Method for analyzing stability of large underwater double-wall steel cofferdam structure without back cover on floodplain

Abstract

The invention discloses a stability analysis method for an underwater large double-wall steel cofferdam structure without a back cover on a floodplain, and aims at the stability analysis method for the large double-wall steel cofferdam structure without the back cover, main size parameters of steel sheet piles of the cofferdam structure are determined through a parameter analysis method; then, the modes of separated modeling of the steel sheet piles and the foundation soil layer, independent grid division and assembly and combination are adopted, so that the number of finite elements of the foundation soil layer with the largest size is reduced, and the problem of low model calculation efficiency caused by too many finite elements of the foundation soil layer is solved; and finally, through an anti-upheaval checking calculation formula of a foundation soil layer at the bottom of the cofferdam, the overall anti-overturning stability and the pit bottom anti-upheaval stability of the non-bottomed concrete cofferdam structure are analyzed, and therefore the defect that a calculation analysis method for the stability of the double-wall steel cofferdam structure is lacked currently is overcome.

Description

Technical Field

[0001] The invention relates to the technical field of cofferdam structure design, and is particularly applicable to a stability analysis method of a large double-wall steel cofferdam structure without a bottom underwater in a floodplain. Background Art

[0002] The double-walled steel cofferdam structure is a temporary water retaining structure commonly used in projects such as river regulation, dam construction, reservoir reinforcement, and wading bridge pier construction. It has the unique advantages of short construction period, low cost, safety and stability, good durability, and reusability. At present, there are a large number of successful application cases, which have achieved good social and economic benefits. The stability calculation and analysis of the double-walled steel cofferdam structure is a prerequisite for the comparison and selection of structural design schemes, technical and economic evaluation, and approval of construction schemes, and is directly related to the safety, progress, cost, and quality of project construction.

[0003] At present, the stability calculation method of double-walled steel cofferdam structure is suitable for the conditions where the soil layer has good anti-seepage stability, the cofferdam area is small, and there are bottom concrete structural measures. However, in the case of floodplain with strong water permeability, thick sedimentation, mixed soil layer, and gravel sedimentation, the bottom concrete cannot be poured due to engineering characteristics, and large cofferdam construction is required, there is a lack of calculation and analysis methods for the stability of double-walled steel cofferdam structure. The main reasons are: (1) the existing design specifications for foundation bearing, foundation pit support, temporary structure, etc., mainly calculate the anti-seepage and stability characteristics of the bottom concrete in the anti-floating analysis of the cofferdam structure, and do not consider the anti-seepage stability of the soil at the bottom of the foundation pit of the cofferdam without bottom; (2) the existing design specifications will The active and passive earth pressures are concentrated on both sides of the single-layer steel sheet piles, and the overall stability analysis of the cofferdam structure is carried out. However, for large-scale water retaining structures, there is a possibility of local instability of the inner and outer steel sheet piles. When performing the calculation and analysis of the double-walled steel cofferdam structure, the passive earth pressure of the middle fill on the outer steel sheet piles and the active earth pressure of the middle fill on the inner steel sheet piles are not considered at the same time; (3) The floodplain is mainly formed by flood overbank deposits, with complex soil layer components, uneven riverbeds, and soil interactions. The foundation bearing calculation method of the existing design specifications assumes homogeneous soil layers, average parameters, and separate soil bodies, but does not consider irregular stratification, cohesion parameter variation, and correlation of multiple soil body parameters. Summary of the invention

[0004] The present invention aims to provide a stability analysis method for a large double-walled steel cofferdam structure without a bottom underwater in a floodplain, so as to solve the problem of stability analysis of a large double-walled steel cofferdam structure without a bottom underwater in a floodplain.

[0005] To achieve the above object, the present invention adopts the following technical solutions: The stability analysis method of the floodplain underwater bottomless large double-walled steel cofferdam structure of the present invention comprises the following steps: S1, parameter analysis method is used to determine the steel sheet pile size of the double-wall steel cofferdam structure; S2, constructing the first finite element model and the second finite element model of the double-walled steel cofferdam according to the size of the steel sheet piles; S3, assembling the first finite element model and the second finite element model, applying constraints and boundaries, and forming a finite element model of the double-walled steel cofferdam; S4, based on the finite element model of the double-walled steel cofferdam, determine the water pressure, water flow force, and active and passive earth pressure of the soil layer on the double-walled steel cofferdam; S5, construct the soil spring model of the double-walled steel cofferdam without bottom soil layer and determine the boundary conditions of the double-walled steel cofferdam; S6, determine the combination of the ultimate limit state effect of the double-walled steel cofferdam and the combination of the normal serviceability limit state effect; S7, based on the water pressure, water flow force, earth pressure, boundary conditions and action effects of the double-walled steel cofferdam, the overall anti-overturning stability of the structure and the anti-uplift stability of the pit bottom are verified.

