River-facing deep foundation pit composite supporting structure and construction method thereof

Abstract

The invention discloses a river-facing deep foundation pit composite supporting structure, a foundation pit comprises a river-facing side, the composite supporting structure comprises an enclosure structure and an inner supporting structure, the enclosure structure comprises an underground diaphragm wall, the inner supporting structure comprises latticed columns and a plurality of supporting beams arranged in the vertical direction of the latticed columns, and the latticed columns are arranged on the side of the foundation pit. The bottoms of the latticed columns are fixed to the supporting piles, the supporting piles are arranged along the supporting beams at intervals, and the lengths of the supporting piles are distributed from long to short in a stepped mode from the side away from the river-facing side to the river-facing side. The supporting beams act on the latticed columns, the latticed columns are fixed to the supporting piles, the supporting piles are arranged in a step shape according to the distance between the supporting piles and the river facing side and the length of the supporting piles, unbalanced pressure is dispersed to the corresponding latticed columns, and deformation of the enclosure structure is reduced.

Description

Technical Field

[0001] The invention relates to a composite supporting structure for a deep foundation pit near a river and a construction method thereof. Background Art

[0002] Many factors influence foundation pit deformation, among which groundwater seepage is a significant one. When excavating a foundation pit in areas with high groundwater levels, a head difference often exists inside and outside the pit. This differential head causes groundwater to seep, causing changes in the pore water pressure and effective stress in the soil inside and outside the pit. When excavation depth is large, the large head difference increases the seepage force and velocity of the groundwater, further driving soil deformation. Finally, under the effects of gravity and external loads, the foundation pit is prone to instability.

[0003] The soil layers in the riverside area are rich in groundwater. Compared to general urban areas, the groundwater level in the riverside area is higher, and the geological conditions are more complex. During the excavation of the foundation pit, the large head difference between the inside and outside of the pit on the riverside side not only increased the water pressure acting on the retaining structure, but also further squeezed the soil within the pit, increasing the earth pressure on the supporting structure and causing it to deform. Furthermore, the permeability of the abundant groundwater on the riverside side also drove the soil outside the pit into the pit, increasing the uplift of the pit bottom. This uplift of the soil at the pit bottom increased the vertical displacement of the vertical support structure, adversely affecting the foundation pit support structure. Summary of the Invention

[0004] The technical problem to be solved by the present invention is the deformation problem of the supporting structure of the deep foundation pit of Linjiang. The purpose of the present invention is to provide a supporting structure suitable for the deep foundation pit of Linjiang. The purpose of the present invention is also to provide a construction method of the above-mentioned supporting structure.

[0005] To this end, the present invention provides a composite support structure for a deep foundation pit facing a river, wherein the foundation pit includes a river-facing side, the composite support structure includes a retaining structure and an internal support structure, the retaining structure includes an underground continuous wall, the internal support structure includes lattice columns and a plurality of support beams vertically arranged along the lattice columns, the bottom of the lattice columns is fixed to support piles, and the support piles are arranged at intervals along the support beams, and the lengths of the support piles are distributed in a stepped manner from long to short from the side away from the river-facing side to the river-facing side.

[0006] Furthermore, the support piles are reinforced concrete cast-in-place piles, and the lattice columns and support piles are densely arranged near the underground continuous wall.

[0007] Furthermore, the inner support structure adopts a prestressed steel composite inner support structure, including several groups of support beams arranged horizontally.

[0008] Furthermore, the underground continuous wall on the river side penetrates deep into the aquiclude, and the lower ends of the support piles adjacent to the continuous wall on the river side are flush with or deeper than the corresponding underground continuous wall.

[0009] Furthermore, a reinforcement structure is provided on the side of the underground continuous wall facing the river that faces the foundation pit. The reinforcement structure is formed by high-pressure rotary grouting and is arranged at a position 15 to 20 meters away from the upper surface of the underground continuous wall.

[0010] Furthermore, cement mixing piles are used to reinforce both sides of the underground continuous wall on the river side, wherein the length of the cement mixing piles on the river side is shorter than the length of the cement mixing piles on the foundation pit side.

