A composite support structure for a deep foundation pit with a covering layer and a design and construction method thereof

Abstract

This invention relates to the field of foundation pit support technology, and particularly to a composite support structure for deep foundation pits with overburden layers, its design, and construction method. The structure includes an ultra-thick continuous wall and prestressed anchor cables. The ultra-thick continuous wall is composed of alternating first and second unit walls. The first unit wall is formed by pouring concrete into a pre-placed reinforcing cage component in a sequential groove, and its two end faces are concave curved surfaces. The second unit wall is formed by pouring concrete into a pre-placed reinforcing cage component in a second sequential groove, and its two end faces are convex curved surfaces. The convex and concave curved surfaces are interlocked. A positioning pipe for the prestressed anchor cables is pre-embedded in the upper part of the reinforcing cage component. The anchoring end of the prestressed anchor cable is anchored to the outer stratum, and the tensioning end is fixed to the side of the ultra-thick continuous wall facing the foundation pit. This invention provides a simple composite support structure layout and eliminates the need for internal supports, thus solving problems such as occupying construction space within the foundation pit.

Description

Technical Field

[0001] This invention relates to the field of foundation pit support technology, and in particular to a composite support structure for deep foundation pits with overburden and its design and construction method. Background Technology

[0002] Deep foundation pits with overburden layers have extremely poor soil self-stabilization capacity, are difficult to control in terms of pit deformation, have long construction periods and complex procedures, and are usually located in sensitive surrounding environments. A single structure is often insufficient to meet both safety and economic requirements. In practical engineering, composite support systems combining multiple technologies are typically employed.

[0003] One type is a composite support system that uses piles, connectors, and seepage prevention structures. For example, CN202010607322.8 uses a three-row pile ultra-deep foundation pit support structure with high lateral stiffness. The pile structure is fixedly connected by multiple support components and reinforced with steel bars to enhance the overall integrity of the piles. To solve the seepage problem, a curtain wall is required, and a retaining structure is set up to deal with the soil between the piles. Its structure and construction process are complex.

[0004] Another approach is a composite support system using retaining plates (or retaining walls) and internal bracing structures. For example, CN202311703996.8 uses a mechanical device consisting of retaining plates, limiting mechanisms, and fixing mechanisms; CN202322286184.X uses a complex structure composed of vertical retaining walls, diagonal supports (or triangular platforms), and piers; CN202321858184.6 uses retaining walls and internal bracing structures. However, the internal bracing in these schemes occupies space within the foundation pit, affecting the construction of the main structure. Especially in hydropower projects, where the foundation pit serves as both the boundary of the main structure and a temporary construction passage, the impact of complex support structures is even more pronounced. Furthermore, in practical implementation, some building projects have large foundation pits that lack the conditions for horizontal bracing, making it impossible to use the aforementioned composite support system with internal bracing. Summary of the Invention

[0005] In view of this, the present invention provides a composite support structure for deep foundation pits with overburden and a design and construction method. The composite support structure has a simple layout and has the combined functions of retaining soil and water during construction and retaining soil, water, preventing seepage and erosion during operation. Moreover, it does not require internal support and can solve the problems of complex structure, occupying construction space in the foundation pit and high cost of the prior art.

[0006] This invention is achieved through the following technical solution: a composite support structure for deep foundation pits with overburden, comprising an ultra-thick continuous wall and prestressed anchor cables; The ultra-thick continuous wall is composed of alternating first unit wall and second unit wall. The first unit wall is formed by pouring concrete into a sequential groove in which a steel cage component is pre-placed. Two adjacent sequential grooves are arranged alternately. The end faces of the first unit wall are concave curved surfaces. The second unit wall is formed by pouring concrete into the second-order slots that are pre-placed in the steel cage component. Each second-order slot is placed between two adjacent first-order slots. The end faces of the second unit wall are convex curved surfaces. The convex curved surfaces and the concave curved surfaces are interlocked and connected. A positioning pipe is pre-embedded in the upper part of the steel cage component. The positioning pipe extends downward from the inside of the foundation pit toward the outer stratum and penetrates the steel cage component. The prestressed anchor cable is inserted through the positioning pipe. The anchoring end of the prestressed anchor cable is anchored to the outer stratum, and the tensioning end of the prestressed anchor cable is fixed to the side of the ultra-thick continuous wall facing the foundation pit.

[0007] Furthermore, the wall thickness of the ultra-thick continuous wall is 1.5-2.0m.

[0008] Furthermore, the steel cage component is assembled from one or more steel cage units; the steel cage component is reinforced according to elevation zones, and the difference in total reinforcement weight between the backwater side and the frontwater side of the steel cage component does not exceed the design allowable value.