[0006] Furthermore, step S1 specifically includes: S1.1, collect the engineering parameters of the floodplain where the double-walled steel cofferdam is located, and set steel sheet piles of different lengths and different riverbed insertion depths based on Darcy's law; S1.2, calculate the anti-seepage stability of steel sheet pile structures with different pile lengths and riverbed insertion depths; S1.3. Determine the length of the steel sheet piles and the minimum depth of insertion into the riverbed of the double-walled steel cofferdam that meets the requirements of anti-seepage stability through parameter analysis.

[0007] Furthermore, the engineering parameters described in step S1.1 include the static water level under the design flood condition of the floodplain, the geological conditions of the riverbed rock strata, the water level elevation, the characteristic value of the foundation bearing capacity, the internal friction angle, the compression modulus, and the natural and saturated weights.

[0008] Furthermore, the first finite element model building step in step S2 includes: S2.1.1, according to the length of steel sheet piles and the depth of riverbed insertion, the plane dimensions of double-walled steel cofferdam, and the layout of purlins and tie rods, establish the geometric model of inner cofferdam, outer cofferdam, steel sheet piles, purlins and tie rods; S2.1.2, use eight-node hexahedron, plate elements for steel sheet piles, beam elements for purlins and tie rods, and divide the finite element grid into layers with a size of one twentieth of the length of the steel sheet piles.

[0009] Furthermore, the second finite element model building step in step S2 includes: S2.2.1, establish the geometric model of the soil layer at the bottom of the inner wall of the double-walled steel cofferdam and the fill between the inner and outer steel sheet piles; S2.2.2, using a size of one-tenth of the total thickness of the bottom soil layer, using eight-node hexahedral solid elements, and using the sweeping method to divide the finite element grid from top to bottom.

[0010] Furthermore, the combined assembly of the first finite element model and the second finite element model in step S3 specifically includes a hinged connection between the steel sheet pile and the bottom soil layer, a common node connection between the purlin and the steel sheet pile, and an elastic connection between the tension rod and the purlin.

[0011] Furthermore, the water pressure in step S4 is calculated using the formula Sure, , , They represent water pressure, water density, and head difference respectively; the water flow force is represented by the formula Determine, among which, , , , , They represent the standard value of water flow force, water flow resistance coefficient, water density, water flow design velocity, and the projection area on the plane vertical to the calculation structure and flow direction; the calculation formulas for the standard value of active earth pressure strength and the standard value of passive earth pressure strength of each soil layer below the groundwater level are: in, They respectively represent the standard value of the active earth pressure strength of the i-th layer of soil outside the cofferdam, the standard value of the passive earth pressure strength of the i-th layer of soil inside the cofferdam, the standard value of the vertical stress of the soil outside the cofferdam, the standard value of the vertical stress of the soil inside the cofferdam, the active earth pressure coefficient of the i-th layer of soil, the passive earth pressure coefficient of the i-th layer of soil, the cohesion of the i-th layer of soil, the internal friction angle of the i-th layer of soil, the vertical total stress generated by the deadweight of the soil outside the cofferdam, the vertical total stress generated by the deadweight of the soil inside the cofferdam, and the standard value of the additional vertical stress of the j-th layer of soil outside the cofferdam.

[0012] Furthermore, in step S5, the soil spring model without bottom soil layer is constructed according to the 0.5m thickness division; the boundary condition of the double-walled steel cofferdam is the soil spring stiffness, and the calculation formula is: ,in, , , , They respectively represent the thickness of each soil layer, the calculated width of the steel sheet pile, the proportional coefficient of the foundation soil, and the distance from the midpoint of each soil layer to the ground.