[0011] Furthermore, a portion of the underground continuous wall that penetrates deep into the aquiclude is provided with a stepped structure, the stepped structure including a water stop plate and a water stop film, a permeable pipe is provided below the water stop film, and the permeable pipe is connected to the ground through a drainage pipe.

[0012] The present invention also includes a construction method for a composite support structure of a deep foundation pit near a river, comprising the following steps: S1: pre-analyzing and calculating a setting model; S2: setting an underground continuous wall, bored piles and lattice columns according to step S1; S3: setting a first support after the foundation pit is excavated to a predetermined depth, dewatering the interior of the foundation pit, and setting a corresponding internal support structure after the foundation pit continues to be excavated to a corresponding depth until the excavation reaches the bottom of the foundation pit.

[0013] Furthermore, step S1 includes: establishing a local model and a fluid-solid coupling model of the foundation pit, and the boundary conditions of the local model of the foundation pit are set as follows: the displacement boundary conditions of the four sides of the model are all set to normal fixed, the bottom surface is set to completely fixed, and the top surface is set to free; the seepage boundary conditions of the two side surfaces and the top surface of the model in the width direction are set to open, and the two side surfaces and the bottom surface of the foundation pit in the length direction are set to closed; dividing the foundation pit into multiple grid units, and establishing a local model of the foundation pit, and performing rationality verification; using PLAXIS 3D finite element software to perform meshing of the model, the basic assumptions, structural parameters, model size and boundary conditions of the fluid-solid coupling model of the foundation pit are the same as those of the local model of the foundation pit, and establishing the fluid-solid coupling model.

[0014] Furthermore, step S2 includes: when excavating the underground continuous wall, a stepped shape is dug in the waterproof layer area at the bottom, a water stop plate is set at the corresponding position on the steel cage, a water stop film and a square tube with water permeable holes are set below the water stop plate, after cleaning the groove, when the steel cage is lowered, the water stop plate cooperates with the upper surface of the stepped soil layer, the square tube cooperates with the middle surface of the stepped soil layer, and a certain gap is provided between the side wall of the square tube and the side wall of the soil layer. After the steel cage is in place, the concrete underground continuous wall is poured.

[0015] Beneficial technical effects of the present invention:

[0016] (1) The retaining structures around the traditional foundation pit are subjected to roughly equal forces, while the riverside deep foundation pit of the present invention is affected by groundwater, and the pressure is significantly greater than that of the retaining structures on the mountainside. Therefore, when the traditional support structure (one end is supported on the retaining structure on the riverside, and the other side is supported on the retaining structure on the mountainside) is applied to the riverside deep foundation pit, the retaining structure will be deformed too much due to the unbalanced pressure on both sides. In this application, the support beam acts on the lattice column, and the lattice column is fixed on the support pile. The support pile is arranged in a stepped manner according to the distance from the riverside, so as to disperse the unbalanced pressure to the corresponding lattice column and reduce the deformation of the retaining structure.

[0017] (2) In a specific embodiment of the present invention, the underground continuous wall on the river side penetrates deep into the clay. The clay layer has a low permeability coefficient and can be regarded as an aquiclude, which reduces the phenomenon of groundwater on the river side seeping from the bottom of the underground continuous wall to the foundation pit. The permeable pipes, water stop plates and water stop membranes provided can reduce the seepage in the gaps between the underground continuous wall and the soil layer, and reduce the deformation of the foundation pit support. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is a schematic diagram of the foundation pit support structure of the present invention;

[0019] Figure 2 for Figure 1 A schematic cross-sectional view of a

[0020] Figure 3 for Figure 1 A schematic cross-sectional view of another part of the

[0021] Figure 4 Schematic diagram of underground continuous wall and reinforcement structure;

[0022] Figure 5 This is a schematic diagram of the cement mixing pile and the underground connection wall;

[0023] Figure 6 This is a schematic diagram of the bottom of the underground continuous wall;

[0024] Explanation of the accompanying symbols: 1. Internal supporting structure; 2. Underground continuous wall; 3. Support beam; 4. Lattice column; 5. Support pile; 6. Connecting beam; 7. High-pressure rotary jet pile; 8. Cement mixing pile; 9. Water stop plate; 10. Permeable pipe; 11. Water stop membrane; 12. River side. DETAILED DESCRIPTION

[0025] In order to further illustrate the technical means and effects adopted by the present invention to achieve the predetermined purpose of the invention, the specific implementation methods, structures, features and effects of the present invention are described in detail below in conjunction with the accompanying drawings and preferred embodiments.

