Deep foundation pit excavation method applicable to areas surrounded by existing buildings

Abstract

This invention discloses a method for deep foundation pit excavation applicable to areas surrounded by existing buildings, comprising: S1, dividing the area surrounding the foundation pit into high, medium, and low load zones based on the properties of the existing buildings; S2, dividing the total excavation depth of the foundation pit vertically into multiple excavation layers; S3, for each excavation layer, dividing its profile into two asymmetrical wedge-shaped excavation sub-zones according to the distribution of the load zones; S4, starting from the low / medium load zone side, excavating the first wedge-shaped sub-zone to form a temporary slope facing that side; S5, starting from the high load zone side, excavating the second wedge-shaped sub-zone in layers and blocks; S6, after the first layer of excavation in the second wedge-shaped sub-zone, installing a horizontal support system at the top of the foundation pit; and installing an anchor cable system on the foundation pit wall when the excavation depth reaches a preset value; S7, repeating steps S3 to S6 until the base elevation is reached. This invention relates to the field of foundation pit construction technology and can solve the technical problems existing in the prior art.

Description

Technical Field

[0001] This invention relates to the field of foundation pit construction technology, and in particular to a method for deep foundation pit excavation in areas surrounded by existing buildings. Background Technology

[0002] With the deepening of urbanization, urban construction land is becoming increasingly scarce. Many new construction projects inevitably involve deep foundation pits located adjacent to existing buildings, municipal roads, and underground pipelines, creating a complex construction environment surrounded by existing structures. In such a complex environment, accurately coordinating and controlling the deformation of the foundation pit, ensuring its stability, and minimizing the impact of construction on the surrounding environment have become core challenges that urgently need to be addressed in the field of geotechnical engineering.

[0003] Currently, for such deep foundation pit projects, the conventional approach is to use symmetrical, layered, and segmented excavation methods, supplemented by pile foundations combined with prestressed anchor cables or internal bracing. The design concept typically treats the surrounding environment of the foundation pit as homogeneous or simply zoned, and the excavation sequence often follows the principles of layering, symmetry, and balance. The determination of support parameters is also often based on overall safety factor considerations, lacking a refined and differentiated response to specific surrounding building complexes.

[0004] The aforementioned existing technologies have revealed many shortcomings in practical applications, making it difficult to meet the high-standard control requirements in complex environments. These shortcomings are manifested in the following five aspects:

[0005] (1) Existing technologies rely on engineers’ qualitative experience to make rough judgments on the load impact of buildings around the foundation pit. They have failed to establish a quantitative assessment system that integrates multiple factors such as building height, structural type, foundation form and distance, and cannot accurately identify high-risk areas. This results in insufficient targeting of subsequent design and construction, leaving safety hazards.

[0006] (2) Traditional symmetrical and balanced excavation methods are beneficial for controlling the overall displacement of the foundation pit, but they cannot actively guide and pre-set the direction of deformation. Under conditions of severe asymmetry in the surrounding loads, stress may be concentrated and released at unexpected locations, posing a threat to existing buildings in the surrounding area.

[0007] (3) Large-scale full-scale excavation can easily cause large-scale stress release and soil disturbance. In conventional layered excavation, the determination of single-layer thickness and excavation form lacks matching with the support system and environmental risks, which increases the uncontrollability of deformation.

[0008] (4) Existing support designs, whether anchor cable spacing or prestress, usually use uniform parameters around the foundation pit. In high load areas, insufficient support strength may lead to excessive deformation, while in low load areas, excessive support may cause economic waste.

[0009] Therefore, there is a need for a deep foundation pit excavation method applicable to areas surrounded by existing buildings, which can solve the above-mentioned technical problems. Summary of the Invention

[0010] The purpose of this invention is to provide a method for deep foundation pit excavation in areas surrounded by existing buildings, which can solve the above-mentioned technical problems.

[0011] This invention is implemented as follows:

[0012] A method for deep foundation pit excavation in areas surrounded by existing buildings includes the following steps:

[0013] S1. Based on the properties of the existing buildings around the foundation pit, the area around the foundation pit is divided into three load zones: high, medium, and low.

[0014] S2. Divide the total excavation depth of the foundation pit into multiple excavation layers along the vertical direction;

[0015] S3. For each excavation layer, based on the distribution of the load zone, its profile is divided into two asymmetrical wedge-shaped excavation sub-zones, namely the first wedge-shaped sub-zone and the second wedge-shaped sub-zone.

[0016] S4. Starting from the low / medium load zone side, excavate the first wedge-shaped sub-zone to form a temporary slope facing that side;

[0017] S5. Starting from the high-load area side, the second wedge-shaped sub-area is excavated in layers and blocks;

[0018] S6. After the first layer of excavation in the second wedge-shaped sub-zone, a horizontal support system is installed at the top of the pit; and when the excavation depth reaches the preset value, an anchor cable system is installed on the pit wall.

[0019] S7. Repeat steps S3 to S6 until the base elevation is reached.

[0020] Step S1 includes the following sub-steps:

[0021] Step S11: The zoning of the load zone needs to be based on factors such as the height, structural type, foundation type, and distance from the edge of the existing buildings around the foundation pit, and determined by the load influence coefficient K.

[0022]

[0023] In the formula: K is the load influence coefficient; H is the height of the existing buildings in the area; H0 is the reference height, which is 1.5 to 2.5 times the excavation depth h of the foundation pit, i.e., H0 = (1.5 to 2.5)h; D0 is the reference distance, which is 0.5 to 1.0 times the excavation depth h of the foundation pit, i.e., D0 = (0.5 to 1.0)h; T is the structural system influence coefficient; F is the foundation form influence coefficient; D is the minimum distance between the building and the edge of the foundation pit in the area; α, β, λ, and δ are weighting coefficients.