[0009] A construction method for a composite support structure for deep foundation pits with overburden layers as described above, the construction method comprising the following steps: S101: The first trench is constructed in the soil layer outside the foundation pit. Two adjacent first trenches are arranged at a certain distance. The steel cage component is hoisted and sunk into the first trench. The positioning pipe pre-embedded in the upper part of the steel cage component is equipped with sealing plates at both ends. S102: Connecting pipes are arranged at both ends of the steel cage component in the first-order slot. Concrete is poured in the first-order slot and outside the connecting pipes. After the concrete enters the setting state, the connecting pipes set at both ends of the steel cage component are rotated and pulled out of the slot. S103: After the concrete of the first-order trench reaches the design strength, a first unit wall with protrusions at both ends is formed. The soil between the adjacent first-order trenches is excavated to construct the second-order trench. The protrusions are removed so that the two ends of the first unit wall form an inward curved surface structure that is recessed towards the middle of the first-order trench. The inward curved surface structure located at both ends of the second-order trench is the boundary of the two ends of the second-order trench. S104: The steel cage component is hoisted and sunk into the second-order trench. The positioning pipe pre-embedded in the upper part of the steel cage component is equipped with sealing plates at both ends. Concrete is poured into the second-order trench to form a second unit wall with the steel cage component. After the concrete in the second-order trench reaches the design strength, the two ends of the second unit wall form an outward convex curved surface structure that interlocks with the concave curved surface structure, so that the first unit wall and the second unit wall are alternately connected to form an ultra-thick continuous wall. S105: Excavate the foundation pit until the positioning pipe is exposed, then continue excavating to the prestressed anchor cable operation elevation; S106: Remove the sealing plate at one end of the positioning pipe inside the foundation pit, drill through the positioning pipe, and after cleaning the hole, install the prestressed anchor cable. The prestressed anchor cable passes through the positioning pipe, and its anchoring end is anchored in the outer stratum. The tensioning end of the prestressed anchor cable is fixed to the side of the ultra-thick continuous wall facing the foundation pit. Apply initial tension force to the prestressed anchor cable, and seal the anchor after tensioning and locking.

[0010] Furthermore, in S101, the two ends of the first-order groove are outwardly convex semi-circular structures protruding towards the second-order groove, and the diameter of the circle corresponding to the outwardly convex semi-circular structure is the thickness of the ultra-thick continuous wall. In S102, the connector pipe is located on the outer side of the end of the reinforcing cage component, and the side of the connector pipe away from the reinforcing cage component is tangent to the convex semi-circular structure.

[0011] Furthermore, in S103, the concave curved surface structure at both ends of the first unit wall is a circular arc surface, and the diameter of the circle corresponding to the circular arc surface is the thickness of the ultra-thick continuous wall; the opening end of the concave curved surface structure faces the second-order groove, and the line connecting the two endpoints of its opening end serves as the boundary line of the lowering position of the steel cage component in the second-order groove.

[0012] A design method for a composite support structure for deep foundation pits with overburden layers as described above, the design method comprising the following steps: S201: Based on the excavation depth and geological conditions of the foundation pit, determine the various working conditions of the composite support structure during the construction and operation phases; propose the initial values ​​of the thickness and anchorage depth of the ultra-thick continuous wall, as well as the number of rows, spacing, and initial values ​​of the prestressed anchor cables. S202: Calculate the internal forces, displacements, and prestressed anchor cable tension of the ultra-thick continuous wall under various construction and operation conditions, and determine the most unfavorable conditions for controlling the wall top displacement, wall bending moment, anchor cable tension, and lateral pressure stress of the anchorage section respectively. For all the most unfavorable working conditions, the following constraints are used: the maximum lateral compressive stress of the anchorage section is not greater than the lateral allowable bearing capacity of the foundation; the displacement of the top of the wall does not exceed the deformation control standard determined based on the safety level of the foundation pit; the absolute difference between the maximum bending moment M+ on the open side of the wall and the maximum bending moment M- on the back side of the wall is not greater than the design allowable value; the total amount of reinforcement in the steel cage component does not exceed the reinforcement; and the tension of the prestressed anchor cable does not exceed the bearing capacity of the selected grade of anchor cable. The initial values ​​of each parameter are iteratively adjusted to obtain multiple sets of candidate parameter schemes that meet all the constraints. With the goal of minimizing project investment, the thickness of the ultra-thick continuous wall, the anchorage depth, the number of rows of prestressed anchors, the spacing between rows, and the final value of prestress are determined from the multiple sets of candidate parameter schemes. S203: Based on the final values ​​of each parameter, calculate the structural internal forces and prestressed anchor cable tension of the ultra-thick continuous wall under various working conditions, draw the bending moment envelope diagram of the ultra-thick continuous wall, and determine the bending moment distribution range of the free face and the bending moment distribution range of the back face of the ultra-thick continuous wall. S204: Based on the distribution range of bending moments on the free face and the back face, the steel cage components are segmented and reinforced according to elevation zones, and steel cage units that constitute the steel cage components are fabricated according to the reinforcement.

[0013] Furthermore, in S202, the deformation control standard is 0.005h-0.01h, where h is the total height of the ultra-thick continuous wall; the initial value of the anchoring depth is 1 / 4-1 / 3 of the height of the ultra-thick continuous wall.

[0014] Furthermore, in S204, the segmented reinforcement specifically includes: The steel cage member is divided into several reinforcement sections along the elevation. The amount of reinforcement on the free side and the back side of each reinforcement section is calculated according to the maximum bending moment of that section. The difference between the cumulative amount of reinforcement on the free side and the cumulative amount of reinforcement on the back side of each reinforcement section at the entire elevation shall not exceed the design allowable value.