[0013] Furthermore, in step S6, the bearing capacity limit state effect combination is a combination of the bearing capacity limit state of the double-walled steel cofferdam and the basic load effect; the normal use limit state effect combination is a combination of the normal use limit state of the double-walled steel cofferdam and the standard load effect, and the calculation formulas are: in, They respectively represent the combination of effects of the ultimate limit state of bearing capacity, the combination of effects of the normal use limit state, the importance coefficient of the steel cofferdam structure, the partial coefficient of the i-th permanent action, the standard value of the i-th permanent action, the largest partial coefficient of variable action, the standard value of the largest variable action, the coefficient of the combination value of other variable actions in the action combination except the largest variable action, the j-th variable action partial coefficient, the standard value of the j-th variable action in the action combination except the largest variable action, the design value of the i-th permanent action, the quasi-permanent value coefficient of the variable action effect, and the design value of the j-th variable action except the largest variable action in the action combination.

[0014] Furthermore, step S7 includes respectively calculating the design normal water level, the highest water level, the inner and outer cofferdams with fill and without fill, the anti-overturning stability of the double-walled steel cofferdam structure and the anti-uplift stability of the foundation pit bottom; the calculation formula for the anti-overturning stability of the double-walled steel cofferdam structure is: in, They respectively represent the safety factor of the embedded stability of the steel sheet pile, the standard value of the combined pressure force inside the steel sheet pile, the standard value of the combined pressure force outside the steel sheet pile, the distance from the point of action of the combined pressure force inside the steel sheet pile to the support point, and the distance from the point of action of the combined pressure force outside the steel sheet pile to the support point; The calculation formula for the anti-uplift stability of the bottom of the double-wall steel cofferdam foundation pit is: in, They represent the anti-uplift safety factor, the natural weight of the soil above the outer bottom of the steel cofferdam, the natural weight of the soil above the inner bottom of the steel cofferdam, the embedded depth of the steel cofferdam, the depth of the steel cofferdam foundation pit, the uniformly distributed load on the bottom, the bearing capacity coefficient, the cohesion, and the internal friction angle. Output the local stress, strain, and deformation of the steel sheet piles, purlins, and tie rods on the inner and outer walls of the cofferdam under different working conditions, and verify the overall anti-overturning stability of the structure and the anti-uplift stability of the pit bottom.

[0015] The advantage of the present invention is that it provides a stability analysis method for a large double-walled steel cofferdam structure without a bottom. Through a parameter analysis method, the main size parameters of the steel sheet piles of the cofferdam structure are clarified, and then the steel sheet piles and the foundation soil layer are separately modeled, meshed separately, and assembled and merged to reduce the number of finite elements of the foundation soil layer with the largest volume, thereby avoiding the problem of low model calculation efficiency caused by too many finite elements of the foundation soil layer; finally, through the anti-uplift calculation formula of the foundation soil layer at the bottom of the cofferdam, the overall anti-overturning stability of the concrete cofferdam structure without a bottom and the anti-uplift stability of the pit bottom are analyzed, thereby making up for the current lack of a calculation and analysis method for the stability of the double-walled steel cofferdam structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 The present invention is a flow chart of the stability analysis method of the underwater large double-walled steel cofferdam structure without bottom in the floodplain.

[0017] Figure 2 It is the finite element model of the large double-walled steel cofferdam structure without bottom cover in the embodiment of the present invention.

[0018] Figure 3 It is the maximum deformation along the river and across the river of the large double-walled steel cofferdam structure without bottom cover in the embodiment of the present invention.

[0019] Figure 4 The local stress and strain of the large double-walled steel cofferdam structure without bottom cover in the embodiment of the present invention.

[0020] Figure 5 The anti-overturning stability calculation results of the large double-walled steel cofferdam structure without bottom cover in the embodiment of the present invention. DETAILED DESCRIPTION

[0021] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] For a double-arm steel cofferdam project, the flow chart of the stability analysis method of the floodplain underwater large double-wall steel cofferdam structure without bottom cover described in the present invention is as follows: Figure 1 shown.