[0024] S12. Along the perimeter of the foundation pit, an assessment section ΔS is defined as 20-30m. i The K value of each evaluation section is calculated sequentially. When K ≥ 2.0, the evaluation section is marked as a high load section; when 1.0 ≤ K < 2.0, the evaluation section is marked as a medium load section; when K < 1.0, the evaluation section is marked as a low load section.

[0025] S13. Let S be the total length of each side of the foundation pit, and let ΔS be the length of each evaluation section on each side. i The calculated K value is marked on the foundation pit plan:

[0026] like Where j is the ΔS corresponding to K≥2.0 on one side of the foundation pit. i If the number of sections is specified, then the area outside that side of the foundation pit is a high-load zone.

[0027] like Where k is the ΔS corresponding to 1.0 ≤ K < 2.0 on one side of the foundation pit. i If the number of sections is specified, then the area outside that side of the foundation pit is a medium-load zone.

[0028] like Where m is the ΔS corresponding to K < 1.0 i If the number of sections indicates that the area outside that side of the foundation pit is a low-load zone, then the area outside the section is a low-load zone.

[0029] The structural system influence coefficient T is determined according to the building's structural type: for brick-concrete / masonry structures, T=0.8; for frame structures, T=1.0; for shear wall structures / frame-shear wall structures, T=1.3; for low-rise steel structures, T=0.9; for high-rise / super high-rise steel structures, T=1.5.

[0030] The foundation type influence coefficient F should be determined according to the foundation type of the building: for isolated foundations / strip foundations, F=1.2; for raft foundations, F=1.0; for pile foundations with pile ends below the excavation surface of the foundation pit, F=0.7; for pile foundations with pile ends above the excavation surface of the foundation pit, F=1.4.

[0031] The values ​​of the weight coefficients α, β, λ, and δ satisfy δ>λ>β>α, α+β+λ+δ=1.

[0032] In step S2, based on the overturning stability control requirements of the foundation pit on one side of the high load area, the area to be excavated in the foundation pit is divided into layers, namely the first excavation layer, the second excavation layer, ..., the nth excavation layer; among them, the thickness of the first excavation layer is the smallest, and as the excavation depth increases, the thickness of each subsequent excavation layer gradually increases.

[0033] The excavation thickness h1 of the first excavation layer is determined according to the following formula:

[0034]

[0035] In the formula: ΣM r The unit width overturning moment provided for the foundation pit support system, in kN·m / m; γ is the unit weight of the soil, in kN / m. 3 ;K q The overload factor is the overload factor for the high load zone. q represents the additional load generated by the building in the high-load area, in kPa; L a B is the equivalent arm of the overturning moment, in meters (m), and is taken as 2 / 3 of the excavation depth h of the foundation pit; B is the width of the foundation pit, in meters (m); F s To ensure a safety factor, a value of 1.3 to 1.5 is adopted.

[0036] The excavation thickness h of the nth excavation layer below the first excavation layer n Add to h1 to satisfy Furthermore, the excavation thickness of each excavation layer shall not exceed 5m.

[0037] In step S3, the excavation area of ​​each excavation layer is further divided into two excavation sub-areas, namely sub-area n-① and sub-area n-②, from the top of the high load area to the bottom of the load area on the opposite side.

[0038] From the cross-section, the starting point is located at the top end of the nth excavation layer on the high load zone side, and the ending point is located at the bottom end of the nth excavation layer on the opposite side of the high load zone. The starting point and the ending point are connected to form a bisecting diagonal. The bisecting diagonal formed by connecting the points on the cross-section is defined as the preset deformation guide surface on the plane.

[0039] From a plan view, the starting line is the edge line of the foundation pit on the side where the nth excavation layer is located in the high load zone, and the ending line is the bottom line of the foundation pit on the opposite side of the high load zone of the nth excavation layer.

[0040] In step S4, the n-① sub-region is excavated using a dual-sided synchronous limited-advance excavation method under three-dimensional spatial grid control. The specific steps are as follows:

[0041] S41. Spatial calibration: The high-load area is marked as area A, the low / medium-load area on the opposite side is marked as area B, and the low / medium-load areas on the adjacent sides are marked as area C and area D respectively.

[0042] S42. Vertical stratification: In the vertical direction, the n-① subregion is divided into several micro-stratifications y. a The thickness of each micro-layer is d, where 1m ≤ d ≤ 2m;

[0043] S43. Horizontal Division: In the horizontal direction, sub-region n-① is divided into several sections x along the direction perpendicular to the edges of the foundation pits in regions C and D. b The width of each section is w, 2m≤w≤4m, thus discretizing the n-① sub-region into a series of gridded excavation units;

[0044] S44. Symmetrical Restricted Excavation: Using the central axis of the foundation pit perpendicular to the edges of the foundation pits in areas A and B as boundaries, excavation is carried out simultaneously and in opposite directions from the edges of the foundation pits in areas C and D respectively; during the excavation process, the excavation is carried out in a step-by-step manner from the side of area B to the side of area A, and from the center of the foundation pit to the direction of the pit wall; when the excavation reaches the intersection of each section with the preset deformation guide surface, the slope of that section is immediately trimmed;

[0045] S45. Circular Progression: The current micro-layer y a All gridded excavation units x b Only after all excavation and slope trimming are completed can the next micro-layer excavation cycle begin, until the entire n-① sub-region is excavated.