[0015] Furthermore, in S204, for high bending moment sections where the bending moment exceeds the predetermined threshold, light rail steel is used to replace steel bars with equal cross-sectional area; light rail steel is connected to each other through standard connecting plates, and light rail steel and steel bars are lapped together according to the reserved lap length in the specification.

[0016] Compared with existing technologies, the beneficial effects of this invention are: 1. The composite support structure of the present invention adopts a composite structure of ultra-thick continuous wall and prestressed anchor cable. The ultra-thick continuous wall is formed by interlocking the first unit wall and the second unit wall to form an integral retaining wall. The positioning pipes with inclined penetrations are pre-embedded on the steel cage components for the installation of prestressed anchor cables. Soil retention and seepage prevention can be achieved without setting up internal support in the foundation pit, freeing up the construction space inside the foundation pit. It is especially suitable for permanent-temporary combined support conditions of deep foundation pits with hydropower cover.

[0017] 2. The thickness of the ultra-thick continuous wall in this invention is 1.5 mm. At 2.0m, compared to conventional diaphragm walls, the overall rigidity of the wall is significantly improved. It is equipped with a matching curved interlocking special joint technology and a zoned reinforcement method for the steel cage. At the same time, the prestressed anchor cables are used to share the load and partially replace the load-bearing capacity of the wall anchorage section. It can adapt to the huge earth pressure of deep foundation pits under the overburden stratum, effectively reduce wall deformation, and ensure the overall safety of the support system.

[0018] 3. The construction method of the present invention achieves reliable interlocking of unit walls by first constructing a first-order trench, chiseling away the protruding part at the end to form a concave curved surface, and then pouring a second-order trench to form an interlocking joint. The joint has good anti-seepage and shear resistance performance, and the process is suitable for the on-site trenching and hoisting construction conditions of thick underground continuous walls.

[0019] 4. The design method of this invention comprehensively considers various working conditions in the construction and operation stages, sets multiple constraints such as the bearing capacity of the anchorage section foundation, wall top deformation, bending moment matching, no over-reinforcement, and anchor cable bearing capacity, and determines the final value of the parameters by comparing multiple sets of candidate parameter schemes with the goal of minimizing project investment, thus taking into account both the safety of the support structure and the economy of the project.

[0020] 5. The design method of this invention clearly provides the wall top deformation control index and the initial value selection range of the anchoring depth that are suitable for this support system, providing constraints for the initial parameter estimation and iterative verification of ultra-thick continuous walls and anchor cable composite support.

[0021] 6. The design method of this invention divides the steel cage components into zones along the elevation, controls the difference in cumulative reinforcement weight between the backwater side and the water-facing side across the entire elevation, which not only meets the stress requirements of sections at different elevations, but also ensures the overall weight balance of the steel cage components, and improves the feasibility of hoisting and lowering the steel cage. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the construction of the first unit wall of the present invention.

[0023] Figure 2 This is a schematic diagram of the construction of the second unit wall of the present invention.

[0024] Figure 3 This is a schematic diagram showing the relative positions of the prestressed anchor cable and the reinforcing cage component of the present invention.

[0025] Among them, 1-reinforcing cage component, 11-first-order reinforcing cage, 12-second-order reinforcing cage; 2-positioning pipe; 3-prestressed anchor cable; 4-joint pipe; 5-first-order groove; 6-second-order groove; 7-protrusion. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0027] Example 1 This embodiment provides a composite support structure for deep foundation pits with overburden, such as... Figures 1 to 3 As shown, the composite support structure includes an ultra-thick diaphragm wall and prestressed anchor cables 3. The thickness of the ultra-thick diaphragm wall is 1.5 mm. The 2.0m ultra-thick continuous wall is composed of alternating interlocking first and second unit walls.

[0028] The first unit wall is formed by pouring concrete into the first-order groove 5 in which the pre-placed steel cage component 1 is placed. A gap is reserved between two adjacent first-order grooves 5 for subsequent construction of the second-order groove 6. The end faces of the first unit wall are concave curved surface structures in the shape of arcs. The diameter of the corresponding circle of the arc is equal to the wall thickness of the ultra-thick continuous wall. The opening of the concave curved surface structure faces the second-order groove 6.

[0029] The second unit wall is formed by pouring concrete into the second-order groove 6 in which the steel cage component 1 is pre-placed. Each second-order groove 6 is placed between two adjacent first-order grooves 5. The end faces of the second unit wall are convex curved surfaces. The convex curved surface of the second unit wall and the concave curved surface of the first unit wall are interlocked and connected to each other, so as to realize the alternating connection of the first unit wall and the second unit wall to form an integral ultra-thick continuous wall.

[0030] The steel cage component 1 is assembled from at least one steel cage unit. For ease of explanation below, the steel cage component 1 placed in the first-order slot 5 is called the first-order steel cage 11, and the steel cage component 1 placed in the second-order slot 6 is called the second-order steel cage 12.