[0026] Reference Figures 1 to 6 As shown, a composite support structure for a deep foundation pit facing a river according to the present invention comprises a foundation pit including a riverside 12, a composite support structure comprising a retaining structure and an internal support structure 1, the retaining structure comprising an underground continuous wall 2, and an internal support structure 1 comprising lattice columns 4 and a plurality of support beams 3 arranged vertically along the lattice columns 4. The bottoms of the lattice columns 4 are fixed to support piles 5, and the support piles 5 are arranged at intervals along the length of the support beams 3. The lengths of the support piles 5 are arranged in a stepped manner from long to short from the side away from the riverside to the riverside. The foundation pit of this embodiment is set on one side of the riverbank, with one side facing the river and the other side facing the mountain. Before the construction of the foundation pit, the model set by the pre-analysis calculation can be combined with the existing technology for the calculation of the model, and then the underground continuous wall 2 can be constructed. Before the construction of the underground continuous wall 2, the foundation can be drained and consolidated first, the construction point can be determined, trenches can be dug to obtain soil, and the river side can be excavated to the silty clay layer or the gneiss below. The permeability coefficient of the clay layer can be measured with a permeameter during the geological survey stage. A steel skeleton is placed in the trench and poured to form the underground continuous wall 2 as a retaining structure. The thickness of the underground continuous wall 2 is 800mm, the concrete strength grade is C30, and the wall depth is in the range of 19m to 56m. The internal support structure 1 is a prestressed steel composite internal support structure, including perimeter purlins, support piles 5, and support beams 3. The support piles 5 are bored reinforced concrete cast-in-place piles. Before the concrete solidifies, one end of the lattice column 4 is inserted into the cast-in-place pile to form the integral structure. The support piles 5 on the riverside extend deep into the bedrock. From the riverside to the mountainside, the length of the support columns decreases in a stepped arrangement. The length of the support piles on the mountainside can be equivalent to the depth of the underground diaphragm wall. The support beams 3 are fixed to the lattice columns 4 so that some of the unbalanced horizontal forces acting on the support beams 3 act on the lattice columns 4. The support beams 3 are arranged in several layers from shallow to deep in the foundation pit, and the horizontal support beams 3 are arranged in several groups.

[0027] In the above embodiment, referring to Figure 2 As shown, the lattice columns 4 and support piles 5 corresponding to the support beams 3 are densely arranged near the underground continuous wall 2 to improve the ability of the internal support structure 1 to resist deformation. Figure 3 As shown, the underground continuous wall 2 between adjacent groups of support beams 3 is supported by connecting beams 6, and the connecting beams 6 are fixed to the corresponding lattice columns 4, and the lattice columns 4 are fixed on the supporting piles 5, and the length of the supporting piles 5 is the same as the length of the supporting piles 5 at the corresponding positions of the adjacent support beams 3.

[0028] In the above embodiment, referring to Figure 4As shown, the underground continuous wall 2 on the river side is provided with a reinforcement structure toward the foundation pit side. The reinforcement structure is formed by high-pressure rotary jet grouting, and can form a continuous structure or high-pressure rotary jet piles 7 arranged at a certain interval. The thickness and height are both set to 5m. The reinforcement structure is set at a position 15 to 20m away from the upper surface of the underground continuous wall 2. The horizontal deformation of the underground continuous wall 2 toward the foundation pit is the largest in the range of 15 to 20 meters underground. The upper surface of the reinforcement structure is the bottom of the foundation pit or slightly higher than the bottom of the foundation pit.