[0046] In step S5, the synchronous progressive excavation method based on the extended operation platform is used to excavate the n-② sub-area. The specific steps are as follows:

[0047] S51. Temporary Slope Stratification: The temporary slope is divided into several micro-stratifications according to its vertical height, that is, in the vertical direction, the n-② sub-region is divided into several micro-stratifications y'. a Each micro-layer has a thickness of d', where 1m ≤ d' ≤ 2m;

[0048] In the horizontal direction, each micro-layer y' a Corresponding horizontal excavation width x' b Determined by the following formula:

[0049]

[0050] In the formula: θ is the slope of the temporary slope, which is determined by the width of the foundation pit B and the depth of the current excavation layer h. n Sure, z represents the total number of sub-regions n-②.

[0051] S52. Excavating an extended working platform: Starting from the first micro-layer at the top of the temporary slope, excavate an initial working trench along the direction parallel to the edges of the pits in areas C and D, from the central axis of the pit perpendicular to the edges of the pits in areas A and B. The depth l of the initial working trench is 2m≤l≤4m, and the width s of the initial working trench is s≤5m. The plane formed after the initial working trench is excavated is the extended working platform.

[0052] S53. Two-way synchronous back-to-back excavation: Two excavators are located on the extended working platform and start simultaneously from the central axis of the foundation pit, moving in opposite directions to excavate towards the edges of the foundation pit in areas C and D respectively. During the excavation, micro-temporary support is applied to the newly formed temporary slope facade facing the unexcavated area, following the excavator's advance. When the excavator advances to the edge of the foundation pit in areas C and D, the soil removal within the current depth l range is completed. Subsequently, the excavator returns to the central axis of the foundation pit and extends the depth towards area A, repeating the above operation of excavating the initial working trench - two-way synchronous back-to-back excavation, until the current micro-layer excavation is completed.

[0053] S54. Only after the current micro-layer is completed and a complete working plane is formed can the construction of the next micro-layer be carried out, and steps S52 and S53 are repeated until the entire n-② sub-area excavation is completed.

[0054] In step S6, after the first excavation layer is completed, a reinforced concrete capping beam is immediately constructed along the perimeter of the foundation pit, and a closed horizontal support system is erected on the capping beam.

[0055] The horizontal support system consists of two orthogonal main supports, namely the first main support and the second main support. The first main support is erected between the capping beams of the high load zone A and the low / medium load zone B, and the second main support is erected between the capping beams of the low / medium load zones C and D on both sides. The first and second main supports are intersected and connected in the central area of ​​the foundation pit to form an integral spatial support frame.

[0056] Anchor cable system is installed every 5m of cumulative excavation depth; when the excavation reaches the corresponding elevation, the excavation is immediately stopped, and steel walers are installed along the pit wall at that elevation and the anchor cables are tensioned and locked.

[0057] The arrangement of the anchor cable system is matched with the load zones, and a differentiated design is implemented: in the high load zone, the anchor cables adopt reinforced design parameters, that is, the anchor cable spacing D1=1.2m and the prestress P1=1.3P0, where P0 is the standard prestress design value of the anchor cable; in the medium load zone, the anchor cables adopt standard design parameters, that is, the anchor cable spacing D2=1.5m and the prestress P2=P0; in the low load zone, no anchor cables are installed.

[0058] Compared with the prior art, the present invention has the following advantages:

[0059] 1. This invention establishes a load influence coefficient (K) that integrates multiple factors, transforming fuzzy experience into precise calculations, thereby accurately identifying high, medium, and low risk load zones around the foundation pit. This gives the load zoning a clear mathematical basis and repeatability, ensuring the pertinence, scientificity, and reliability of all subsequent design and construction decisions from the source, and avoiding safety hazards or resource waste caused by misjudging risk levels.

[0060] 2. By setting a preset deformation guiding surface, the present invention actively guides the main stress release and initial deformation to the low-load zone side that allows for a certain displacement at the beginning of the excavation stage, while creating a favorable state of pressure pre-tightening for the soil on the high-load zone side, thereby realizing the preset deformation path, ensuring the safety and stability of the area where the existing building is located, and effectively protecting the most sensitive building area.

[0061] 3. This invention adopts a three-dimensional spatial gridding construction concept, decomposing the entire excavation body into discrete micro-units in three-dimensional space. Through micro-layering, segmentation, and a strict synchronous advancement sequence, a large-scale, high-risk stress release is decomposed into multiple small, controllable stress adjustments, reducing disturbance to the soil, effectively suppressing the development of the plastic zone, and achieving precise management of deformation rate and total amount, thereby maintaining the stability of the foundation pit excavation process.

[0062] 4. This invention utilizes a synchronous progressive excavation method based on an extended working platform. Starting from the top of the slope, it excavates layer by layer, segment by segment, and in opposite directions, while simultaneously applying micro-temporary support. This method eliminates unstable bodies layer by layer from the most dangerous top of the slope, transforming the tall and dangerous slope into a series of stable, low steps. This avoids large-scale landslide accidents during excavation, ensures stable construction of the foundation pit, and fundamentally protects the safety of construction personnel and the surrounding environment.

[0063] 5. This invention couples the anchor cable system with the load zoning results, that is, it directly links the anchor cable spacing, prestress and other parameters with the load zoning. By using denser spacing and higher prestress anchor cables in high load areas for key reinforcement, and simplifying and optimizing by not using or sparsely using anchor cables in low load areas, it achieves on-demand allocation of support resources. While ensuring the overall safety of the foundation pit and effectively controlling deformation, it reduces the cost of support engineering and achieves a balance between safety and economy. Attached Figure Description

[0064] Figure 1 This is a schematic diagram of the excavation cross section of the deep foundation pit excavation method applicable to the surrounding area of ​​existing buildings according to the present invention.