[0023] Step 1. According to the distribution characteristics of riverbed layered rock and soil in the floodplain area where a double-arm steel cofferdam project is located, and the static water level in the floodplain area under the design flood conditions, the water level elevation, characteristic value of foundation bearing capacity, internal friction angle, compression modulus, natural and saturated weight and other parameter indicators are extracted, and the following data are obtained: the riverbed elevation in the floodplain area is 70.2 m, the normal water level is 73.5 m, and the water level corresponding to the 20-year flood is 78.2 m. The riverbed gravel soil in the floodplain area is divided into six layers. The characteristic values ​​of foundation bearing capacity from top to bottom are 120, 190, 250, 290, 300, and 250 kPa, the internal friction angles are 16, 22, and 26 °C, the compression modulus is 15, 22, 22, 20, and 25 MPa, and the natural weight is 9.6, 10.1, 10.6, 10.6, and 10.6 kN / m 3 , saturated density 19.6, 20.1, 20.6, 20.6, 20.6 kN / m 3 , the cohesion is 0, and the lengths of the steel sheet piles are set to 20 m, 24 m, 28 m, and 32 m, corresponding to the insertion depths of 10 m, 14 m, 18 m, and 22 m.

[0024] Step 2: Based on Darcy's law, calculate the anti-seepage stability of the steel sheet pile structure. Through parameter analysis, determine the length of the steel sheet pile that meets the anti-seepage stability and the minimum depth requirement for its insertion into the riverbed.

[0025] Step 3: According to the design plan of the double-arm steel cofferdam project, the plane of the double-arm steel cofferdam is similar to a trapezoid, the length of the outer cofferdam along the river is 370 m, the width of the outer cofferdam across the river is 190 m, and the distance between the inner and outer cofferdams is 10 m. Based on the structural parameters such as the length of the steel sheet piles of the double-wall steel cofferdam and the riverbed insertion depth in step 1, the steel sheet pile length is selected as 24 m and the riverbed insertion depth is 14 m. The geometric models of the inner cofferdam, outer cofferdam, purlin and tie rod of the double-wall steel cofferdam structure are established respectively. An eight-node hexahedron is used, the steel sheet piles are plate units, and the purlins and tie rods are beam units. The steel sheet piles are layered and sectioned with a length of 1.2 m, and the finite element grid is divided to construct a refined finite element model, i.e., the first finite element model.

[0026] Then, the geometric models of the soil layer at the bottom of the inner steel sheet pile and the fill between the inner and outer steel sheet piles were established. The total thickness of the soil layer was 48 m. The foundation soil model was simplified into a regular, isotropic, semi-infinite body with a thickness of 4.8 m. An eight-node hexahedral solid unit was used, and the sweeping method was used for unit mesh division to construct a large-scale finite element model of the soil foundation, namely the second finite element model.

[0027] The finite element models of the inner and outer steel sheet piles, the waling, the tie rods, the soil layer at the bottom of the inner steel sheet piles, and the filling between the inner and outer steel sheet piles are merged and assembled. A hinge connection is adopted between the steel sheet piles and the foundation soil layer, a co - node connection is adopted between the waling and the steel sheet piles, and an elastic connection is adopted between the tie rods and the waling to form a finite element model of the overall double - wall steel cofferdam structure, as Figure 2 shown.

[0028] Figure 2 Figure a is a schematic diagram of the geometric model of the inner and outer steel sheet piles. Figure 2 Figure b is a schematic diagram of the geometric model of the tie rods. Figure 2 Figure c is a schematic diagram of the geometric model of the waling. Figure 2 Figure d is a schematic diagram of the geometric model of the filling at the bottom of the inner steel sheet piles and the filling between the inner and outer steel sheet piles. Figure 2 Figure e is the whole assembled from the geometric models of the above - mentioned four components, that is, the finite element model of the double - wall steel cofferdam.

[0029] Step 4: Based on the established finite element model of the double - wall steel cofferdam, according to the important parameter indicators of the hydrogeology in the floodplain area, calculate the load effects such as water pressure, water flow force, and earth pressure borne by the steel cofferdam structure. The water pressure calculation of the cofferdam structure is based on the water head difference and uses the following formula: where, the unit weight of water takes a value of 10 kN / m 3 , and the water head difference takes a value determined according to the difference between the water level and the riverbed elevation.

[0030] The calculation of the water flow force acting on the steel cofferdam structure uses the following formula: where, the water flow resistance coefficient takes a value of 1.5, the water density takes a value of 1000 kg / m 3 , the designed water flow velocity takes a value of 2 m / s, and the projected area on the plane perpendicular to the flow direction of the calculation structure takes a value of 570 m 2 .