[0029] In the above embodiment, referring to Figure 5 As shown, cement mixing piles 8 can also be used to reinforce both sides of the underground continuous wall 2 on the riverside. The length of the cement mixing piles 8 on the riverside is shorter than that on the side close to the foundation pit. In this embodiment, the cement mixing piles 8 on the riverside are 6 meters long, while the cement mixing piles 8 on the side close to the foundation pit are approximately 10 to 12 meters below the bottom of the foundation pit. In this embodiment, the cement mixing piles 8 can be integrated with the high-pressure rotary jet grouting piles 7 to jointly resist the deformation tendency of the underground continuous wall 2.

[0030] In the above embodiment, referring to Figure 6 As shown, a portion of the underground continuous wall 2 extending deep into the aquiclude is provided with a stepped structure. The stepped structure includes a waterstop plate 9 fixed to a steel frame. A waterstop membrane 11, such as a waterproof film, is fixed below the waterstop plate 9. A permeable pipe 10 is provided below the waterstop membrane 11. The permeable pipe 10 is connected to the ground through a drainage pipe. The side of the permeable pipe 10 has permeable holes. The waterstop plate 9 extends outward from the ground surface of the permeable pipe 10. Another waterstop membrane 11 is provided on the bottom surface of the permeable pipe 10 and the waterstop plate 9 connected to the bottom of the permeable pipe 10. The interface between the underground continuous wall 2 and the soil layer may contain subtle seepage channels. The waterstop plate 9 can reduce the amount of seepage that bypasses the bottom of the underground continuous wall 2. The permeable pipe 10 can collect the seepage and pump it out through the drainage pipe. This embodiment is mainly suitable for underground continuous wall 2 with the bottom located in a clay layer. If the bottom of the underground continuous wall 2 is located on bedrock, the seepage will be even less.

[0031] The present invention also includes the construction method of the composite support structure of the deep foundation pit near the river, which includes the following steps: S1: pre-analyze and calculate the model setting; S2: set the underground continuous wall 2, bored piles and lattice columns 4 according to step S1; S3: after the foundation pit is excavated to a predetermined depth, set the first support, dewater the interior of the foundation pit, and after the foundation pit continues to be excavated to the corresponding depth, set the corresponding internal support structure 1 until the bottom of the foundation pit is excavated. In this embodiment, the part with an intermediate length of 35m is selected to establish a local model of the foundation pit without considering the fluid-solid coupling effect, and then establish a fluid-solid coupling model considering the fluid-solid coupling effect. The model establishment of step S1 of this embodiment includes:

[0032] Basic assumptions: Since the actual construction process is relatively complicated, in order to simplify the calculation, the following assumptions are made: (1) the foundation pit has been consolidated under its own weight stress before excavation; (2) the soil and structural materials are isotropic; (3) during the construction phase of the underground continuous wall 2, bored piles and lattice columns 4 before foundation pit excavation, the displacement zeroing function is used to ignore the disturbance during the construction process.

[0033] Model size and boundary conditions: The impact range and impact depth of foundation pit excavation are 3 to 4 times and 2 to 4 times the maximum depth of the foundation pit, respectively. The impact range and impact depth of the foundation pit are calculated based on the maximum excavation depth of the foundation pit. At the same time, combined with the calculation scale of the model and the results of the preliminary trial calculation, the calculation range of the model is finally determined: the length, width, and standard height of the complete foundation pit model. The model is meshed to determine the units and nodes. The boundary conditions of the complete model and the local model are set as follows: the displacement boundary conditions on the four sides of the model are set to normal fixed, the bottom surface is set to completely fixed, and the top surface is set to free. The seepage boundary conditions on the four sides and top surface of the model are set to open, and the bottom surface is set to closed. The initial groundwater is set to -6m.