[0065] Figure 2 This is a schematic diagram of the excavation sequence of sub-zone n-① in the deep foundation pit excavation method applicable to the surrounding area of ​​existing buildings in this invention;

[0066] Figure 3 This is a schematic cross-sectional view of the excavation sequence of sub-zone n-① in the deep foundation pit excavation method applicable to the surrounding area of ​​existing buildings in this invention;

[0067] Figure 4 This is a schematic cross-sectional view of the excavation sequence of sub-zone n-② in the deep foundation pit excavation method applicable to the surrounding area of ​​existing buildings in this invention;

[0068] Figure 5 This is a schematic diagram of the excavation sequence of sub-zone n-② in the deep foundation pit excavation method applicable to the surrounding area of ​​existing buildings in this invention;

[0069] Figure 6 This is a plan view of the extended working platform in the deep foundation pit excavation method applicable to the surrounding area of ​​existing buildings in this invention. Detailed Implementation

[0070] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0071] A method for deep foundation pit excavation in areas surrounded by existing buildings includes the following steps:

[0072] S1. Based on the properties of the existing buildings around the foundation pit, the area around the foundation pit is divided into three load zones: high, medium, and low.

[0073] Step S1 includes the following sub-steps:

[0074] Step S11: The zoning of the load zone needs to be based on factors such as the height, structural type, foundation form, and distance from the edge of the foundation pit of existing buildings around the pit. Specifically, it is determined by defining and calculating the load influence coefficient K.

[0075]

[0076] In the formula: K is the load influence coefficient; H is the height of the existing buildings in the area; H0 is the reference height, which is 1.5 to 2.5 times the excavation depth h of the foundation pit, i.e., H0 = (1.5 to 2.5)h; D0 is the reference distance, which is 0.5 to 1.0 times the excavation depth h of the foundation pit, i.e., D0 = (0.5 to 1.0)h; T is the structural system influence coefficient; F is the foundation form influence coefficient; D is the minimum distance between the building and the edge of the foundation pit in the area; α, β, λ, δ are weighting coefficients, α + β + λ + δ = 1.

[0077] The structural system influence coefficient T should be determined based on the structural type of the building.

[0078] 1) For brick-concrete / masonry structures with poor overall integrity and sensitivity to deformation, T=0.8.

[0079] 2) For the frame structure, T=1.0.

[0080] 3) For shear wall structures / frame-shear wall structures with strong integrity but large self-weight and stiffness, T=1.3.

[0081] 4) For low-rise steel structures that are lightweight but have high foundation requirements, T=0.9.

[0082] 5) For high-rise / super high-rise steel structures that are relatively lightweight but have large loads and are sensitive to differential settlement, T=1.5.

[0083] The foundation type influence coefficient F should be determined based on the building foundation type:

[0084] 1) For isolated foundations / strip foundations, these foundations are generally shallow and are more sensitive to lateral displacement and settlement caused by excavation of the foundation pit, F=1.2.

[0085] 2) For raft foundations, this type of foundation has good integrity and strong deformation adjustment ability, but it has a large additional load on the adjacent foundation pit, F=1.0.

[0086] 3) For pile foundations with the pile tip located below the excavation surface of the foundation pit, the load of the superstructure is transferred to the deep part, and the direct impact on the excavation of the foundation pit is small, F=0.7.

[0087] 4) For pile foundations where the pile tip is above the excavation surface of the foundation pit, the excavation of the foundation pit is prone to cause failure of the bearing capacity of the pile tip and negative skin friction of the pile body, which is extremely unfavorable to the superstructure. F=1.4.

[0088] The specific values ​​of the weighting coefficients α, β, λ, and δ can be determined by the analytic hierarchy process (AHP) that compares the importance of each influencing factor pairwise. The final values ​​satisfy δ>λ>β>α to reflect that the proximity has the most significant impact on the stability of the foundation pit, followed by the foundation form, structural system, and building height.

[0089] S12. Along the perimeter of the foundation pit, an assessment section ΔS is defined as 20-30m. i The K value of each evaluation section is calculated sequentially. When K ≥ 2.0, the evaluation section is marked as a high load section; when 1.0 ≤ K < 2.0, the evaluation section is marked as a medium load section; when K < 1.0, the evaluation section is marked as a low load section.

[0090] S13. Let S be the total length of each side of the foundation pit, and let ΔS be the length of each evaluation section on each side. i The calculated K value is marked on the foundation pit plan:

[0091] like Where j is the ΔS corresponding to K≥2.0 on one side of the foundation pit.i If the number of sections is specified, then the area outside that side of the foundation pit is a high-load zone.

[0092] like Where k is the ΔS corresponding to 1.0 ≤ K < 2.0 on one side of the foundation pit. i If the number of sections is specified, then the area outside that side of the foundation pit is a medium-load zone.

[0093] like Where m is the ΔS corresponding to K < 1.0 i If the number of sections indicates that the area outside that side of the foundation pit is a low-load zone, then the area outside the section is a low-load zone.

[0094] By constructing a load influence coefficient K, the four parameters of surrounding existing building height, structural type, foundation type, and distance from the edge of the foundation pit are normalized through benchmark values ​​and weighting coefficients to form a comprehensive evaluation index.

[0095] S2. Divide the total excavation depth of the foundation pit into multiple excavation layers along the vertical direction.

[0096] Please see the appendix Figure 1 In step S2, based on the overturning stability control requirements of the foundation pit on one side of the high load area, the area to be excavated in the foundation pit is divided into layers, namely the first excavation layer, the second excavation layer, ..., the nth excavation layer; among them, the thickness of the first excavation layer is the smallest, and as the excavation depth increases, the thickness of each subsequent excavation layer gradually increases.