[0031] For the earth pressure calculation, for the soil mass below the groundwater level, the method of combining water and soil is adopted to calculate the standard values of the active earth pressure intensity and the passive earth pressure intensity of each soil layer, as shown in the following formulas: Among them, the cohesion of the six layers of soil The internal friction angle of the 1st to 4th layer of soil is 0. The vertical total stress generated by the deadweight of the soil outside the cofferdam is 16, 22, and 26°C. The value is 103.4 kPa, and the vertical total stress generated by the deadweight of the soil inside the cofferdam is Take the value 0, the outer side of the cofferdam j Standard value of additional vertical stress in soil layer The value is accumulated based on each layer.

[0032] For the unsealed soil layer of the cofferdam structure, according to the thickness of the soil layer, 0.5m thickness is divided into a soil spring, and the soil spring stiffness is calculated using the following formula to construct the boundary conditions of the cofferdam structure: The thickness of each soil layer Take the value of 0.5 m and calculate the width of the steel sheet pile Take the value of 1 m, the proportional coefficient of foundation soil Take the value as 9000, the distance between the midpoint of each soil layer and the ground The value depends on the thickness.

[0033] The design is carried out based on the combination of the effects of the two ultimate states of cofferdam structure bearing capacity and normal use. If the design is based on the ultimate state of bearing capacity, the basic combination of load effects should be considered; if the design is based on the ultimate state of normal use, the standard combination of load effects should be considered. The formula is as follows: in, , They represent the combination of the ultimate bearing capacity state effect, the combination of the normal use limit state effect, and the importance coefficient of the steel cofferdam structure. The value is 1.0, i Partial coefficient of permanent action The value is 1.0, i Standard value of permanent effect Calculated based on earth pressure. Maximum variable action partial factor The value is 1.0, the maximum variable effect standard value Calculated according to water pressure, the coefficient of the combined value of other variable actions in the action combination except the maximum variable action The value is 0.5, j Variable action partial coefficient The value is 1.0, which means that the action combination is the first action except the maximum variable action. j Variable action standard value According to the calculation of water flow force, i Design value of permanent effect The value is calculated based on the earth pressure, the quasi-permanent value coefficient of the variable action effect The value is 1.0, which means that the action combination is the first action except the maximum variable action. j Variable action design value Calculated based on water pressure and water flow force.

[0034] Step 5: Based on the calculation of the combination of soil and water pressure and action effect, the anti-overturning stability of the double-wall steel cofferdam structure is calculated according to the four action conditions: the riverbed elevation of the floodplain is 70.2 m, the design normal water level is 73.5 m, the maximum water level of 20 years is 78.2 m, and the inner and outer cofferdams have fill and no fill, as shown below: Among them, the safety factor of steel sheet pile embedment stability is The value is 1.25, the standard value of the combined pressure inside the steel sheet pile , Standard value of combined force of outer pressure The values ​​are determined according to the above-mentioned combination of effects. The distance from the point of action of the combined pressure on the inner side of the steel sheet pile to the support point is , the distance from the outer pressure resultant force point to the fulcrum The values ​​are determined according to the force position.

[0035] The calculation of the anti-uplift stability of the bottom of the double-wall steel cofferdam foundation pit is as follows: Among them, the anti-uplift safety factor Take the value of 1.4, the natural weight of the soil above the bottom of the steel cofferdam Value, natural weight of soil above the bottom surface of the steel cofferdam Value, embedment depth of steel cofferdam The value is 14 m, the depth of the steel cofferdam foundation pit Take the value as 9 m, and the bottom surface is uniformly loaded. Value 0, cohesion The value is 0, the internal friction angle The values ​​are 16, 22, and 26℃.

[0036] Through the above calculation process, the local stress, strain and deformation of the steel sheet piles, purlins and tie rods on the inner and outer walls of the cofferdam under different working conditions are calculated and output, such as Figure 3 , Figure 4shown. Figure 3 a is a schematic diagram of the deformation of the double-walled steel sheet pile cofferdam perpendicular to the water flow direction. Figure 3 b is the enlarged view of the deformation of the steel sheet pile at the corner; Figure 3 c is a schematic diagram of the double-walled steel sheet pile cofferdam parallel to the water flow direction. Figure 3 d is the enlarged view of the deformation of the steel sheet pile at the corner. Figure 4 a is the axial force distribution diagram of the double-wall steel sheet pile cofferdam. Figure 4 b is the axial force distribution diagram of the steel sheet pile at the corner. Figure 4 c is the shear force distribution diagram of the double-wall steel sheet pile cofferdam. Figure 4 d is the shear force distribution diagram of the steel sheet pile at the corner.