[0031] A positioning pipe 2, which is a steel pipe, is pre-embedded in the upper part of the reinforcing cage component 1. The positioning pipe 2 is arranged inclined downwards along the inner side of the foundation pit towards the outer stratum, and it penetrates the reinforcing cage component 1. Both ends of the positioning pipe 2 are equipped with sealing plates. The sealing plates are used to seal the opening of the positioning pipe 2 when pouring the wall concrete to prevent concrete mortar from entering the cavity of the positioning pipe 2. The sealing plates on the side of the foundation pit can be removed later to allow for anchor cable drilling operations.

[0032] The prestressed anchor cable 3 is inserted inside the positioning tube 2. The anchoring end of the prestressed anchor cable 3 is anchored in the outer stratum, and the tensioning end of the prestressed anchor cable 3 is fixed on the side of the ultra-thick continuous wall facing the foundation pit.

[0033] The steel cage component 1 is divided into multiple elevation reinforcement sections along the height direction. A zoned reinforcement design is adopted. The total reinforcement weight of the back water side and the front water side of the steel cage component 1 is basically the same. In specific implementation, the difference in the total reinforcement weight of the back water side and the front water side of the steel cage component 1 does not exceed the design allowable value.

[0034] As a further preferred option of this scheme, for high bending moment sections with large wall bending moments, light rail steel with the same cross section is used to replace some of the ordinary reinforcing bars; the light rail steel is connected with standard connecting plates, and the light rail steel and ordinary reinforcing bars are lapped according to the current specifications to solve the problem of insufficient concrete compaction caused by excessively dense reinforcement in high bending moment sections.

[0035] Example 2 This embodiment is used for the construction of the composite support structure for deep foundation pits with overburden in Embodiment 1. The thickness of the ultra-thick diaphragm wall in Embodiment 1 is 1.5-2.0m, while the design thickness of diaphragm walls in China is generally 0.3m-1.2m. This embodiment provides a construction method for ultra-thick wall structures, combined with... Figures 1 to 3 As shown, it includes the following steps: S101: On the soil layer outside the foundation pit, trenching equipment is used to construct the first trench 5. The two ends of the first trench 5 are convex semi-circular structures that protrude towards the second trench 6. The diameter of the circle corresponding to the convex semi-circular structure is the thickness of the ultra-thick continuous wall. The middle area between the two ends of the first trench is a straight section. The width of the straight section is the thickness of the ultra-thick continuous wall. A gap is reserved between adjacent first-order trenches 5, which is used for the later excavation of second-order trenches 6. The first-order steel cage 11 is hoisted and lowered into the first-order trench 5; the positioning pipe 2 embedded in the first-order steel cage 11 is pre-installed with sealing plates at both ends.

[0036] S102: In the first-order groove 5, joint pipes 4 are respectively placed on the outer sides of both ends of the first-order steel cage 11. The joint pipes 4 are placed vertically and their height is not lower than that of the first-order steel cage 11. The outline of the side of the joint pipe 4 away from the first-order steel cage 11 is tangent to the outward convex semi-circular outline of the end of the first-order groove 5. Concrete is poured inside the first-order groove 5 and outside the joint pipe 4; after the concrete has set, the joint pipe 4 is rotated and pulled out of the first-order groove 5.

[0037] S103: After the concrete in the first-order trench 5 reaches the design strength, the concrete and the first-order steel cage 11 form the original state of the first unit wall. The original state of the first unit wall is the first unit wall with the protrusion 7. The protrusion 7 is located on both sides of the first unit wall. The sum of the areas occupied by the protrusion 7 and the joint pipe 4 is equal to or less than the inner area of ​​the outer semi-circular structure of the first-order trench 5.

[0038] The soil between two adjacent first unit walls is excavated to form a second-order trench 6. The middle part of the second-order trench 6 is a straight section, and the width of the straight section is the thickness of the ultra-thick continuous wall. The two ends of the second-order trench 6 are connected to the adjacent first-order trench 5.

[0039] Subsequently, the protruding portion 7 of the original state of the first unit wall is removed, resulting in concave curved surface structures at both ends of the first unit wall that face inward toward the center of the wall, thus forming the final state of the first unit wall. In this embodiment, the concave curved surface structure of the first unit wall is an arc surface, and the diameter of the circle corresponding to the arc surface is the thickness of the ultra-thick continuous wall.

[0040] After removing the protrusion 7, the final secondary groove 6 is formed simultaneously. At this time, the two ends of the secondary groove 6 are convex curved surface structures. The opening end of the convex curved surface structure is connected to the straight section of the secondary groove 6. In this embodiment, the convex curved surface structure is a semi-circular arc surface, and the diameter of the circle corresponding to the semi-circular arc surface is the thickness of the ultra-thick continuous wall. That is to say, the two ends of the final secondary groove 6 are convex semi-circular structures protruding towards the primary groove 5, and the middle area between the two ends of the secondary groove 6 is a straight section.

[0041] During the above process, the semi-circular protrusions 7 at both ends of the first unit wall are removed. The opening end of the concave curved surface structure faces the second-order groove 6. The line connecting the two endpoints of the opening end of the concave curved surface structure serves as the boundary line of the lowering position of the steel cage component 1 in the second-order groove 6.