[0097] The excavation thickness h1 of the first excavation layer is determined according to the following formula:

[0098]

[0099] In the formula: ΣM r The unit width overturning moment provided for the foundation pit support system, in kN·m / m; γ is the unit weight of the soil, in kN / m. 3 ;K q The overload factor is the overload factor for the high load zone. q represents the additional load generated by the building in the high-load area, in kPa; L a B is the equivalent arm of the overturning moment, in meters (m), and can be taken as 2 / 3 of the excavation depth h; B is the width of the excavation pit, in meters; F s For the safety factor, it can be taken as 1.3~1.5.

[0100] The excavation thickness h of the nth excavation layer below the first excavation layer n It can be appropriately increased based on h1, but it must meet the following requirements. Furthermore, the excavation thickness of each excavation layer shall not exceed 5m.

[0101] S3. For each excavation layer, based on the distribution of the load zone, its profile is divided into two asymmetrical wedge-shaped excavation sub-zones, namely the first wedge-shaped sub-zone and the second wedge-shaped sub-zone.

[0102] Please see the appendix Figure 1 In step S3, the excavation area of ​​each excavation layer is further divided into two excavation sub-areas, namely sub-area n-① and sub-area n-②, from the top of the high load area to the bottom of the load area on the opposite side.

[0103] From the cross-section, the starting point is located at the top endpoint of the nth excavation layer on the high-load zone side, and the ending point is located at the bottom endpoint of the nth excavation layer on the opposite side of the high-load zone. Connecting the starting point and the ending point forms a bisecting diagonal. This bisecting diagonal formed on the cross-section is defined as a preset deformation guide surface on the plane.

[0104] By defining a specific diagonal line from the top of the high-load zone to the bottom of the low / medium-load zone as the boundary for the initial excavation, asymmetric active deformation control is achieved using a pre-set deformation guiding surface. This pre-set deformation guiding surface is used to actively guide the foundation pit support structure to generate an initial displacement towards the low / medium-load zone during the excavation of sub-zone n-①, thereby creating a favorable mechanical state of pre-compacted soil on the high-load zone side from the initial stage of excavation.

[0105] The preset deformation guide surface can realize the active control of the mechanical state of the excavation area. Its orientation setting geometrically forces the unloading amount of the n-① sub-zone to be much greater than that of the high load area side on the low / medium load side. When a large amount of soil support in the pit is removed on the low / medium load side, the soil pressure acting on the support structure on this side decreases rapidly and tends to be active earth pressure. The support system as a whole will rotate and translate towards the low / medium load side, causing the soil on the high load side to be compressed. Its earth pressure value develops from static earth pressure to passive earth pressure. This trend improves the stability and safety margin of the soil on the high load side.

[0106] From a plan view, the starting line is the edge line of the foundation pit on the side where the nth excavation layer is located in the high load zone, and the ending line is the bottom line of the foundation pit on the opposite side of the high load zone of the nth excavation layer.

[0107] S4. Starting from the low / medium load zone side, excavate the first wedge-shaped sub-zone to form a temporary slope facing that side.

[0108] In step S4, the n-① sub-region is excavated using a dual-sided synchronous limited-advance excavation method under three-dimensional spatial grid control. The specific steps are as follows:

[0109] Please see the appendix Figure 2 S41. Spatial calibration: The high-load area is marked as area A, the low / medium load area on the opposite side is marked as area B, and the low / medium load areas on the adjacent sides are marked as area C and area D respectively.

[0110] Please see the appendix Figure 3 S42, Vertical Stratification: In the vertical direction, the n-① subregion is divided into several micro-stratifications y a The thickness of each micro-layer is d, where 1m ≤ d ≤ 2m.

[0111] Please see the appendix Figure 2 S43, Lateral Division: In the horizontal direction, the n-① sub-area is divided into several sections x along the direction perpendicular to the edges of the foundation pits in areas C and D. b The width of each section is w, where 2m ≤ w ≤ 4m, thus discretizing the n-① sub-region into a series of gridded excavation units.

[0112] S44. Symmetrical Restricted Excavation: The central axis of the excavation pit is perpendicular to the edges of the pits in areas A and B (as shown in the attached diagram). Figure 2 (As shown in the attached diagram) Excavation will proceed simultaneously and in opposite directions from the edges of the pits in areas C and D. During excavation, the excavation will proceed in a step-by-step manner, from the side of area B towards the side of area A, and from the center of the pit towards the pit wall, as shown in the attached diagram. Figure 2 As indicated by the arrows in the diagram; when excavation reaches the boundary between each section and the preset deformation guide surface, the slope of that section is immediately trimmed.

[0113] S45. Circular Progression: The current micro-layer y a All gridded excavation units x b Only after all excavation and slope trimming are completed can the next micro-layer excavation cycle begin, until the entire n-① sub-region is excavated.

[0114] The dual-sided synchronous limited-advance excavation method achieves precise control over the soil unloading process in both spatial scale and temporal sequence by discretizing the excavated body into a grid in three-dimensional space. First, the gridded units decompose large-scale unloading into a series of localized, minute stress release events, effectively suppressing the penetration and development of the overall plastic zone. Second, the synchronous, opposing excavation of zones C and D causes the generated lateral earth pressures to cancel each other out near the central axis of the pit, forming a dynamic equilibrium and thus suppressing potential torsional deformation of the entire pit. Finally, the progression from zone B to zone A, and from the center to the pit wall, ensures that the temporary slope formation process is a gradual compression and conformation to the pre-set deformation guide surface from bottom to top, from the stable zone to the sensitive zone. This process utilizes the spatial support effect of the unexcavated soil and provides multiple stress redistribution and stabilization opportunities for the temporary slope through micro-layered cycles, thereby continuously maintaining the overall stability of the pit wall during dynamic excavation and ensuring stability on the high-load side.