[0034] Model Parameters: The model was simplified, combining soil layers with similar properties. The retaining structure was constructed using a diaphragm wall 2 with a concrete strength grade of C30, a wall thickness of 800 mm, and a depth ranging from 19 m to 56 m. The diaphragm wall was simulated using plate elements; when entering the plate element weight, the soil weight should be subtracted. Both sides of the diaphragm wall were reinforced with C30 triaxial cement mixing piles 8 with a diameter of 850 mm and a spacing of 600 mm. The reinforcement depth of the diaphragm wall on the mountainside ranged from 10 m to 12 m, while the depth on the riverside was 6 m. In the model, the triaxial cement mixing piles 8 were converted to diaphragm wall 2 using equivalent stiffness. The triaxial cement mixing piles 8 were also simulated using plate elements. When setting up the diaphragm wall in the model, the thickness of the diaphragm wall within the reinforcement range was added to the converted thickness of the triaxial cement mixing piles 8 to simulate the triaxial cement mixing piles 8.

[0035] Interface elements are set on both sides of the ground-connected wall and triaxial cement mixing piles 8 to simulate the contact surface between the soil and the slab. In PLAXIS finite element software, when soil elements are removed to simulate earthwork excavation, the interface elements at the excavated soil are also removed, making the foundation pit model closer to the actual project, which is one of the advantages of PLAXIS finite element software. The lateral support in the foundation pit support system uses three C30 reinforced concrete supports, each with a reinforced concrete ring beam purlin. The purlin cross-section of the first support is 1400mm × 1000mm, and the cross-section of support beam 3 is 800mm × 1300mm. Connecting beams 6 with cross-sectional dimensions of 600mm × 800mm and 600mm × 1000mm are installed between the three support beams, respectively. The purlin cross-sectional dimensions of the second support are 1600mm × 1000mm, and the cross-sectional dimensions of support beam 3 and connecting beam 6 are the same as those of the first support. The cross-sectional dimensions of the purlin for the third support are 1600mm × 1200mm, the cross-sectional dimensions of support beam 3 are 800mm × 1500mm, and the cross-sectional dimensions of tie beam 6 are the same as those of the first two supports. The vertical supports in the support system are lattice columns 4 with cross-sectional dimensions of 480mm × 480mm. Both the lateral and vertical supports are modeled using beam elements. The design bearing capacity of beam 3 for each of the three supports is calculated based on the axial force warning value of the support, which is 70% of the design bearing capacity of the component. The axial force warning values for beam 3 for the first, second, and third supports can be obtained. The foundation pit consists of C30 bored piles with a diameter of 1200mm. The bored piles are embedded to a depth similar to that of diaphragm wall 2 and are modeled using embedded pile elements. In the model, since the embedded pile elements overlap with the soil elements, the soil weight should be subtracted when setting the weight of the embedded pile elements.

[0036] In the above embodiment, referring to Figure 6 As shown, step S2 includes: when the underground continuous wall 2 is constructed and the trench is excavated, a stepped soil interface is dug out in the waterproof layer (clay layer) area at the bottom of the trench, and a waterstop plate 9 is set at the corresponding position on the steel cage. A waterstop film and a square tube with water-permeable holes are set below the waterstop plate 9. The bottom of the square tube is welded with a waterstop plate 9 extending outward. The upper waterstop plate 9, the waterstop membrane and the step surface of the soil layer, as well as the waterstop plate 9 and the waterstop membrane connected to the bottom of the square tube and the soil body are squeezed against each other to form a double-layer anti-seepage structure to avoid the formation of a seepage gap between the waterstop membrane and the soil layer. After the trench is cleaned, when the steel cage is lowered, the water stop plate 9 cooperates with the upper surface of the stepped soil layer, the square tube cooperates with the middle surface of the stepped soil layer, and a certain gap is provided between the side wall of the square tube and the side wall of the soil layer. After the steel cage is put in place, the concrete underground continuous wall 2 is poured. After the concrete is poured, the clay will fill the gap under the action of gravity and seepage. On the other hand, the gap can also lengthen the length of the lower step surface to facilitate construction when it is difficult to ensure accurate construction during trenching.