[0037] Verify the overall anti-overturning stability of the structure and the anti-uplift stability of the pit bottom, such as Figure 5 shown. Figure 5 a is the bending moment distribution diagram of the double-walled steel sheet pile cofferdam, Figure 5 b is the bending moment distribution diagram of the steel sheet pile at the corner; Figure 5 c is the normal stress distribution diagram of the double-wall steel sheet pile cofferdam, Figure 5 d is the normal stress distribution diagram of the steel sheet pile at the corner; Figure 5 e is the shear stress distribution diagram of the double-wall steel sheet pile cofferdam, Figure 5 f is the shear stress distribution diagram of the steel sheet pile at the corner.

Claims

1. A stability analysis method for a large double-walled steel cofferdam structure without bottom cover in a floodplain, characterized in that: The following steps are involved: S1, parameter analysis method is used to determine the steel sheet pile size of the double-wall steel cofferdam structure; S2, constructing the first finite element model and the second finite element model of the double-walled steel cofferdam according to the size of the steel sheet piles; S3, assembling the first finite element model and the second finite element model, applying constraints and boundaries, and forming a finite element model of the double-walled steel cofferdam; S4, based on the finite element model of the double-walled steel cofferdam, determine the water pressure, water flow force, and active and passive earth pressure of the soil layer on the double-walled steel cofferdam; S5, construct the soil spring model of the double-walled steel cofferdam without bottom soil layer and determine the boundary conditions of the double-walled steel cofferdam; S6, determine the combination of the ultimate limit state effect of the double-walled steel cofferdam and the combination of the normal serviceability limit state effect; S7, based on the water pressure, water flow force, earth pressure, boundary conditions and action effects of the double-walled steel cofferdam, the overall anti-overturning stability of the structure and the anti-uplift stability of the pit bottom are verified.

2. The stability analysis method of the floodplain underwater bottomless large double-walled steel cofferdam structure according to claim 1 is characterized by: Step S1 specifically includes: S1.1, collect the engineering parameters of the floodplain where the double-walled steel cofferdam is located, and set steel sheet piles of different lengths and different riverbed insertion depths based on Darcy's law; S1.2, calculate the anti-seepage stability of steel sheet pile structures with different pile lengths and riverbed insertion depths; S1.

3. Determine the length of the steel sheet piles and the minimum depth of insertion into the riverbed of the double-walled steel cofferdam that meets the requirements of anti-seepage stability through parameter analysis.

3. The stability analysis method of the floodplain underwater large double-walled steel cofferdam structure without bottom cover according to claim 2 is characterized by: The engineering parameters described in step S1.1 include the static water level under the design flood conditions of the floodplain, the geological conditions of the riverbed rock strata, the water level elevation, the characteristic value of the foundation bearing capacity, the internal friction angle, the compression modulus, and the natural and saturated weights.

4. The stability analysis method of the floodplain underwater bottomless large double-walled steel cofferdam structure according to claim 1 is characterized by: The first finite element model building step in step S2 includes: S2.1.1, according to the length of steel sheet piles and the depth of riverbed insertion, the plane dimensions of double-walled steel cofferdam, and the layout of purlins and tie rods, establish the geometric model of inner cofferdam, outer cofferdam, steel sheet piles, purlins and tie rods; S2.1.2, use eight-node hexahedron, plate elements for steel sheet piles, beam elements for purlins and tie rods, and divide the finite element grid into layers with a size of one twentieth of the length of the steel sheet piles.

5. The stability analysis method of the floodplain underwater large double-walled steel cofferdam structure without bottom cover according to claim 1 is characterized by: The second finite element model building step in step S2 includes: S2.2.1, establish the geometric model of the soil layer at the bottom of the inner wall of the double-walled steel cofferdam and the fill between the inner and outer steel sheet piles; S2.2.2, using a size of one-tenth of the total thickness of the bottom soil layer, using eight-node hexahedral solid elements, and using the sweeping method to divide the finite element grid from top to bottom.

6. The stability analysis method of the floodplain underwater bottomless large double-walled steel cofferdam structure according to claims 4 and 5 is characterized by: The combined assembly of the first finite element model and the second finite element model in step S3 specifically includes a hinged connection between the steel sheet pile and the bottom soil layer, a common node connection between the purlin and the steel sheet pile, and an elastic connection between the tension rod and the purlin.