[0115] S5. Starting from the high-load area side, the second wedge-shaped sub-area is excavated in layers and blocks.

[0116] In step S5, the synchronous progressive excavation method based on the extended operation platform is used to excavate the n-② sub-area. The specific steps are as follows:

[0117] Please see the appendix Figure 4 S51. Temporary Slope Layering: The temporary slope is divided into several micro-layers according to its vertical height. The thickness of each micro-layer is d', where 1m ≤ d' ≤ 2m. That is, in the vertical direction, the n-② sub-region is divided into several micro-layers y'. a Each micro-layer has a thickness of d', where 1m ≤ d' ≤ 2m;

[0118] Please see the appendix Figure 5 In the horizontal direction, each micro-layer y' a Corresponding horizontal excavation width x' b Determined by the following formula:

[0119]

[0120] In the formula: θ is the slope of the temporary slope, which is determined by the width of the foundation pit B and the depth of the current excavation layer h. n Sure, z represents the total number of sub-divisions in sub-region n-②.

[0121] S52. Excavation and Construction of an Extended Working Platform: Starting from the first micro-layer at the top of the temporary slope, from the centerline of the excavation pit perpendicular to the edges of areas A and B (as shown in the attached diagram). Figure 5 At the location indicated by "---", excavate an initial working trench parallel to the edges of the foundation pits in areas C and D. The depth l (i.e., the length towards the high-load area of ​​area A) of this initial working trench is 2m ≤ l ≤ 4m, and the width s (i.e., the length of the edges of the foundation pits in areas C and D) is s ≤ 5m. The plane formed after the excavation of this initial working trench is the extended working platform, as shown in the attached figure. Figure 6 As shown.

[0122] S53. Two-way synchronous back-to-back excavation: Two excavators are located on the extended working platform and start simultaneously from the central axis of the foundation pit, moving in opposite directions to excavate towards the edges of the foundation pit in areas C and D, respectively. During the excavation, micro-temporary support is applied to the newly formed temporary slope facade facing the unexcavated area, following the advance of the excavators. When the excavators advance to the edges of the foundation pit in areas C and D, the soil removal within the current depth l range is completed. Subsequently, the excavators return to the central axis of the foundation pit and extend the depth towards area A, repeating the above operation of excavating the initial working trench - two-way synchronous back-to-back excavation, until the current micro-layer excavation is completed.

[0123] S54. Only after the current micro-layer is completed and a complete working plane is formed can the construction of the next micro-layer be carried out, and steps S52 and S53 are repeated until the entire n-② sub-area excavation is completed.

[0124] The synchronous progressive excavation method based on the extended working platform constructs a symmetrical stress field to suppress torsional deformation of the foundation pit by starting bidirectional back-to-back excavation operations from the central axis of the foundation pit. It guides the development of the free surface generated by the excavation of the temporary slope to a non-sensitive area to achieve risk transfer. Relying on the layered extended working platform system, through micro-temporary support and unloading control, it achieves the gradual excavation of the temporary slope soil layer by layer and a smooth transition of mechanical state.

[0125] S6. After the first layer of excavation in the second wedge-shaped sub-zone, a horizontal support system is installed at the top of the pit; and when the excavation depth reaches the preset value, an anchor cable system is installed on the pit wall.

[0126] In step S6, after the first excavation layer is completed, a reinforced concrete capping beam is immediately constructed along the perimeter of the foundation pit, and a closed horizontal support system (preferably a prestressed concrete horizontal support system) is erected on the capping beam.

[0127] The horizontal support system consists of two orthogonal main supports (i.e., the first main support and the second main support). The first main support is erected between the capping beams of the high-load zone (zone A) and the low / medium-load zone (zone B), while the second main support is erected between the capping beams of the low / medium-load zones (zones C and D) on both sides. The first and second main supports intersect and connect in the central area of ​​the pit, forming an integral spatial support frame that can effectively transfer and balance the loads outside the pit.

[0128] Preferably, the anchor cable system is installed every 5m (i.e., the preset value) of the cumulative excavation depth; when the excavation reaches the corresponding elevation, the excavation is immediately stopped, and a steel waler is installed along the pit wall at that elevation and the anchor cable is tensioned and locked.

[0129] In step S6, the arrangement of the anchor cable system is matched with the load zoning, implementing a differentiated design. The specific design scheme is as follows:

[0130] In the high-load zone, the anchor cables adopt reinforced design parameters, namely, anchor cable spacing D1=1.2m and prestress P1=1.3P0, where P0 is the standard prestress design value of the anchor cable; in the medium-load zone, the anchor cables adopt standard design parameters, namely, anchor cable spacing D2=1.5m and prestress P2=P0; in the low-load zone, no anchor cables are installed.

[0131] The support system, comprised of a horizontal support system and an anchor cable system, achieves a balance between safety and economy through the principles of overall spatial frame coordination and precise zoned anchor cable deployment. The closed spatial support frame (i.e., the horizontal support system) installed in the first excavation layer integrates lateral earth pressures into an internal self-balancing system. Through orthogonal primary and secondary supports, a portion of the load from the high-load zone is directly transferred to the low-load zone on the opposite side, effectively suppressing the initial overall torsional deformation of the excavation pit. Meanwhile, the anchor cable system, installed at fixed depths, concentrates the support stiffness and resistance to the high-load zone, actively compensating for additional loads, while omitting anchor cables in the low-load zone to avoid redundant construction. Together, this support system constitutes a flexible yet robust active control network that is compatible with the risks posed by the surrounding environment.