[0042] S104: The second-order steel cage 12 is hoisted and lowered into the second-order trench 6; similarly, sealing plates are set at both ends of the top positioning pipe 2 of the second-order steel cage 12; concrete is poured into the second-order trench 6, the concrete encapsulates the second-order steel cage 12 and fills the internal space of the concave curved surface structure of the first unit wall on both sides; the concrete hardens and forms the second unit wall, and the two ends of the second unit wall form an outward convex curved surface structure that interlocks with the concave curved surface of the first unit wall, thus completing the ultra-thick continuous wall with alternating arrangement and connection of the first unit wall and the second unit wall.

[0043] S105: Carry out layered excavation of the foundation pit, excavate until the positioning pipe 2 is exposed, and continue to excavate downwards to the elevation where the prestressed anchor cable operation will be carried out.

[0044] S106: Remove the sealing plate at one end of the positioning pipe 2 inside the foundation pit; drill through the cavity of the positioning pipe 2 to the outer stratum, and install the prestressed anchor cable 3 after cleaning the hole; the prestressed anchor cable 3 passes through the positioning pipe 2, the anchoring end of the prestressed anchor cable 3 is anchored to the outer stratum, and the tensioning end of the prestressed anchor cable 3 extends out of the ultra-thick continuous wall towards the foundation pit side; apply initial tension force to the prestressed anchor cable 3, and after tensioning and locking, carry out the anchor sealing operation.

[0045] Example 3 This embodiment 3 is a design method for the composite support structure of deep foundation pit with overburden layer described in embodiment 1. The design method includes the following steps.

[0046] S201: Based on the excavation depth of the foundation pit and the on-site geological survey data, determine all working conditions that the support structure needs to withstand during the construction and operation stages; initially propose the initial values ​​of the wall thickness and anchorage depth of the ultra-thick continuous wall, as well as the number of rows, spacing, and initial prestress of the prestressed anchor cables 3; in this embodiment, the initial value of the anchorage depth is taken as 1 / 4 to 1 / 3 of the height of the ultra-thick continuous wall.

[0047] S202: Calculate the internal forces, wall displacement, and prestressed anchor cable tension of the ultra-thick continuous wall under various working conditions during the construction and operation stages; determine the most unfavorable working conditions for controlling the wall top displacement, wall bending moment, anchor cable tension, and lateral pressure stress of the anchorage section.

[0048] For all the most unfavorable operating conditions, the following conditions are used as iterative constraints: The maximum lateral compressive stress in the anchorage section shall not exceed the allowable lateral bearing capacity of the foundation. The displacement of the wall top shall not exceed the deformation control standard corresponding to the safety level of the foundation pit. In this embodiment, the deformation control standard is taken as 0.005h. 0.01h, where h is the total height of the ultra-thick continuous wall; Maximum bending moment M on the open side of the wall + maximum bending moment M on the back side of the wall The absolute values ​​are roughly the same. In specific implementation, the maximum bending moment M+ on the open side of the wall and the maximum bending moment M on the back side of the wall are approximately the same. The absolute difference does not exceed the design allowable value; The reinforcement of steel cage member 1 does not exceed the allowable reinforcement limit; The tension of the prestressed anchor cable 3 shall not exceed the bearing capacity of the selected grade anchor cable.

[0049] The initial parameters were iteratively adjusted to obtain multiple candidate parameter schemes that satisfied all constraints. With the minimum project investment as the optimization objective, the candidate parameter schemes were selected to determine the final values ​​of wall thickness, anchorage depth, number of anchor cable rows, anchor cable spacing, and anchor cable prestress.

[0050] As a further preferred embodiment, in the process of determining the prestress of the anchor cable, the wall thickness, the number of anchor cable rows and the spacing between rows are kept constant, and the initial tension of the anchor cable is changed to carry out structural calculations multiple times. The corresponding relationship curves of the initial tension of the anchor cable, the wall stress and the reinforcement area are established. Combined with various safety constraints, the optimal initial locking value of the prestressed anchor cable 3 is obtained, which is used to help determine the final prestress value of the prestressed anchor cable 3.

[0051] S203: Using the final values ​​of the parameters determined in the previous step, perform structural calculations for each working condition again to obtain the internal forces of the ultra-thick continuous wall structure and the anchor cable tension results. Draw the bending moment envelope diagram of the ultra-thick continuous wall along the elevation direction and determine the bending moment distribution range of the wall's open face and back face respectively.

[0052] S204: Based on the bending moment distribution range of the free face and the back face, the steel cage member 1 is divided into several reinforcement segments along the elevation. Within each reinforcement segment, the reinforcement amount for the free face and the back face is calculated according to the maximum bending moment of that segment. Within the entire elevation range, the cumulative reinforcement amount of each segment on the free face is basically equivalent to the cumulative reinforcement amount on the back face. In specific implementation, this difference does not exceed the design allowable value. After completing the reinforcement calculation, the corresponding steel cage units are fabricated and assembled to obtain the steel cage member 1.

[0053] When the reinforcement is segmented, if the bending moment of a certain segment exceeds the preset threshold, light rail steel is used to replace part of the ordinary steel bars with equal cross-sectional area. The light rail steel bars are connected with standard connecting clamps, and the light rail steel bars and ordinary steel bars are lapped according to the specifications.

