

[0054] Step S4, refer to Figure 4 Install the retaining support structure, which includes: bracket 1, waler 2 and inner support beam 3. First, install bracket 1 and waler 2 on both sides of the foundation pit, and then install tension arm that can apply prestress to the pit wall outside the inner support beam 3.

[0055] Step S4.1, bracket 1 installation: Before installation, calculate the elevation of the top of bracket 1 based on the center elevation of each inner support beam 3, and use a level to measure and lay it out on the steel sheet pile. Bracket 1 is fixed to the steel sheet pile using double-sided welding process to ensure that its top is on the same plane. Bracket 1 is prefabricated on the ground and assembled on site to speed up the construction.

[0056] Step S4.2, waler 2 installation: use a truck crane to lift and place it onto the welded bracket 1. Construction workers splice the two waler sections 2 in the foundation pit. The spliced ​​waler 2 is made of 10mm thick steel plate. The waler 2 is close to the pile body plane. If there is a gap between individual waler 2 and steel sheet pile, a 10mm thick steel plate is used as a pad.

[0057] Step S4.3: Install the tensioning arm by fitting it onto the outside of the inner support beam 3. For details, refer to... Figure 5The tension arm includes a left arm 7 and a right arm 8, and both the left arm 7 and the right arm 8 include a support plate 9, a top plate 10, a force transmission rod 11, and a meshing plate 12. The support plate 9 and the top plate 10 are connected by the force transmission rod 11. The support plate 9 and the top plate 10 are both provided with through holes 13 in the middle. The meshing plate 12 is provided on the opposite side of the top plate 10 to the support plate 9, and there are two meshing plates 12. The two meshing plates 12 are arranged opposite to each other and parallel on the top plate 10. The ends of the meshing plates 12 are provided with interlocking teeth that are adapted to the retaining wall surface formed by continuously driven steel sheet piles. The support plate 9 of the left arm 7 is located between the support plate 9 of the right arm 8 and the top plate 10. At the same time, the support plate 9 of the right arm 8 is located... Between the support plate 9 and the top plate 10 of the left arm 7, the force transmission rod 11 of the left arm 7 passes through the support plate 9 of the right arm 8, and the force transmission rod 11 of the right arm 8 passes through the support plate 9 of the left arm 7. There are at least four force transmission rods 11 to ensure stability. There are two force transmission rods 11 on the left arm 7 and the right arm 8 respectively, and the four force transmission rods 11 are cross-shaped when viewed from the axial direction to avoid interference between multiple force transmission rods 11. An energy storage spring 14 is provided between the support plate 9 of the right arm 8 and the top plate 10 of the left arm 7. Correspondingly, an energy storage spring 14 is also provided between the support plate 9 of the left arm 7 and the top plate 10 of the right arm 8. The energy storage spring 14 is fitted on the corresponding force transmission rod 11.

[0058] Step S5, refer to Figure 3 Before the tension arm enters the foundation pit along with the inner support beam 3, the tension arm is adjusted so that both ends of the tension arm contract in a concentrated manner to store energy. The tension arm is then hoisted into the foundation pit along with the inner support beam 3. The tension arm is adjusted again to release energy, so that both ends of the tension arm extend synchronously toward the corresponding foundation pit wall. Thus, the tension arm in the released energy state applies prestress to the foundation pit wall.

[0059] Step S5.1: Energy storage for the tension arm. The through hole 13 matches the inner support beam 3, allowing the tension arm to be fitted onto the inner support beam 3. Initially, the length of the tension beam is longer than the width of the pipe gallery pit. Energy storage is needed to shorten its length so it can be lowered into the pit along with the inner support beam 3. Specifically, in the initial state, space must be reserved between the support plates 9 of the left arm 7 and the right arm 8 of the tension arm. Two jacks are placed horizontally between the two support plates 9, keeping them parallel to balance the force applied by the jacks to the support plates 9. Then, the two jacks are controlled to simultaneously output force to the support plates 9, moving them synchronously in opposite directions. During this process, the left arm 7 will pull... The top plate 10 of the body moves towards the right lever arm 8. Correspondingly, the right lever arm 8 will also pull its own top plate 10 towards the left lever arm 7, so that the two top plates 10 move synchronously towards the center of the tension arm. By moving the two top plates 10 synchronously towards the center of the tension arm, the length of the tension arm can be changed, thereby shortening the length of the tension arm to match the length of the inner support beam 3 or the width between the walers 2 on both sides of the pipe gallery pit, ensuring that the tension arm can enter the pipe gallery pit together with the inner support beam 3. At this time, the energy storage spring 14 located on the left is squeezed by the top plate 10 of the left lever arm 7 and the support plate 9 of the right lever arm 8 to achieve energy storage. The energy storage spring 14 located on the right is squeezed by the top plate 10 of the right lever arm 8 and the support plate 9 of the left lever arm 7 to achieve energy storage.