[0132] S7. Repeat steps S3 to S6 until the base elevation is reached.

[0133] The above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the invention. Therefore, any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for deep foundation pit excavation in areas surrounded by existing buildings, characterized by: Includes the following steps: S1. Based on the properties of the existing buildings around the foundation pit, the area around the foundation pit is divided into three load zones: high, medium, and low. S2. Divide the total excavation depth of the foundation pit into multiple excavation layers along the vertical direction; S3. For each excavation layer, based on the distribution of the load zone, its profile is divided into two asymmetrical wedge-shaped excavation sub-zones, namely the first wedge-shaped sub-zone and the second wedge-shaped sub-zone. S4. Starting from the low / medium load zone side, excavate the first wedge-shaped sub-zone to form a temporary slope facing that side; S5. Starting from the high-load area side, the second wedge-shaped sub-area is excavated in layers and blocks; S6. After the first layer of excavation in the second wedge-shaped sub-zone, a horizontal support system is installed at the top of the pit; and when the excavation depth reaches the preset value, an anchor cable system is installed on the pit wall. S7. Repeat steps S3 to S6 until the base elevation is reached; Step S1 includes the following sub-steps: Step S11: The zoning of the load zone needs to be based on factors such as the height, structural type, foundation type, and distance from the edge of the existing buildings around the foundation pit, and determined by the load influence coefficient K. ； In the formula: K is the load influence coefficient; H is the height of existing buildings in the area; H0 is the reference height, which is 1.5 to 2.5 times the excavation depth h of the foundation pit, i.e., H0 = (1.5 to 2.5)h; D0 is the reference distance, which is 0.5 to 1.0 times the excavation depth h of the foundation pit, i.e., D0 = (0.5 to 1.0)h; T is the structural system influence coefficient; F is the foundation form influence coefficient; D is the minimum distance between the building and the edge of the foundation pit in the area; α, β, λ, and δ are weighting coefficients. S12. Along the perimeter of the foundation pit, an assessment section ΔS is defined as 20-30m. i The K value of each evaluation section is calculated sequentially. When K ≥ 2.0, the evaluation section is marked as a high load section; when 1.0 ≤ K < 2.0, the evaluation section is marked as a medium load section; when K < 1.0, the evaluation section is marked as a low load section. S13. Let S be the total length of each side of the foundation pit, and let ΔS be the length of each evaluation section on each side. i The calculated K value is marked on the foundation pit plan: like Where j is the ΔS corresponding to K≥2.0 on one side of the foundation pit. i If the number of sections is specified, then the area outside that side of the foundation pit is a high-load zone. like Where k is the ΔS corresponding to 1.0 ≤ K < 2.0 on one side of the foundation pit. i If the number of sections is specified, then the area outside that side of the foundation pit is a medium-load zone. like Where m is the ΔS corresponding to K < 1.0 i If the number of sections indicates that the area outside that side of the foundation pit is a low-load zone, then the area outside the section is a low-load zone.

2. The method for deep foundation pit excavation in areas surrounded by existing buildings according to claim 1, characterized in that: The structural system influence coefficient T is determined according to the building's structural type: for brick-concrete / masonry structures, T=0.8; for frame structures, T=1.0; for shear wall structures / frame-shear wall structures, T=1.

3. For low-rise steel structures, T=0.9; for high-rise / super high-rise steel structures, T=1.

5.

3. The method for deep foundation pit excavation in areas surrounded by existing buildings according to claim 1, characterized in that: The foundation type influence coefficient F should be determined according to the foundation type of the building: for isolated foundations / strip foundations, F=1.2; for raft foundations, F=1.0; for pile foundations with pile ends below the excavation surface of the foundation pit, F=0.7; for pile foundations with pile ends above the excavation surface of the foundation pit, F=1.

4.

4. The method for deep foundation pit excavation in areas surrounded by existing buildings according to claim 1, characterized in that: The values ​​of the weight coefficients α, β, λ and δ satisfy δ>λ>β>α, α+β+λ+δ=1.

5. The method for deep foundation pit excavation in areas surrounded by existing buildings according to claim 1, characterized in that: In step S2, based on the overturning stability control requirements of the foundation pit on one side of the high load area, the area to be excavated in the foundation pit is divided into layers, namely the first excavation layer, the second excavation layer, ..., the nth excavation layer; among them, the thickness of the first excavation layer is the smallest, and as the excavation depth increases, the thickness of each subsequent excavation layer gradually increases. The excavation thickness h1 of the first excavation layer is determined according to the following formula: ； In the formula: ΣM r The unit width overturning moment provided for the foundation pit support system, in kN·m / m; γ is the unit weight of the soil, in kN / m. 3 ;K q The overload factor is the overload factor for the high load zone. q represents the additional load generated by the building in the high-load area, in kPa; L a B is the equivalent arm of the overturning moment, in meters (m), and is taken as 2 / 3 of the excavation depth h of the foundation pit; B is the width of the foundation pit, in meters (m); F s To ensure a safety factor, a value of 1.3 to 1.5 is adopted. The excavation thickness h of the nth excavation layer below the first excavation layer n Add to h1 to satisfy Furthermore, the excavation thickness of each excavation layer shall not exceed 5m.