[0054] This invention provides a composite support structure for deep foundation pits with overburden layers, along with its construction and design methods. The composite support structure is composed of an ultra-thick diaphragm wall and adjustable prestressed anchor cables. The ultra-thick diaphragm wall (1.5-2.0m thick) serves as a rigid core, bearing over 80% of the horizontal earth pressure, controlling the horizontal displacement of the foundation pit, and simultaneously achieving water stoppage (permeability coefficient ≤10). - 7 (cm / s). Adjustable prestressed anchor cable 3 serves as a flexible supplement. By applying prestress, the anchor cable penetrates deep into stable strata, balances the remaining earth pressure, and inhibits soil creep. Prestressed anchor cable 3 can replace part of the continuous wall embedment section, reducing costs by 20%-30%.

[0055] Application Examples For example, in a large-scale hydropower project located in an urban area, influenced by traffic and urban development on both banks, the foundation pit adopts a vertical excavation and support method. This support method serves as temporary support during construction and as a permanent structure for flow passage and seepage prevention during operation. During construction, from ground excavation to the designed bottom of the foundation pit, the stress on the support structure gradually increases to its maximum at this stage; the stress decreases when the flow passage bottom slab structure is completed. During operation, the stress on the support structure may increase or decrease under the combined action of water pressure on the flow passage side and groundwater and soil pressure on the mountain side. To achieve a combination of temporary and permanent support, this vertical support structure must be able to adapt to deformation and internal force changes caused by different actions under both conditions.

[0056] Based on the aforementioned boundaries, this project adopts the composite support structure of ultra-thick reinforced concrete diaphragm wall and adjustable prestressed anchor cable proposed in this invention. The specific structural design and construction method are as follows: The working conditions that the support structure may encounter during the construction and operation phases are analyzed. Based on the depth of the foundation pit, the thickness of the ultra-thick diaphragm wall is initially estimated using empirical formulas. Based on engineering experience, the anchoring depth is initially proposed. Combining the foundation pit depth, geological conditions, and engineering experience, the anchor cable layout elevation, spacing, initial prestress, and anchor cable grade are initially proposed. The width of the diaphragm wall unit wall is determined based on the anchor cable spacing.

[0057] Based on existing anchor pile design theory and current code requirements, the internal forces, wall top displacement, and anchor cable tension of the diaphragm wall under various working conditions are calculated to preliminarily determine the most unfavorable working condition. By controlling the structural reinforcement to prevent over-reinforcement, the range of acceptable wall thicknesses is determined. Combining existing construction technology, 2-3 wall thicknesses are initially proposed, and structural calculations are performed for each.

[0058] The anchorage depth of the wall is determined by controlling the maximum lateral compressive stress of the anchorage section to not exceed the lateral allowable bearing capacity of the foundation; the spacing between anchor cables and the initial prestress are determined by controlling the wall top displacement to not exceed 0.005h and the absolute values ​​of M+ on the exposed side and M- on the back side of the wall to be approximately equal; based on the geological conditions, the anchor cable grade used in this project should not exceed 3000kN, and the spacing between anchor cables is determined by controlling the anchor cable tension to not exceed the bearing capacity of the selected grade anchor cable and the section control stress requirements.

[0059] Based on the above calculation results of anchor cables and steel bars corresponding to different wall thicknesses, the concrete volume, anchor cable volume and steel bar volume were initially calculated, and the investment was calculated. With the minimum investment as the optimization objective, the wall thickness was finally determined to be 2m. The wall was constructed using C35 underwater concrete. The average rock penetration depth of the anchor cable section of the protective seepage prevention wall was 6m. Three rows of 2000kN prestressed anchor cables were designed, with initial horizontal tension of 1050kN, 900kN and 700kN for the three rows of anchor cables, respectively.

[0060] After determining the above parameters, the internal forces and anchor cable tensions under each working condition were calculated, and the bending moment envelope diagram was drawn. The type of steel reinforcement or steel material was determined by ensuring the clear spacing of the reinforcement bars was not less than the required value. Based on the standard length of the reinforcement bars or steel material and the bending moment distribution on both sides, the amount of reinforcement bars or steel material arranged along the entire length on one side and in segments (two to four segments) was initially determined. Considering the symmetrical and uniform position of the continuous reinforcement bars or steel material on both sides, the guarantee of the overall stiffness of the steel cage during hoisting, and the roughly equal total weight of the reinforcement bars on both sides, the final amount of reinforcement bars or steel material on both sides of the steel cage component was determined. The final scheme uses 25 Q30 light rails for the tension zone, 4 Q30 light rails and 9 C36 steel bars for the compression zone, and 6C16@400 shear reinforcement.

[0061] Then, based on the anchor cable spacing and existing diaphragm wall construction equipment, the width of the trench for each diaphragm wall section was determined, and construction was carried out in two phases. The 2m thick diaphragm wall consists of 56 sections, constructed in two phases. Each section is 6m long. The main reinforcement of the steel cage uses steel rails and steel bars. The first section of the steel cage is approximately 38m long and weighs approximately 95 tons.