Claims

1. A composite support structure for a deep foundation pit near a river, wherein the foundation pit includes a riverside, the composite support structure includes a retaining structure and an inner support structure, the retaining structure includes an underground continuous wall, and the inner support structure includes lattice columns and a plurality of support beams arranged vertically along the lattice columns, characterized in that: The bottom of the lattice column is fixed to the supporting piles, and the supporting piles are arranged at intervals along the supporting beam. From the side away from the river side to the river side, the lengths of the supporting piles are distributed in steps from long to short.

2. The composite support structure for a deep foundation pit near a river according to claim 1, characterized in that: The supporting piles are reinforced concrete cast-in-place piles, and the lattice columns and supporting piles are densely arranged near the underground continuous wall.

3. The composite support structure for a deep foundation pit near a river according to claim 2, characterized in that: The inner support structure adopts a prestressed steel composite inner support structure, including a plurality of groups of support beams arranged horizontally.

4. A composite support structure for a deep foundation pit near a river according to any one of claims 1 to 3, characterized in that: The underground continuous wall on the river side penetrates into the aquiclude, and the lower ends of the support piles adjacent to the continuous wall on the river side are flush with or deeper than the corresponding underground continuous wall.

5. The composite support structure for a deep foundation pit near a river according to claim 4, characterized in that: The underground continuous wall on the river side is provided with a reinforcement structure on the side facing the foundation pit. The reinforcement structure is formed by high-pressure rotary grouting and is arranged at a position 15 to 20 meters away from the upper surface of the underground continuous wall.

6. The composite support structure for a deep foundation pit near a river according to claim 5, characterized in that: Both sides of the underground continuous wall on the river side are reinforced with cement mixing piles, wherein the length of the cement mixing piles on the river side is shorter than the length of the cement mixing piles on the foundation pit side.

7. The composite support structure for a deep foundation pit near a river according to claim 4, characterized in that: A portion of the underground continuous wall that penetrates deep into the water-proof layer is provided with a stepped structure, which includes a water-stop plate and a water-stop film. A permeable pipe is provided below the water-stop film, and the permeable pipe is connected to the ground through a drainage pipe.

8. A construction method for a composite support structure for a deep foundation pit near a river according to claim 1, characterized in that: The following steps are involved: S1: Preliminary analysis and calculation of the model; S2: Arrange underground continuous walls, bored piles and lattice columns according to step S1; S3: After the foundation pit is excavated to the predetermined depth, the first support is set up to dewater the interior of the foundation pit. After the foundation pit continues to be excavated to the corresponding depth, the corresponding internal support structure is set up until the bottom of the foundation pit is excavated.

9. The construction method of a composite support structure for a deep foundation pit near a river according to claim 8, characterized in that: The step S1 includes: establishing a local model and a fluid-solid coupling model of the foundation pit, and setting the boundary conditions of the local model of the foundation pit as follows: the displacement boundary conditions of the four sides of the model are all set to normal fixed, the bottom surface is set to completely fixed, and the top surface is set to free; the seepage boundary conditions of the two sides and the top surface of the model in the width direction are set to open, and the two sides and the bottom surface of the foundation pit in the length direction are set to closed; dividing the foundation pit into multiple grid units, establishing a local model of the foundation pit, and performing rationality verification; using PLAXIS 3D finite element software to perform meshing of the model, the basic assumptions, structural parameters, model dimensions and boundary conditions of the fluid-solid coupling model of the foundation pit are the same as those of the local model of the foundation pit, and establishing the fluid-solid coupling model.

10. The construction method of a composite support structure for a deep foundation pit near a river according to claim 8, characterized in that: The step S2 includes: when the underground continuous wall is excavated, a stepped shape is dug in the waterproof layer area at the bottom, a water stop plate is set at the corresponding position on the steel cage, a water stop film and a square tube with water permeable holes are set below the water stop plate, after the groove is cleaned, when the steel cage is lowered, the water stop plate cooperates with the upper surface of the stepped soil layer, the square tube cooperates with the middle surface of the stepped soil layer, and a certain gap is provided between the side wall of the square tube and the side wall of the soil layer. After the steel cage is placed in place, the concrete underground continuous wall is poured.

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