[0060] Step S5.2, refer to Figure 6 and Figure 7 After determining the length of the tension arm, the jack is removed. Multiple stress grooves 15 are distributed in a ring on the surface of the support plate 9. Holes 16 are formed at the bottom of each stress groove 15, allowing the support plate 9 to be opened through the holes 16. The diameter of the holes 16 is smaller than the opening of the stress grooves 15, so that the bottom of the stress grooves 15 forms a stress groove platform 17. Following this, a shaping arm 20 is placed horizontally between the two support plates 9. The size of the shaping arm 20 matches the stress grooves 15, with one end of the shaping arm 20 inserted into the corresponding stress groove 15 of the support plate 9. Then, the pressure from the jack is gradually released. As the two support plates 9 are brought closer together, the other end of the shaping arm 20 will automatically insert into the stress groove 15 of its corresponding support plate 9. The shaping arm 20 resists the energy released by the energy storage spring 14, temporarily shaping the length of the tension arm. Finally, the pressure of the jack is released until the jack can be removed from between the two support plates 9, so that the operation can be repeated to store energy for the tension arms on other inner support beams 3. The stress groove 15 can prevent the shaping arm 20 from being misaligned by the pressure applied by the two support plates 9, thereby eliminating safety hazards.

[0061] Step S5.3, refer to Figure 3 and Figure 7The inner support beam 3 is hoisted and lowered. The tensioning arm is then used to lift the tensioning arm along with the inner support beam 3 into the foundation pit, aligning the end of the inner support beam 3 with the side of the waler 2. Two jacks are then symmetrically placed between the two support plates 9, applying pressure to the two support plates 9 to remove the shaping arm 20 from between them. After removing the shaping arm 20, the jacks are operated again to release the pressure. During this process, under the action of the energy storage spring 14, the two support plates 9 gradually move closer together, while the two top plates 10 move further apart, thus increasing the total length of the tensioning arm. Gradually increase the pressure until the meshing teeth at the end of the meshing plate 12 are embedded in the retaining wall formed by the continuously driven steel sheet piles. At this point, the energy released by the energy storage spring 14 will be transferred to the retaining wall, generating prestress on the wall of the pipe gallery pit. This prestress can actively resist the lateral pressure from the pit wall on the retaining wall, thereby effectively controlling the pit displacement and reasonably controlling the lateral pressure released by the pit wall to avoid threatening the retaining structure of the pit and keep the retaining support system of the pit stable. Finally, the jack can be removed.

[0062] Step S6: Connect the inner support beam 3 to the waler 2 to complete the uppermost layer of the foundation pit enclosure.

[0063] Example 2 is basically the same as Example 1, except that it can add a protective layer to the inner support beam 3, increasing the support strength of the inner support beam 3, thereby resisting the extremely strong lateral pressure generated by the large deformation of the soil in the foundation pit wall. Specifically, refer to... Figure 4 , Figure 6 and Figure 8 A resisting arm 18 is provided between the support plate 9 of the left lever arm 7 and the support plate 9 of the right lever arm 8. Both ends of the resisting arm 18 pass through the corresponding support plate 9 through the slot 16 and protrude outward. The protruding part of the resisting arm 18 is threadedly connected to a resisting block 19, and a pressure gap is reserved between the resisting block 19 and the adjacent support plate 9. This pressure gap interacts with the energy storage spring 14. When the soil of the pit wall undergoes large deformation, the stress added by the energy storage spring 14 to the two top plates 10 will be mutually transmitted, so that regardless of whether it is a single side of the pit wall or a single side of the pit wall, the stress will be mutually transmitted. The large deformation of the pit walls on both sides causes the two top plates 10 to be compressed simultaneously, and the corresponding two energy storage springs 14 will also be compressed. The above-mentioned pressure gap can provide a certain amount of energy storage for the energy storage springs 14, so that the compressed energy storage springs 14 can be compressed in the opposite direction, maximizing the role of the tension arm. If this method still cannot resist the side pressure of the pit walls, rigid support can be achieved by the cooperation of the resistance block 19 and the adjacent support plate 9, so as to absorb the load for the inner support beam 3 as much as possible and avoid the risk of the inner support beam 3 breaking.