[0062] Drilling equipment such as CZ-7 impact drills were arranged at the boundary of the foundation pit. The hammer assembly was a modified and welded UKA series. At the same time, the first-order trench 5 was constructed. After the trench reached the design elevation, the hole was cleaned and, after meeting the specifications, one or more steel cage units of the first-order steel cage 11 (with positioning pipe 2 fixed in the steel cage) were hoisted, spliced, and sunk in sequence. The steel cage installation was carried out using SC2800 crawler cranes and SC3200 crawler cranes. Concrete was poured, and when the concrete entered the setting state, the joint pipes 4 on both sides of the first-order steel cage 11 were rotated and pulled out.

[0063] After the concrete strength of the first-order trench 5 reaches the design strength, the second-order trench 6 is formed between the existing concrete walls using drilling equipment. The concrete parts (protruding parts 7) at both ends of the first-order trench 5 are removed to form a concave semi-circular curved surface structure with a diameter equal to the wall thickness. After cleaning the hole, one or more steel cage units of the second-order steel cage 12 (positioning pipe 2 is fixed in the steel cage) are hoisted, spliced ​​and sunk in sequence; concrete is poured.

[0064] Finally, after the concrete strength of the second-order trench 6 reaches the design strength, the foundation pit excavation is carried out. Excavation continues until the elevation of the positioning pipe 2 is set and exposed, and then continues to the elevation of the prestressed anchor cable 3. The foundation pit is excavated to an elevation of 571m, at which point excavation is stopped, and the construction of the three rows of anchor cables is carried out. After the anchor cables are tensioned, excavation continues to the design elevation of 556m. The sealing plate of the exposed end of the positioning pipe 2 is removed, and a drilling rig is used to drill through the positioning pipe 2. After the hole is cleaned, the prestressed anchor cable 3 is installed, the initial tension is applied, the anchor is sealed, and the subsequent foundation pit excavation continues.

[0065] After completion, it successfully withstood the most dangerous conditions during the excavation of the foundation pit, with a maximum vertical height of 32m. To date, no cracks or seepage points have been observed on the exposed wall surface, nor have any scouring pits been found. Monitoring data shows that the deformation of the ultra-thick continuous wall as a protective seepage barrier conforms to general laws, and both the deformation value and the stress of the reinforcing steel are within the design allowable range.

[0066] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A composite support structure for deep foundation pits with overburden, characterized in that, Including ultra-thick continuous walls and prestressed anchor cables; The ultra-thick continuous wall is composed of alternating first unit wall and second unit wall. The first unit wall is formed by pouring concrete into a sequential groove in which a steel cage component is pre-placed. Two adjacent sequential grooves are arranged alternately. The end faces of the first unit wall are concave curved surfaces. The second unit wall is formed by pouring concrete into the second-order slots that are pre-placed in the steel cage component. Each second-order slot is placed between two adjacent first-order slots. The end faces of the second unit wall are convex curved surfaces. The convex curved surfaces and the concave curved surfaces are interlocked and connected. A positioning pipe is pre-embedded in the upper part of the steel cage component. The positioning pipe extends downward from the inside of the foundation pit toward the outer stratum and penetrates the steel cage component. The prestressed anchor cable is inserted through the positioning pipe. The anchoring end of the prestressed anchor cable is anchored to the outer stratum, and the tensioning end of the prestressed anchor cable is fixed to the side of the ultra-thick continuous wall facing the foundation pit.

2. The composite support structure for deep foundation pits with overburden layer as described in claim 1, characterized in that, The thickness of the ultra-thick continuous wall is 1.5-2.0m.

3. The composite support structure for deep foundation pits with overburden layer as described in claim 1 or 2, characterized in that, The steel cage component is assembled from one or more steel cage units; the steel cage component is reinforced according to elevation zones, and the difference in total reinforcement weight between the backwater side and the frontwater side of the steel cage component does not exceed the design allowable value.

4. A construction method for a composite support structure for deep foundation pits with overburden as described in any one of claims 1 to 3, characterized in that, The construction method includes the following steps: S101: The first trench is constructed in the soil layer outside the foundation pit. Two adjacent first trenches are arranged at a certain distance. The steel cage component is hoisted and sunk into the first trench. The positioning pipe pre-embedded in the upper part of the steel cage component is equipped with sealing plates at both ends. S102: Connecting pipes are arranged at both ends of the steel cage component in the first-order slot. Concrete is poured in the first-order slot and outside the connecting pipes. After the concrete enters the setting state, the connecting pipes set at both ends of the steel cage component are rotated and pulled out of the slot. S103: After the concrete of the first-order trench reaches the design strength, a first unit wall with protrusions at both ends is formed. The soil between the adjacent first-order trenches is excavated to construct the second-order trench. The protrusions are removed so that the two ends of the first unit wall form an inward curved surface structure that is recessed towards the middle of the first-order trench. The inward curved surface structure located at both ends of the second-order trench is the boundary of the two ends of the second-order trench. S104: The steel cage component is hoisted and sunk into the second-order trench. The positioning pipe pre-embedded in the upper part of the steel cage component is equipped with sealing plates at both ends. Concrete is poured into the second-order trench to form a second unit wall with the steel cage component. After the concrete in the second-order trench reaches the design strength, the two ends of the second unit wall form an outward convex curved surface structure that interlocks with the concave curved surface structure, so that the first unit wall and the second unit wall are alternately connected to form an ultra-thick continuous wall. S105: Excavate the foundation pit until the positioning pipe is exposed, then continue excavating to the prestressed anchor cable operation elevation; S106: Remove the sealing plate at one end of the positioning pipe inside the foundation pit, drill through the positioning pipe, and after cleaning the hole, install the prestressed anchor cable. The prestressed anchor cable passes through the positioning pipe, and its anchoring end is anchored in the outer stratum. The tensioning end of the prestressed anchor cable is fixed to the side of the ultra-thick continuous wall facing the foundation pit. Apply initial tension force to the prestressed anchor cable, and seal the anchor after tensioning and locking.