7. The stability analysis method of the floodplain underwater large double-walled steel cofferdam structure without bottom cover according to claim 1 is characterized by: The water pressure in step S4 is calculated using the formula Sure, , , They represent water pressure, water density, and head difference respectively; the water flow force is represented by the formula Determine, among which, , , , , They represent the standard value of water flow force, water flow resistance coefficient, water density, water flow design velocity, and the projection area on the plane vertical to the calculation structure and flow direction; the calculation formulas for the standard value of active earth pressure strength and the standard value of passive earth pressure strength of each soil layer below the groundwater level are: ； ； ； ； ； ； in, , , , , , , , , , , They respectively represent the standard value of the active earth pressure strength of the i-th layer of soil outside the cofferdam, the standard value of the passive earth pressure strength of the i-th layer of soil inside the cofferdam, the standard value of the vertical stress of the soil outside the cofferdam, the standard value of the vertical stress of the soil inside the cofferdam, the active earth pressure coefficient of the i-th layer of soil, the passive earth pressure coefficient of the i-th layer of soil, the cohesion of the i-th layer of soil, the internal friction angle of the i-th layer of soil, the vertical total stress generated by the deadweight of the soil outside the cofferdam, the vertical total stress generated by the deadweight of the soil inside the cofferdam, and the standard value of the additional vertical stress of the j-th layer of soil outside the cofferdam.

8. The stability analysis method of the floodplain underwater bottomless large double-walled steel cofferdam structure according to claim 1 is characterized by: In step S5, the soil spring model without bottom soil layer is constructed according to the 0.5m thickness division; the boundary condition of the double-walled steel cofferdam is the soil spring stiffness, and the calculation formula is: ,in, , , , They respectively represent the thickness of each soil layer, the calculated width of the steel sheet pile, the proportional coefficient of the foundation soil, and the distance from the midpoint of each soil layer to the ground.

9. The stability analysis method of the floodplain underwater bottomless large double-walled steel cofferdam structure according to claim 1 is characterized by: In step S6, the ultimate bearing capacity state effect combination is a combination of the ultimate bearing capacity state of the double-walled steel cofferdam and the basic load effect; the normal use limit state effect combination is a combination of the normal use limit state of the double-walled steel cofferdam and the standard load effect, and the calculation formulas are: ； ； in, , , , , , , , , , , , , They respectively represent the combination of effects of the ultimate limit state of bearing capacity, the combination of effects of the normal use limit state, the importance coefficient of the steel cofferdam structure, the partial coefficient of the i-th permanent action, the standard value of the i-th permanent action, the largest partial coefficient of variable action, the standard value of the largest variable action, the coefficient of the combination value of other variable actions in the action combination except the largest variable action, the j-th variable action partial coefficient, the standard value of the j-th variable action in the action combination except the largest variable action, the design value of the i-th permanent action, the quasi-permanent value coefficient of the variable action effect, and the design value of the j-th variable action except the largest variable action in the action combination.

10. The stability analysis method of a large double-walled steel cofferdam structure without bottom cover in a floodplain according to claim 1, characterized in that: Step S7 includes calculating the design normal water level, the highest water level, the inner and outer cofferdams with fill and without fill, the anti-overturning stability of the double-walled steel cofferdam structure and the anti-uplift stability of the foundation pit bottom; the calculation formula for the anti-overturning stability of the double-walled steel cofferdam structure is: ； in, , , , , They respectively represent the safety factor of the embedded stability of the steel sheet pile, the standard value of the combined pressure force inside the steel sheet pile, the standard value of the combined pressure force outside the steel sheet pile, the distance from the point of action of the combined pressure force inside the steel sheet pile to the support point, and the distance from the point of action of the combined pressure force outside the steel sheet pile to the support point; The calculation formula for the anti-uplift stability of the bottom of the double-wall steel cofferdam foundation pit is: ； ； ； in, , , , , , , , , , They respectively represent the anti-uplift safety factor, the natural weight of the soil above the outer bottom surface of the steel cofferdam, the natural weight of the soil above the inner bottom surface of the steel cofferdam, the embedding depth of the steel cofferdam, the depth of the steel cofferdam foundation pit, the uniformly distributed load on the bottom surface, the bearing capacity coefficient, the cohesion, and the internal friction angle.

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