[0064] It is understood that the above description is merely exemplary and the embodiments of this application do not limit the scope of the application.

[0065] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention shall be within the scope of protection of the pending claims of the present invention.

Claims

1. A method for constructing retaining walls for a utility tunnel foundation pit, characterized in that, Includes the following steps: Step S1: Determine the boundary line of the foundation pit excavation and determine the drilling location of the piles; Step S2: Install a guide frame along the boundary line to ensure the neatness of the sheet piles after installation and prevent displacement during the installation process. Step S3: Drive steel sheet piles, place the steel sheet piles opposite each other on both sides of the foundation pit, and make the upper end of the steel sheet piles protrude from the ground surface. Then excavate the foundation pit between the two rows of steel sheet piles. Step S4: Install the retaining support structure, which includes a bracket, a waler, and an inner support beam. First, install the bracket and the waler on both sides of the pit. Then, install a tension arm that can apply prestress to the pit wall outside the inner support beam. Step S5: Before the tension arm enters the foundation pit together with the inner support beam, the tension arm is adjusted so that both ends of the tension arm contract in a concentrated manner to store energy. The tension arm is then hoisted into the foundation pit together with the inner support beam. The tension arm is adjusted again to release energy, so that both ends of the tension arm extend synchronously towards the corresponding foundation pit wall. Thus, the tension arm in the released energy state applies prestress to the foundation pit wall. In step S4, the tension arm includes a left arm and a right arm, and both the left arm and the right arm include a support plate, a top plate, a force transmission rod, and a meshing plate. The support plate and the top plate are connected by the force transmission rod. Both the support plate and the top plate have through holes in their middle sections, and the through holes are adapted to the inner support beam. The top plate, on the opposite side of the supporting plate, is in close contact with the waler; The meshing plate is located on the side of the top plate that is in close contact with the waler; The meshing plate has meshing teeth at one end facing the pit wall to adapt to the retaining wall formed by continuously driven steel sheet piles. The support plate of the left lever arm is located between the support plate of the right lever arm and the top plate, and at the same time, the support plate of the right lever arm is located between the support plate of the left lever arm and the top plate; The force transmission rod of the left lever arm passes through the support plate of the right lever arm; The force transmission rod of the right lever arm passes through the support plate of the left lever arm; An energy storage spring is provided between the support plate of the right lever arm and the top plate of the left lever arm; correspondingly, an energy storage spring is also provided between the support plate of the left lever arm and the top plate of the right lever arm. The energy storage spring is fitted onto the corresponding force transmission rod; Step S6: Connect the inner support beam to the waler to complete the uppermost layer of protection for the foundation pit.

2. The construction method for retaining wall of a pipe gallery foundation pit according to claim 1, characterized in that, In step S2, the guide frame includes guide piles and guide beams. The guide piles are driven at the beginning and end according to a set distance, and the two guide piles are kept in the same straight line. Then, the guide beam is connected to the two guide piles at the same time, and the guide beam is made to contact the ground surface. The two guide piles are connected together through the guide beam.

3. The construction method for retaining wall of a pipe gallery foundation pit according to claim 2, characterized in that, The ground at the location of the guide beam is leveled manually, and the guide beam extends in the same direction as the boundary line; During the installation of sheet piles, the piles are driven along the side of the guide beam facing the foundation pit.

4. The construction method for retaining wall of a pipe gallery foundation pit according to claim 1, characterized in that, In step S3, when encountering corners and final angles during the installation of sheet piles, irregular angle piles are used to connect sheet piles in different directions. The irregular angle pile is made by dividing the steel sheet pile along its length, so as to use the divided parts of the steel sheet pile to adjust the splicing direction of the steel sheet assembly.

5. The construction method for retaining wall of a pipe gallery foundation pit according to claim 1, characterized in that, The surface of the support plate is provided with stress grooves, and a slot hole is opened at the bottom of the stress groove, through which the support plate is opened; The diameter of the slot is smaller than the opening of the stress groove, so that the bottom of the stress groove forms a stress groove platform. A resisting arm is provided between the support plate of the left lever arm and the support plate of the right lever arm. Both ends of the resisting arm pass through the slots of the corresponding support plates and protrude outwards. The protruding portion of the resistance arm is provided with a resistance block.

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