5. The construction method of the composite support structure for deep foundation pits with overburden as described in claim 4, characterized in that, In S101, the two ends of the first-order groove are outwardly convex semi-circular structures that protrude toward the second-order groove, and the diameter of the circle corresponding to the outwardly convex semi-circular structure is the thickness of the ultra-thick continuous wall. In S102, the connector pipe is located on the outer side of the end of the reinforcing cage component, and the side of the connector pipe away from the reinforcing cage component is tangent to the convex semi-circular structure.

6. The construction method of the composite support structure for deep foundation pits with overburden as described in claim 4 or 5, characterized in that, In S103, the concave curved surface structure at both ends of the first unit wall is a circular arc surface, and the diameter of the circle corresponding to the circular arc surface is the thickness of the ultra-thick continuous wall; the opening end of the concave curved surface structure faces the second-order groove, and the line connecting the two endpoints of its opening end serves as the boundary line of the lowering position of the steel cage component in the second-order groove.

7. A design method for a composite support structure for deep foundation pits with overburden as described in any one of claims 1 to 3, characterized in that, The design method includes the following steps: S201: Based on the excavation depth and geological conditions of the foundation pit, determine the various working conditions of the composite support structure during the construction and operation phases; propose the initial values ​​of the thickness and anchorage depth of the ultra-thick continuous wall, as well as the number of rows, spacing, and initial values ​​of the prestressed anchor cables. S202: Calculate the internal forces, displacements, and prestressed anchor cable tension of the ultra-thick continuous wall under various construction and operation conditions, and determine the most unfavorable conditions for controlling the wall top displacement, wall bending moment, anchor cable tension, and lateral pressure stress of the anchorage section respectively. For all the most unfavorable working conditions, the following constraints are used: the maximum lateral compressive stress of the anchorage section is not greater than the lateral allowable bearing capacity of the foundation; the displacement of the top of the wall does not exceed the deformation control standard determined based on the safety level of the foundation pit; the absolute difference between the maximum bending moment M+ on the open side of the wall and the maximum bending moment M- on the back side of the wall is not greater than the design allowable value; the total amount of reinforcement in the steel cage component does not exceed the reinforcement; and the tension of the prestressed anchor cable does not exceed the bearing capacity of the selected grade of anchor cable. The initial values ​​of each parameter are iteratively adjusted to obtain multiple sets of candidate parameter schemes that meet all the constraints. With the goal of minimizing project investment, the thickness of the ultra-thick continuous wall, the anchorage depth, the number of rows of prestressed anchors, the spacing between rows, and the final value of prestress are determined from the multiple sets of candidate parameter schemes. S203: Based on the final values ​​of each parameter, calculate the structural internal forces and prestressed anchor cable tension of the ultra-thick continuous wall under various working conditions, draw the bending moment envelope diagram of the ultra-thick continuous wall, and determine the bending moment distribution range of the free face and the bending moment distribution range of the back face of the ultra-thick continuous wall. S204: Based on the distribution range of bending moments on the free face and the back face, the steel cage components are segmented and reinforced according to elevation zones, and steel cage units that constitute the steel cage components are fabricated according to the reinforcement.

8. The design method for the composite support structure of deep foundation pits with overburden as described in claim 7, characterized in that, In S202, the deformation control standard is 0.005h-0.01h, where h is the total height of the ultra-thick continuous wall. The initial value of the anchoring depth is 1 / 4 to 1 / 3 of the height of the ultra-thick continuous wall.

9. The design method for the composite support structure of deep foundation pits with overburden as described in claim 7 or 8, characterized in that, In S204, the segmented reinforcement specifically includes: The steel cage member is divided into several reinforcement sections along the elevation. The amount of reinforcement on the free side and the back side of each reinforcement section is calculated according to the maximum bending moment of that section. The difference between the cumulative amount of reinforcement on the free side and the cumulative amount of reinforcement on the back side of each reinforcement section at the entire elevation shall not exceed the design allowable value.

10. The design method for the composite support structure of deep foundation pits with overburden as described in claim 9, characterized in that, In S204, for high bending moment sections where the bending moment exceeds the predetermined threshold, light rail steel is used to replace steel bars with equal cross-sectional area; light rail steel is connected to each other through standard connecting clamps, and light rail steel and steel bars are lapped together according to the reserved lap length in the specification.

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