6. The method for deep foundation pit excavation in areas surrounded by existing buildings according to claim 1, characterized in that: In step S3, the excavation area of ​​each excavation layer is further divided into two excavation sub-areas, namely sub-area n-① and sub-area n-②, from the top of the high load area to the bottom of the load area on the opposite side. From the cross-section, the starting point is located at the top end of the nth excavation layer on the high load zone side, and the ending point is located at the bottom end of the nth excavation layer on the opposite side of the high load zone. The starting point and the ending point are connected to form a bisecting diagonal. The bisecting diagonal formed by connecting the points on the cross-section is defined as the preset deformation guide surface on the plane. From a plan view, the starting line is the edge line of the foundation pit on the side where the nth excavation layer is located in the high load zone, and the ending line is the bottom line of the foundation pit on the opposite side of the high load zone of the nth excavation layer.

7. The method for deep foundation pit excavation in areas surrounded by existing buildings according to claim 1, characterized in that: In step S4, the n-① sub-region is excavated using a dual-sided synchronous limited-advance excavation method under three-dimensional spatial grid control. The specific steps are as follows: S41. Spatial calibration: The high-load area is marked as area A, the low / medium-load area on the opposite side is marked as area B, and the low / medium-load areas on the adjacent sides are marked as area C and area D respectively. S42. Vertical stratification: In the vertical direction, the n-① subregion is divided into several micro-stratifications y. a The thickness of each micro-layer is d, where 1m ≤ d ≤ 2m; S43. Horizontal Division: In the horizontal direction, sub-region n-① is divided into several sections x along the direction perpendicular to the edges of the foundation pits in regions C and D. b The width of each section is w, 2m≤w≤4m, thus discretizing the n-① sub-region into a series of gridded excavation units; S44. Symmetrical Restricted Excavation: Using the central axis of the foundation pit perpendicular to the edges of the foundation pits in areas A and B as boundaries, excavation is carried out simultaneously and in opposite directions from the edges of the foundation pits in areas C and D respectively; during the excavation process, the excavation is carried out in a step-by-step manner from the side of area B to the side of area A, and from the center of the foundation pit to the direction of the pit wall; when the excavation reaches the intersection of each section with the preset deformation guide surface, the slope of that section is immediately trimmed; S45. Circular Progression: The current micro-layer y a All gridded excavation units x b Only after all excavation and slope trimming are completed can the next micro-layer excavation cycle begin, until the entire n-① sub-region is excavated.

8. The method for deep foundation pit excavation in areas surrounded by existing buildings according to claim 1, characterized in that: In step S5, the synchronous progressive excavation method based on the extended operation platform is used to excavate the n-② sub-area. The specific steps are as follows: S51. Temporary Slope Stratification: The temporary slope is divided into several micro-stratifications according to its vertical height, that is, in the vertical direction, the n-② sub-region is divided into several micro-stratifications y'. a Each micro-layer has a thickness of d', where 1m ≤ d' ≤ 2m; In the horizontal direction, each micro-layer y' a Corresponding horizontal excavation width x' b Determined by the following formula: ； In the formula: θ is the slope of the temporary slope, which is determined by the width of the foundation pit B and the depth of the current excavation layer h. n Sure, z represents the total number of sub-regions n-②. S52. Excavating an extended working platform: Starting from the first micro-layer at the top of the temporary slope, excavate an initial working trench along the direction parallel to the edges of the pits in areas C and D, from the central axis of the pit perpendicular to the edges of the pits in areas A and B. The depth l of the initial working trench is 2m≤l≤4m, and the width s of the initial working trench is s≤5m. The plane formed after the initial working trench is excavated is the extended working platform. S53. Two-way synchronous back-to-back excavation: Two excavators are located on the extended working platform and start simultaneously from the central axis of the foundation pit, moving in opposite directions to excavate towards the edges of the foundation pit in areas C and D respectively. During the excavation, micro-temporary support is applied to the newly formed temporary slope facade facing the unexcavated area, following the excavator's advance. When the excavator advances to the edge of the foundation pit in areas C and D, the soil removal within the current depth l range is completed. Subsequently, the excavator returns to the central axis of the foundation pit and extends the depth towards area A, repeating the above operation of excavating the initial working trench - two-way synchronous back-to-back excavation, until the current micro-layer excavation is completed. S54. Only after the current micro-layer is completed and a complete working plane is formed can the construction of the next micro-layer be carried out, and steps S52 and S53 are repeated until the entire n-② sub-area excavation is completed.

9. The method for deep foundation pit excavation in areas surrounded by existing buildings according to claim 1, characterized in that: In step S6, after the first excavation layer is completed, a reinforced concrete capping beam is immediately constructed along the perimeter of the foundation pit, and a closed horizontal support system is erected on the capping beam. The horizontal support system consists of two orthogonal main supports, namely the first main support and the second main support. The first main support is erected between the capping beams of the high load zone A and the low / medium load zone B, and the second main support is erected between the capping beams of the low / medium load zones C and D on both sides. The first and second main supports are intersected and connected in the central area of ​​the foundation pit to form an integral spatial support frame. Anchor cable system is installed every 5m of cumulative excavation depth; when the excavation reaches the corresponding elevation, the excavation is immediately stopped, and steel walers are installed along the pit wall at that elevation and the anchor cables are tensioned and locked. The arrangement of the anchor cable system is matched with the load zones, and a differentiated design is implemented: in the high load zone, the anchor cables adopt reinforced design parameters, that is, the anchor cable spacing D1=1.2m and the prestress P1=1.3P0, where P0 is the standard prestress design value of the anchor cable; in the medium load zone, the anchor cables adopt standard design parameters, that is, the anchor cable spacing D2=1.5m and the prestress P2=P0; in the low load zone, no anchor cables are installed.

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